aj mccormack and son

Flags & Slabs - Page 08
The Brew Cabin
flags and slabs


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Forum Question Lorry damaged driveway - Anji - Jun 10th 2003
I think I know the answer to this but I wanted to share it.......A few months back you kindly confirmed that Bob the Builder had left me with a shoddy crazy paved patio - I had hoped that the drive would fare better as the pointing seems harder. HOWEVER - we have been filling up a skip with builder's rubble (left behind by Bob) over the last few days - the lorry collected said skip today - lo and behold there is now a wheel shaped dip on the drive with cracked and broken pointing all around it.

I know the chaps laid a layer of what they called MOT down first and used a wacka(?) plate on it but I can't recall how they bedded the slabs after that. Not correctly it would seem.

Is there any point in us lifting and rebedding this bit or should we just watch it all crumble around us and get someone in who knows what they are doing? sad

forum answer Tony McCormack - 14th Jun 2003
Well, to be fair to your Bob, most residential driveways could not cope with a skip wagon. They are designed for loads of up to around 3-5 tonnes, whereas a skip lorry, even before it has the skip on its back, is the best part of 10 tonnes - you can double that once it's carrying a fully loaded skip!

I'm not sure how extensive is the damage caused by the skip wagon, but I'd guess that your best bet is to lift and relay whatever has been disturbed, if only for safety reasons, and then consider a full reconstruction next year, once you've had a chance to see how the rest of the driveway performs over the summer. It may be that, once normal traffic resumes, the drive copes adequately, and there would be no need to re-construct, or, if you're really unlucky, it might be that it gradually deteriorates as it's trafficked leaving you no real option other than to start again.

If you've any photos of the damage done, send them in to me and I'll take a look.

Forum Question Overlay quandary - P James - Jun 14th 2003
I'm laying a york stone patio, on top of an old concrete path and newer concrete patio (50mm depth). However the curbstones which form the boundary of the patio only give me 80mm height to work with.

Question is do I go for 50mm flags with 30mm bed, or thinner flags and more there a problem with having the flags proud of the curb boundary by 10mm or so?
Advice gratefully received.

forum answer Tony McCormack - 15th Jun 2003
The bedding is the weakest material, so you should always aim to keep it to no more than 50mm thickness. Ideally, the bedding should be 30-50mm in thickness, so, given the choice, opt for slightly thicker flags.

Laying the flags 6-10mm higher than the top of the kerb is not a problem - that's how they should be laid, although 3-6mm upstand is much safer than a 10mm "trip".

Forum Question Fair Price for laying Indian Sandstone - Chris May - Jun 17th 2003
Knowing how many cowboys there are in the building trade has always put us off using them so when we decided to re-lay and extend our patios, we were resigned to do it ourselves. After having read your excellent web site, we felt confident to get some quotes.

We have been quoted, so far, for a labour only charge, of between £2900 and £6000 to lay 84sq m of Indian Sandstone flags and to build a natural stone mortared wall ¾meter high by 12 meters long. 90% of the preparation work has already been done by us (excavation, clearance, laying of the base and design of the random pattern down to the last flag), as per your instructions. They have quoted for compacting the base, laying the flags on a full mortar bed, for full mortar joints and for building the wall. We are supplying all materials. All the contractors have said it's a straightforward job, and will lay the flags straight onto the base that we have prepared.

We want a first class job, so don't want to take the cheapest quote and get a shoddy job, however, we don't want to be ripped off either, so if you can give us any guidance on what would be a fair price for such a job we will be extremely grateful.

forum answer Tony McCormack - 18th Jun 2003
As I've not seen the job, I can't really say what would be a fair price. Things such as access, location, local trading conditions, etc. all play a part in settting the price of a job such as this.

The paving is straightforward enough, and, from speaking with contractors throughout Britain and Ireland, the 'going-rate' for laying Indian sandstone is around 15 quid per square metre. So, your 84m² patio should cost something around 1250 quid to lay.

The walling is another matter, as the amount of labour involved depends on the type of stone being used - particularly whether its coursed or random, and how much dressing will be required. The price diff between an average 'builder' having a stab at building in stone, and the cost of a genuine stone mason is around 200%, but a stone mason will do a better job in a shorter period of time.

You have to go with your own intuition. Have you checked out previous work by these companies? Have you spoken to previous clients? Is there one that makes you feel more comfortable than the others, or is there one that gives you the jitters?

The proportion of cowboys in the construction industry is actually quite small - it's just unfortunate that one story about a cowboy is more interesting to the general public than 1,000 stories of honest-to-goodness builders who bend over backwards to fulfil the client's wishes. I would suggest that the best way to sort the wheat from the chaff in your case is to ask for a written method statement, in which each of the contractors tendering for the work sets out exactly how the paving will be laid and the wall built. This will give you a better insight into their working methods and, hopefully, will help narrow down the field.

Chris May
Jan 18th 2003
Many thanks for this helpful advice. You've confirmed what we suspected that, although we think that the contractors we've seen so far will do a good job, (they all said the right things about laying, pointing and drainage, as per your web site) they've got a bit carried away with their profit margins!

Location and access to our property is good so there should be any premium being charged there. Generally speaking, in our experience, wages and charges for work tend to be on the lower side in Devon than in the South East say, so that shouldn't affect the quotes. As we were going to lay the patios ourselves originally, we did have a quote from a stonemason for the walling (building stone walls is a bit out of our league) for £500 approx to supply and build, which we thought was very reasonable. In view of your advice, we'll probably use him and then we can be in no doubt about what were being charged for laying the flags.

We can now sort out the silly quotes from the serious ones and we will definitely ask for a written method statement. We've seen pictures of each of the contractors past work but perhaps we should push to see the real thing!

Once again thanks.

Forum Question Uneven slate patio - Tianshu - Jun 17th 2003
Desperately looking for advice. Our patio is poorly installed, the surface is uneven and the water stays at several low spots and makes water puddles.

It is slate on top of concrete. The contractor cannot repair it. Indeed, when they did try, they damaged the surface of slate. Are there any solutions to this problem? Thanks in advance

forum answer Tony McCormack - 18th Jun 2003
There are no 'quick fixes' for such a problem. Slate on concrete is more or less impermeable, so there's nothing you can do to alleviate the ponding problem other than lift the paving and relay it to falls so that the surface water is directed to disposal points, such as gullies or the garden, etc.

It's not so much that the contractor cannot repair it, it's more a case of they can't be bothered to make it right!

Jun 18th 2003
Thank you very much. We are the first-time homeowner and know nothing about these home projects. This site is really helpful to us.

Our contractor is a garden expert and we initially asked him to do our garden and then added the patio project. It seems that he is not experienced in patio. He did a beautiful garden, but the patio is a dissapointment.

The contractor is now proposing to grind the patio. Is this the correct thing to do? Can this solve the ponding problem? What will the slate patio look after the grinding? Should we insist him to relay the patio instead? Should the entire patio be relay or only the puddle areas? I am concerned whether it produces the right slope if only relay the puddle area. Three sides of our patio are surrounded by lawn and the other side is attached to the house.

Many thanks!

Tony McCormack
Jun 19th 2003
Tianshu commented....
Our contractor is a garden expert and we initially asked him to do our garden and then added the patio project. It seems that he is not experienced in patio. He did a beautiful garden, but the patio is a dissapointment.

The moral of the story is that, just because someone can plant flowers and lay turf, it does not mean they can lay paving. You're not the only one to make this error - I hear it time and time again..."Oh, they did such a lovely job of the garden , so I thought they'd be ideal to lay the driveway, but..."

If you employed a painter to re-gloss all the windows, doors and fascias of the house, would you use them to to plumb-in your new cooker, just because they did a good job on the bargeboards?

Each man/woman to his/her trade!   smile

The contractor is now proposing to grind the patio.  Is this the correct thing to do?

NO! It will make a right mess of the slate, rendering it useless and covering the garden in a dangerous, lung-clogging dust!

Can this solve the ponding problem?

Possibly, but it is more likely to cause more problems than it solves, as well as being very expensive, environmentally-unfriendly and somewhat hit-and-miss.

What will the slate patio look after the grinding?

The area that has been grinded will have a radically different surface texture to the rest of the patio, and that difference is permanent. The only way to hide the treated area, is to grind the entire patio, which is just being silly!

Should we insist him to relay the patio instead? Should the entire patio be relay or only the puddle areas?

Yes - re-lay the affected areas and see if that cures the problem. There's no need to re-lay areas that drain adequately at the moment, just the areas that pond and any other areas that need to be adjusted to ensure the surface water has a clear escape route to a disposal point. So, for example, if there was a 1 metre diameter puddle 3 metres from the edge of the patio, the puddle area plus the 2 metres between the puddle and the edge would need to be re-laid to ensure the water can escape in future.

This 'contractor' has already made a mess of the patio, and is now offering to turn it into a total disaster. They should realise and accept the limitations of their talents. They are out of their depth - they do not know how to lay paving properly, nor how to put it right when they've got it wrong. They would be best advised to bring in a specialist to correct the problem. In the long run, it's probably cheaper and quicker to rely on someone who actually knows what they're doing!

Forum Question Pointing Questions - Steve B - Jun 18th 2003
I have just finished a small natural stone patio ( 8 x 9 feet ), gravel sub-base, sharp sand / mortar bedding layer and have a few follow-up questions (horse, stable .. bolted).

The patio is immediately behind our house ( mid-terrace victorian ) and as our neighbour has an extension the patio effectively abuts a wall on 2 sides ( L shape ). I removed a tiny crap cracked concrete patio ( 5 x 8 ) that was originally there.

So here goes....
We aimed for 6:1 bedding mix - supplied bags of portland cement were 25Kg, sand was 40Kg so couldn't do 4:1. I suspect we actually started with 8:1 before strengthening. Do you think this change will cause a problem?

The flags were laid on a baking day. Because we didn't want to walk over the flags, we dry-mortared as we went with the first few rows done with 8:1 mix. These joins seem a bit crumbly. Do you think it is worth re-doing the dodgy bits straight away or only if movement results?

Also we aimed to leave around a 10mm gap but a couple of flags were touching due to us not getting the rows absolutely straight. As these flags are wider at the top than bottom and taking into account the heat I'm not confident that we have the cavity nicely filled. I'm wondering whether we could then get problems with water entering the join and frost damage as well as unwittingly having created a home for ants ( we have many ). Again do you think this is worth re-doing straightaway? With the blocks almost touching ( although the irregular join means a small gap ) how would I add new mix?

Next. The rear wall on our house has been re-rendered down to the level of the old patio. There is now a recess to the original brick where I have removed the old patio and the new flags start about an inch out and below where this render finishes. I plan to fill this gap with 3:1 sand - cement mix plus waterproofing agent. As I have several bags of sharp sand left over could I use this or is it a big no no to use sharp sand instead of builder's sand. Is the builder's sand better for waterproofing? Does the above ratio sound ok?

Finally, when I had removed the original patio I found that it was above our neighbour's dampcourse. There is even a flower bed with the soil also at a higher level! I'm assuming this was possible as the wall had a smooth render ( like ours ) going down and joining the old concrete patio ( 3 inches above DPC in 1 place ) and also going down below the top level of the soil ( and inturn DPC ) where the flower bed was located. Have you come across this before?

Apart from this the patio looks great ! Sorry about the wordy-ness of all this!

forum answer Tony McCormack - 19th Jun 2003
OK - taking your questions one at a time....

An 8:1 mix for bedding is fine. I normally recommend a 10:1 mix for patios, as I found that was always adequate.

I'm not sure what you mean by 'dry-mortared' but I'm assuming you buttered the joints with a mortar and then pointed them immediately, but because of the heat, they mortar was 'parched', ie, it had the water sucked out of it by the thirsty flags and natural evaporation before it had a chance to ensure full hydration of the cement content. This is why the resultant mortar is weak and crumbly, and the only remedy is to cut out all the weak mortar and replace it with fresh, prefeably on a cloudy or overcast day, when it's out of direct sunlight, and late in the afternoon.

The joint width is a problem. When laying these stone flags, or even the imitation stone flags that are popular, it's best to leave the joints unpointed until an area has been laid and the joint widths adjusted by levering with a bar or a spade to even out any differences. Ideally, no joints should be less than 6mm and none wider than 18mm, (12m ± 6mm), but there is always some variation.

If you have flags in direct contact with their neighbours, this is less than ideal, but, if it's only one or two, then it's not worth losing sleep over. However, if there is a lot of very tight joints, it may be better to re-lay a section to even out the joints. Without seeing it, I can't say which is best.

When it comes to filling those tight joints, you could mix a slurry mortar and pour that into the joints to find its own level and then top them up later, once the slurry mortar has settled - it always 'sinks' by 6mm or so as the water content drains out and is used up by the cement. For a slurry mix, use a 4:1 mix with a plasticiser and plenty of water to make it a soup-like consistency. Keep the slurry agitated by stirring and pour it into the joints from an old watering can or similar. Wipe up and sponge down any spillage immediately. Take your time - you will probably get some slight overspill and that may stain the stone slightly, but it can be washed down with an acid wash in 7-10 days and no-one will be any the wiser.   smile

An alternative, and in my opinion less successful method, is to use a dry mortar, ie a mix of very dry sand and cement with no added water. Brush that into the joints using a hand brush, and then pour water into the joint to wash down the dry mortar into every nook and cranny of the joint and initiate hydration. I find that this method always results in a lot of settlement in the jointing material as the water washes through the dry mix, and you end up having to repeat the process two or three times to get properly filled joints with no great gaping voids.

And then the gap in the rendering - using sharp sand will give you a coarser-textured mortar. Structurally, it's fine to use it to make up the 25mm fillet between the old render and the paved surface, and, if you're happy enough with the texture, then there's nowt wrong with it.
Building or plastering sand is normally used for rendering because it is finer and has a higher clay content, which means it makes a 'stickier' mortar and adheres more immediately to the substrate (it's all to do with grain size, the size of the voids between the grains, and capillary action), as well as giving a smoother finish when trowelled. But for a 25mm gap, at the base of a wall, that no-one will ever notice, using sharp sand will not be a problem.

The problem with the levels and your neighbour's DPC is more of a problem, though. It may be that some previous occupant laid the old patio and didn't understand the way that DPCs work, and so, in blissful ignorance, laid paving and constructed flower beds that breached the DPC.
Your idea about the render protecting the DPC is not right - a render would, normally, stop at or above the DPC, not bridge it, as seems to be the case here.

Does your neighbour have any problems with damp in their property? If they do, of if damp problems were to develop, and it could be shown that your patio was the prime cause, you could be left facing a large-ish bill for remedial work and re-decorating.

When you bought the property, did your surveyor not identify this problem of a bridged DPC?

Steve B
Jun 20th 2003
Thanks for the full reply.

Yes, the pointing is as you described and I will gouge out and redo the bad sections. In many places it has actually hardened well. For some reason I have it in my mind that I should have pushed the mortar down having brushed it in - which I didn't.
Does this matter if it's hardened well on top?

One additional point on the pointing ( yup bad pun ). I've noticed that some of it looks wet and there is a damp patch a few cms either side on the flags ( parallel to the join ) even though it's been dry and warm for a while. I remember reading about this in another post but can't find it. Does this indicate a cavity underneath?

The tight joint indeed only affects a couple of edges so I will leave them down I reckon.

As far as the DPC goes we've been in the house for 12 months and our neighbour hasn't mentioned damp problems. Having said that it's not in the weather / trains .. hmm / weather list so I don't think it would come up and I don't think the surveyor looked at the adjoining property as it wasn't mentioned.

As a complete building novice I'm pretty ignorant of DPCs. When I removed the old concrete it showed as a thin black layer of what looks to be slate. It is very low compared to the level of the garden. Yet if the wall is rendered and this forms a neat join with a concrete patio how would you know where it was? This is the case with our section of wall. I just made sure that my flags weren't higher that what I replaced. We have no damp problem. Around the corner ( path to kitchen door ) on our house the unrendered wall has been mortared over the bottom 8 or so inches so I presume the DPC is under there but I can't visibly see it unlike the newly exposed section of our neighbours wall.

Tony McCormack
Jun 20th 2003
Steve B scribbled.....
For some reason I have it in my mind that I should have pushed the mortar down having brushed it in - which I didn't.
Does this matter if it's hardened well on top?

It's essential that the mortar is pressed firmly into the joints and then tooled on the surface to create a full joint that bonds properly to the edges of the flags and forms an effective seal at the top. Simply brushing-in the mortar mix will, of course, fill the joint and help keep out litter and fag-ends, but it doesn't actually help reinforce the construction.

With regard to the mystery DPC, these older properties can often be very confusing as to whether they have a proper DPC at all, and, if they do, where it is and why. I'd suggest that, as long as you're no higher than what was there before, and there's no problem with damp walls for either yourselves or the neighbour, then forget about it and just enjoy your patio.   smile

Forum Question Re-laying Flagstones on Concrete Stairs - Ray Mallon - Jun 21st 2003
I would be very grateful for help on the best method for re-laying existing, loose flagstones on a 1.20 meter wide concrete stairway, which rises some 3 meters from street level to a patio at house level, whilst avoiding the water seepage that affects the plaster on the wall adjacent to the stairs.

To complete the problem description (sorry it is a bit long but I'm guessing that a full description will be better than an outline):

At the bottom of the stairs is a level area comprising several flagstones, which are strongly bonded to the underlying concrete. Several steps take us to the middle 'plate' and several more take us to the upper 'plate' (I'm not sure of the correct terminology), each 'plate' comprising several flagstones and with overall dimensions of approx. 1.25 x 1.25 meters.

The horizontal 'stepping' flagstones are rectangular, they are cut and ground on all surfaces to 123 cm x 25 cm x 3 cm.  The same stone is used for the vertical pieces between the steps – they are also cut and ground to a uniform rectangular shape (121 cm x 13.5 cm x 2 cm).  The flagstones are a natural stone, grey/black speckled in colour, I don't know the stone type.

The right side of the stairs, when looking upwards, is bounded by a wall, which forms the left side of the driveway to the garage. The left side of the stairs is bounded by a number of reconstituted stone slabs (18 cm x 12 cm cross-section and unknown length), which are positioned vertically and form a barrier between the stairs and the garden, which slopes upwards from these slabs. There is a narrow air gap between these vertical slabs and the stairs.

The vertical stones of the steps were originally cemented to the concrete stairs; they are now mostly loose. The horizontal stones are resting on a layer (about 40 mm thick) of what appears to be a loose sand/cement mix, rather like that described in the 'Laying Flags/Slabs' section of the website. The mortar between the stones comprising the 'plates' is badly degraded, as is the mortar between the horizontal and vertical stones.

The effect of the degraded mortar is that the sand/cement mix supporting the steps remains very moist.  This moisture is transmitted to the adjacent wall, causing unsightly discolouration and degradation of the plaster in the area of the wall that is immediately adjacent to the stone steps.  My aim is to prevent this happening.

The second issue is whether or not one should have any 'fall' on the horizontal steps to help clear rainwater (and, in winter, to minimise ice formation) – and if so, how much?

I would be very grateful for any help with these questions.

forum answer Tony McCormack - 21st Jun 2003
Hi Ray,

what you call 'plates' are actually termed 'landings' - it's one of those things that you actually know, but that you can't call to mind on the only occasion in your life you'll ever need the information!

The loose flags can be re-bedded using a good quality, Class II mortar, with an added waterproofing and bonding agent - I'd be tempted to use SBR (Styrene Butadiene Resin), as it imparts excellent adhesive, water-proofing and strengthening properties to a standard mortar. It can be bought from most good BMs for around 20-30 quid for 5 litres, but it's money well spent for a job such as this.

Remove all the loose paving and any other stonework that needs to be re-seated. If in doubt, take it out - it's much easier to chisel out an 'iffy' piece of stone now, rather than wait and hope that it doesn't work itself loose next winter.

Clean off the old, degraded moratr and expose the main structure of the steps as far as possible. Don't leave any soft, degraded or suspect material in place.

Starting at the top of the steps, and working your way down, re-set the treads on a 25-45mm full bed of SBR-enhanced mortar. Spread the mortar with a trowel, ripple the bed to allow seating, then press the flags into the mortar and tap down to the required level using a small rubber hammer. Make sure they flags are well and truly held by the mortar, and point up any joints immediately. You need a fall of around 1:40, that's 50mm over the 1250mm depth of each tread, to ensure there's no standing water or chance of ice formation, so use a spirit level to guide your levels. There's no need for a cross fall if you have sufficient end fall.

Any vertical stonework that needs re-fixing can be stuck on using the same mortar, although you'll need a thinner 'bed'. Again, work from the highest step downwards, so that you're not having to stand on newly-finished work to proceed.

SBR hastens the initial cure of the mortar, and you'll have only 20 minutes or so of working time, so don't mix too much at once. Follow the guidance on the container for dosing quantities, and don't be tempted to overdo it, or add a bit extra just to be safe - it's a waste and will only reduce the working time without improving the end quality of the mortar.

A building sand is fine for making the mortar, and you can use an ordinary portland cement - the SBR acts as its own plasticiser, so no need to add any or to buy pre-plasticised cement. Keep your measures fairly accurate - X ml of SBR per litre of gauguing water, 4 empty paint tins of building sand to one empty paint tin of cement, that sort of thing, and then don't mix the mortar too wet or it starts dribbling everywhere. Keep it stiff but workable, and don't rush!

Good luck!

Ray Mallon
Jun 22nd 2003
Many thanks for your prompt response Tony – and for a wonderfully informative and well laid-out website.  

In the meantime I have found your "Steps" page, among many others, and so I am now up to speed on the terminology for my question!

Sorry to be a pest but I have a three subsidiary questions:-

1) - Concerning the 'fall' for each tread, you advise 50 mm for a 1250 mm tread depth. I'm guessing that this refers only to the landings, which are 1250mm x 1250 mm. The depth of each regular tread is 25 cm (they are 1230 mm wide) and a 40:1 fall equates, I think, to 6.25 mm per tread. Bottom Line: I'm guessing that we want the water to flow down from step to step rather than tipping it over the side of each tread into the garage drive. Is that right or is tipping it into the drive what you had in mind?

2) - Would you recommend using a flexible (elastomeric, such as acryl or silicone or whatever) pointing material for the gaps between (a) the treads and the stone plates that cover the riser area and (b) the gaps between the treads/risers and the (whoops, I've run out of terminology - they'd be called skirting boards inside the house) side bits. (I'm thinking minus 15C in winter in this part of Germany).

3) - Would you recommend using a urethane sealant on top of the Class II mortar that you propose for bedding the flags and pointing of the joints? (To minimise, maybe, wash-out of the pointing when using a high pressure washer for occasional cleaning of the stone?)  Or would it offer no improvement?

Looking forward to hearing from you.

Tony McCormack
Jun 23rd 2003
Q1 - for the steps, use the same sort of fall, 6-10mm per step will be plenty, and yes: we do prefer the water to run down the steps than over the edges, but, as long as the water gets away, it doesn't really matter. The reason why the fall is most commonly arranged to send surface water cascading down the steps rather than over the edge is that, if you slope a flight of steps from one side to t'other, it looks odd to the eye when you're at the foot of the steps. It's an aesthetic thing, rather than anything structural.   smile

Q2 - for the joints between treads and risers, I'd use the same mortar, but, for the joint between the top landing and the side of the house, an elastomeric sealant would be used as long as there no direct tie-in of the step structure to the house structure, i.e., they are two, separate entities.

Q3 - If you use a SBR-improved mortar for the joints, there's no way a power-washer is going to loosen that mortar! We've successfully used SBR mortar to point setts and stone pavings in city centres that are subjected to very high-pressure jet-washing to remove chewing gum, and to the little motorised street cleaning vehicles that use vacuums to collect up any litter and detritus, and the mortar stays put!

Leave out the selant at this stage - I'm pretty confident that you'll never need it.   smile

Forum Question Random slabs and drains - Samantha - Jun 23rd 2003
Need some advice. Just moved into a new property and have just 2 paving slabs outside French doors and turf. Can't afford to have a proper patio done at the moment (have had a few quotes) but need something for my table chairs as the ground is uneven. Could I just dig and level the ground, lay a plastic sheeting (to prevent weeds), have a laying base of sharp sand/dry cement lay the slabs randomly with gaps and infil with gravel? The patio is not going to be raised so would this be ok. Also would I need to do a slope for water drainage.

P.S as it's a new house the ground is literally rock solid.

forum answer Tony McCormack - 23rd Jun 2003
Drop the idea of using a plastic sheet - it's a recipe for disaster. Have a read of the new page I uploaded earlier - FAQ - Do I really need a membrane? - for a fuller explanation of why plastic sheets are a no-no.

Apart from that, your plan sounds feasible. Gradient for drainage is as stated on the site, particularly the Drainage for Pavements page - 1:60 is about right. Bear in mind that there has to be somewhere for the water to drain to.

Jan 11th 2003
Thanks for the reply which was of help. As for drainage was thinking of digging a trench at end of patio area (plastic sheeting and filling the hole with gravel so it looks like an edging (i.e is it called a french drain or something or other). Will this be ok?
Tony McCormack
Jun 23rd 2003
What's this obsession with plastic sheeting??? Plastic sheeting is impermeable - that means it doesn't let water pass through, so why would it ever be used for drainage??

Have a read of the Land Drainage pages of the site. They answer your questions.

Jun 23rd 2003
I read it some where on another gardening site and it said with french drains to dig down to a certain depth and lay some sort of plastic sheeting. This doesn't make sense to me either!!!!

Gonna have another look!!!!!

Have been looking on the site about drainage already.

Forum Question Balcony Patio - Tucker - Jun 25th 2003
I have a balcony at the rear of my house on which i would like to lay textured paving slabs. The balcony is constructed from approx. 8" concrete and part of it forms the roof of my garage and workshop. The previous covering of slabs was in poor condition and allowed water to penetrate into the rooms below. I sealed the concrete with Aquaseal 8 months ago and this has cured all water penetration.

Is it now possible to lay slabs onto this base or is this not advisable from a water-proofing point of view. Any advise would be gratefully appreciated.

forum answer Tony McCormack - 26th Jun 2003
Normally, balconies and roof gardens are paved with what are known as Promenade Pavers, a type of lightweight flags, but, as your balcony is a fairly substantial concrete construction, you could use any flags you choose.

Now that you've Aqua-sealed the surface, I would suggest that you lay a damp proof membrane over the concrete (Visqueen 1200 or similar), then lay the flags on a full bed of Class II mortar, with mortared joints, and adequate fall to a drainage point. That should be fine.   smile

Forum Question Wooden Edge or Concrete Edge - Chrissy K - Jun 26th 2003
I'm the one who is creating a 40m² imported indian sandstone patio.

All the excavation work is nearly completed and the site should hopefully be cleared by 7pm on Sunday (if not I can't go to the pub)

The grit sand is coming the following Friday, which will be dumped and levelled over the week and the stone hopefully the following Friday.

My question is:- I was just going to lay the slabs haunched up to a concrete edge. I have just finished reading your page on timber edge and now I'm not to sure as the timber edge looks slightly easier to install.

Can you please give your advice and the costings of the concrete based on 30m of edge needed

Cheers - Chris

forum answer Tony McCormack - 26th Jun 2003
Maybe I'm missing summat, but why do you need an edging? Unlike block paving, gravel or tarmac, flags don't need an edging to ensure their integrity - they can manage perfectly well without one.

There's no doubt that timber edgings are easier to install, and cheaper than concrete-bedded edgings, but they aren't as secure and tend to have a shorter lifespan.

The cost of a concrete bedded edging has two components. The first is the cost of the edging itself, which can be anything from a few pence per metre if you use old, relaimed bricks or setts, up to several pounds per metre, if you opt for a fancy manufactured unit. Secondly, there's the cost of the concrete needed for the bedding and the haunching - this varies slightly, depending on the dimensions of the edging being used. A 200mm wide brick paver used as an edging needs a bed around 300mm wide, but the haunching will only be 40-50mm high, whereas a 200x50mm concrete edging kerb needs a bed only 150mm wide but the haunching will be three times that needed for blocks, at 150mm wide.

So, I'd suggest you first decide whether an edging really is necessary, and then, if it is, decide on the type.

To calculate the volume and cost of the necessary concrete, take the width of the edging, add 100mm and then multiply by 100mm. That gives a rough quantity of concrete, per metre, needed for bedding. Next, take the height of the edging, subtract 25mm and multiply that by 100mm - that's gives the volume of haunching required. Add the two together, multiply by the total length of edging required (30m in your case), multiply by 1.15 (for wastage and spread) and that gives the total volume of concrete needed to do the job. To find the cost, multiply the volume, in cubic metres, by 75 quid, which is an average sort of price for site-mixed concrete.

How's that?

Chrissy K
Jun 27th 2003

I thought all projects needed to have an "edge". i.e to stop the bedding dispersing around the garden and the flags moving.

In that light, what stops the end flags from moving, if there is no edge to keep them secure, or are they held in place by there own weight?


Tony McCormack
Jun 30th 2003
If the bedding is cement-bound, as is the bedding mix I recommend (10:1 grit sand/cement), then it will not disperse. Full stop. The flags stay put due to their weight, as you surmise, unless they are laid to a significant gradient, in which case the edge flags would be laid on a wet mortar bed to 'anchor' them in position. A few possible solutions to laying flags to free edges are given on a separate page of the main website.

Having said all that, if you want to lay an edging to your flags, then go for it. There's nowt wrong with doing it that way, but it's not essential, which is what you seemed to think originally.

Chrissy K
Jun 30th 2003

My next question is would the roots of fruit tree, plum for instance, planted at the corner of the patio disturb the slabs or are the roots not that powerful? Or would you suggest, either not planting the tree, or encasing the flower bed in old slabs so that the roots cannot travel underneath the new flags?

Thanks again

Tony McCormack
Jun 30th 2003
The roots of fruit trees certainly would disturb the paving, but it will take a few years and it could be 20 years or more before you noticed.

If you're just about to plant these fruit trees, I'd suggest you use a root barrier membrane to line the planting pit and keep roots away from the paving. You only need place the root barrier between the tree(s) and the paving, not all the way around, and it need only extend down around 600mm or so, as once roots are deeper than that, their effect on the surface of the paving is hardly noticeable.

You might like to take a look at Peter Scott's site, as they specialise in just that sort of thing.

Forum Question Slabs stained by Acid - Richard MCG - Jun 30th 2003
I laid some Marshall Heritage buff-coloured slabs. I then used some cleaner to clean everything up. The consistency seems to have been too strong and a number of the slabs are now white. I presume that they have been burnt by the acid. Is there a dye or anything that I can do to restore the colour?
forum answer Tony McCormack - 30th Jun 2003
Nope - I'm afraid you are what is known in the trade as "buggered".

The acid has reacted with the iron oxide based dyes and adversely affected the colouring, although I've never seen a bleaching effect before - the usual reaction with the yellow/buff dye is to darken it to a rusty brown sort of colour.

This is why I always urge extreme caution when using brick cleaning acids, and, to be fair, many of the 'trade' brands do carry a warning that they can affect coloured concrete, but some of the budget 'patio cleaners' can contain up to 2.5% Hydrochloric acid, which is enough to alter the dyes used in concrete paving, but they make no mention of this on the packaging. frown

Is there a lot of flags affected, or would it be feasible to yank them out and slot in replacements?

Jul 15th 2003
I bought slabs, they appear to be concrete base with a "sandstone" coating and a design stamped on. I did not take much notice when they were being loaded but when we were laying the patio, three are badly stained white and most of the rest have white speckles and patches. We tried to power wash them but no luck. What could this be and is there a remedy? Ta.
Tony McCormack
Jul 18th 2003
Sounds like efflorescence, Catherine. Is is a white, powdery deposit? If so, there's nowt you can do about it. It's a fairly common occurence with cementitious materials and it does disappear with time. However, the time taken can be anything from a couple of weeks to a couple of years.

It can be temporarily removed by scrubbing with a brush, but it will return until the supply of salts responsible for the bloom have been spent, an event that occurs as part of the weathering process.

Forum Question How do I stop my flags creeping? - DrG - Jun 30th 2003
With the help of your excellent web site I'm renovating my back yard which is the first time I've really done anything like this. I have a few questions for which I haven't managed to find answers to on the site and would be very appreciative of your advice.

The yard is roughly "L" or perhaps "P" shaped and I intend to lay Indian sandstone flags down the "long arm" and fill most of the rest up with pebbles. Although part of the slab path will be bordered by a wall on either side I'm concerned that the part of the path that abuts the pebbles may slip or creep, over time.


1. Do I need to use an edging or can this be avoided?

2. should I use a 10:1 sand:cement mix rather than just sand in the bedding layer?

Initially I was just going to use sand and was going to lay it all in a "oner", then compact it all and then start to lay the flags. I'm assuming that laying the flags may take me a couple of days and so I'll need to add the cement gradually to avoid it going off too early. Should I also lay and compact the sand in small doses as I go?

3. Finally (for now) if I put a weed barrier below a layer of sub-base beneath the pebbles will it just get ripped (and therefore become useless) when I compact the sub-base? Is there anything I can do to prevent this? I know you're not too keen on these fabrics but the old asphalt in the yard had loads of weeds pushing through it and I'd like to avoid the same fate.

Cheers - Gareth

forum answer Tony McCormack - 30th Jun 2003
Hi Dr G,

1 - No - see the adjacent thread about choice of edgings. It covers this very topic.

2 - yes! because the Indian sandstone varies in thickness so much, it's more or less impossible to lay on a pre-screeded bed, and so you have to use individual bedding, and a cement-bound bedding material (the 10:1 mix I keep banging on about) is the best option.
Just mix enough of the bedding to keep you going for an hour or so, and then mix more as and when you need it.

3 - If you have a sub-base AND a cement-bound bedding material, the weed membrane is a complete and utter waste of time. Treat the existing ground (asphalt???) with a good weedkiller before starting work. I prefer Sodium Chlorate, but I have a feeling it's not generally available to the public, so summat like Round-Up or PathClear will have to do. Once the ground has been doused with that, and overlain with cement-bound bedding and the flags themselves, the weeds will not come through - but see the newly uploaded Membrane FAQ for more info.

Good luck!

Jun 30th 2003
I saw your answer to the other edging thread just before I read your answer above so had realised I didn't need an edge. smile

I take it from your reply to my second question that I should be only bedding a small amount at a time so I should follow the sequence....

mix sand:cement - lay - compact - lay flag

....and then start again when I've got close to the edge of my original bed.

Oops, looks like I didn't explain properly. I wasn't going to use a sub-base beneath the flags (as it's only light use), planning on just putting the bedding mix directly onto compacted soil (I have excavated the old asphalt and soil to approx 100mm). The sub base is only to go under the pebbles. In light of this, is your answer to my question still the same?

Yours - Gareth

Tony McCormack
Jun 30th 2003
When it comes to mixing the bedding, I'd suggest doing a barrowful at a time. Because there's no water added to the bedding mix, you get a reasonable amount of working time - something around 4 hours, so you could knock-up as much as you think you'll need to keep you busy for a couple of hours, rather than mixing just enough to lay one flag at a time.

The membrane - my mistake, I didn't read your post properly. Yes, you could use a membrane beneath the sub-base for the gravelled/pebble-covered area, as that will be prone to weed colonisation. If you level out the sub-grade, compact it and make sure there no sharp stones or bits of glass to puncture the membrane, that will be adequate preparation. These weed membranes are not intended to be 100% weed-proof, but to reduce the opportunities for weeds to get a foothold, so any minor tears or areas not completely covered are not critical.

Forum Question How much mortar? - Chrissy K - Jun 30th 2003

you have a calculator telling me that for a 40msup2; patio with a bed of 40mm I need about 3 tonnes of grit sand.

I'm trying to work out how much sand, cement and plasticiser I need to create the mortar joints.

I think I have 170m of joints, these will be 12mm thick and about 35mm deep = 0.07m³ of mortar.

Thanks again

PS what am I supposed to do with the old paving slabs I removed from around my house? It seems a shame to throw them in the skip.

forum answer Tony McCormack - 1st Jul 2003
The bedding quantity sounds about right at 3 tonnes - a 50mm bed takes around a tonne every 10m²

I don't have an online calculator for jointing mortar; perhaps I should write one, as I have them on my system here. Your calculation is basically correct except for one very important factor - wastage. You need to add 20-60% for wastage, and the reason why there's such a relatively large amount of wastage depends on just how the flags are being jointed.

If you're buttering and pointing, a significant quantity of the buttering mortar is trapped beneath the flags or is squeezed up and lost. With square-edges flags, this might be as little as 20%, but with riven stone flags or those wet-cast flags with imitation fettled edges, the amount lost to wastage can reach 60% or even more.

If you're pointing only, then allowing 20% for wastage is much nearer the mark.

Anyway, to complete your calculation....

170m x12mm x 40mm x 1.4 (wastage) = 0.11m³

0.11m³ @ 2.4T/m³ = 275kg @ 4:1 = 55Kg cement plus 220kg sand

Plasticiser for such a relatively small amount of mortar is negligible. If you have lots of other mortar work, it might be worth buying a 5 litre pack, but, if this is your only requirement, then the old trick of using a squirt of wash-up liquid to the gauging water is probably a better idea.

Forum Question Grouting between slabs & setts - Dermot McE - Jul 1st 2003
How best to grout between textured slabs & 50mm sq granite setts without staining?

I want to add black colorant to darken cement & this will only stain worse. I tried some wet, but impossible not to get over setts& slabs . If dry - is it completely dry - do you 'strike ' joints later to smooth, or just brush in?

forum answer Tony McCormack - 2nd Jul 2003
Textured flags, whether they're shot-blasted or bush-hammered, are a total bloody nightmare to point up without staining, and if you're using a black or a dark mortar (not "cement", note, but MORTAR!!!) it seems to be even harder to get a neat finish.

As you've experienced, if you use a wet mortar it slops everywhere and stains, but if you use a dry 'brush-in' mix, it doesn't set as well as it should to give a good joint. There's an old trick of brushing in dry and then wetting the lot via a fine rose affixed to a watering can, but this still splashes some of the mortar onto the surface of the paving. Further, a dry mix doesn't 'colour' properly. The cement dyes used to make coloured mortars rely on water to disperse the powder and activate their colouring power, so you don't get a good depth of colour with a dry mix.

One method we've used successfully in such situations is to use a semi-dry mix, adding just enough water to bind the mortar and bring out the colour. It's damned hard work mixing a mortar in this way - you have to keep the water content to an absolute minimum, which means it's very, very stiff, but you also have to mix thoroughly to prevent any 'streaking' of the dye within the mortar. Once mixed, the semi-dry mortar can be fed into the joints from the blade of a brick trowel and pressed in with a pointing bar. If the mix is just damp, it tends not to stain the surface of the paving if accidentally spilled, but it gives a good, hard joint of full colour. Any accidental spillage should be sponged down asap. This method is shown on the Pointing: Case Study page.

Other methods you might use include...

- masking: you can use masking tape to protect the surface immediately adjacent to the joint

- retarder chemicals: Grace Pieri have an interesting substance by the oh-so-catchy name of NMP01 that they claim can be painted onto the surface of paving to protect it from mortar stains during pointing. I'm not sure how readily available it is for DIYers or what it's cost might be, but it's certainly made me sit up and pay attention.

- wet grout: this method is illustrated on the website, and it works quite well with 'awkward' pavings such as you have, as long as you are absolutely, totally, 100% methodical in washing off all of the mortar slurry. Could be a good method to combine with the retarder idea mentioned above.

- polymeric compounds: there's a good few of these on the market, and they are becoming more readily available in a range of colours, although I've still to see a genuinely dark-grey/charcoal/black. All of the so-called dark colours I've seen have actually been medium grey at best. Some are dry-brushed in and then cure by exposure to the atmosphere, while others are mixed to a wet consistency and poured into the joints from a watering-can or can be pumped in via specialist mortar guns.

I can't tell you which would be best for your project. You need to think about the skill levels available, the size of the job, the budget, how much time you can afford to spend, the end use....and then decide which is best suited to the job in hand. If it were me, the very stiff mortar would be my choice, but then, I've been pointing flags since I was three years old and have almost got the hang of it now. smile

Dermot McE
Jul 2nd 2003
Thanks for this Masking tape & dry mix looks good. The area was around a raised bed and used paving slabs with 4 setts between each. I like the look of the setts and aim to do adjoining patio area. Leads to next Q.

With patio, I want to lay on 'screed' with 10:1 as your info shows this to be good. How best to incorporate setts into this? I was aiming to do a 1 or 2 deep rectangular band. The slabs are 40mm & setts 50mm deep, but the setts are irregular - do I remove the 10:1 screed and bed the setts in strong mortar? I was hoping to use sand joints for slabs & dark mortar for setts.

If I do a rectangle of setts, is it better looking to cut the inner slabs so that slab joints inner/outer line up, or work from each side & cut in central ones?

I seen from other links, folk complaining about Marshalls products being plain, looking at their slabs, my local firm Tobermore do a far better range of textured stuff - they look like textured stone not textured cement. Pity that they are hard to deal with - no stock, made to order, no guarantee of delivery date, leaving BM & B&Q offering finest Marshalls stuff.

Cheers - Dermot

Tony McCormack
Jul 6th 2003
As there's only 10mm diff between the slabs and the setts, I'd probably leave the 10:1 screeded bed in place and just loosen it with a trowel as I came to lay the setts, and remove some of it to accommodate the extra depth, and then tap down the setts so they're flush with the slabs.

I'm not sure which would be the best way to work with your sett rectangles. Ideally, you'd use full setts and full flags and avoid all the cutting, but it's not always possible. In your situation, I'd do a dry test panel first to see which looked best.

Tobermore only serve Norn Iron, don't they? I've never seen their products on this side of the water. I think they use a shot-blast texturing process which is essentially the same as Marshalls (and lots of other manufacturers) but the big diff is the starting material. The aggs and dyes that Tobermore use are different from those used by Marshalls, which is why the end product looks so different.

I'm surprised to hear you've not much praise for the Tobermore customer services. I was talking with a contractor from Derry last time I was over, and he said they were a great firm to deal with. Maybe they treat private customers less effectively than their trade regulars.

Forum Question Soaked sub-base - A Bancroft - Jul 5th 2003
Hi there - I'm the crazy Scot trying to lay a flagstone patio by hand in a Texas summer.

Not only is it bloody hot here, but it can rain a hell of a lot. And after I barrowed in my sub-base, but before I could compact it, that's exactly what it did - rained a LOT (several inches I'd guess). Now my sub-base is 'squishy' (is that the technical term? wink). When I walk on it, my feet sink in up to 15mm or so. The depth varies - some parts seem pretty solid, others don't (I assume this is due to a crappy distribution of fines).

Can I compact it like this? Or should I wait until it dries out? Or something else???

I went out this morning & the sub-base seems to have sunk about 35mm from yesterday morning! Is this due to the rain? I presume the only way to bring it back up to the required height is to add more sub-base?

(the sub-base is 4 to 6 inches thick, if that's any help)

forum answer Tony McCormack - 6th Jul 2003
If the sub-base has sunk by 35mm or so, then it must be due to that crazy Texan rain (God's revenge on George W, probably!), but it is actually a "good thing". Think about it - it has washed down all the fines to ensure you have a really solid sub-base that is unlikely to settle any further.

Top it up as required with extra sub-base material, re-compact and all will be well (touch chipboard!) smile

Forum Question Flags and Dry-slate Walls - Barney Blarney - Jul 8th 2003
I'm in early stages of planning for a new patio to replace old grey concrete slabbed area of the garden. I'm thinking sandstone flags. Also, a raised bed surround, formed by newly laid dry-slate effect walls built about 18 inches from the garden perimeter walls.

Questions - (1) what criteria should I use to decide whether to lay on top of existing slabs?

(2) If ok to leave the old slabs in situ, do I use a lesser thickness of mortar/sand?

(3) Should I build the new dry-slate walls on top of old slabs, new flags, and must I dig a spearate foundation for these, and build them first before laying flags?

Thanks - and great site - I'm in the (web) business and this is one of the best I've seen.

forum answer Tony McCormack - 9th Jul 2003
1 - the most important factor is the level of the new paving in relation to the dpc. Will you be able to maintain a minimum of 150mm below dpc? After that, you need to consider the condition of the existing flagged surface and whether paving over the top of it is likely to cause any long term problems.

2 - the bedding layer should be at least 25mm thick. Now, if you're using sandstone flags, you'll probably find that you have a bed of variable thickness to accommodate the difference in thickness of the sandstone flags, but, in all cases, 25mm should be the minimum to aim for.

3 - Depends on the height of the walls and the condistion of the flags, as noted in (1) but I always prefer a separate foundation for walls, even dwarf walls on gardens. I know some contractors and certain publications advocate building dwarf walls on top of paving, but that assumes that the paving is sound and will not move at all during the life of the wall - how does that tie-in with your plans?

Forum Question Anyone ever built a firepit in a patio? - Barnsley Jon - Jul 8th 2003
I'm about to build a slightly raised, 300mm or so, patio in our back garden. My other (better) half has seen somewhere one with a firepit incororated in it - sounds like an Ideal Home exhibition or Chealsea Flower Show kind of thing to me. Anyway, she wants one in the patio.

My thinking is to incorporate a 600x600 galvanised pavior manhole cover and frame into the patio to cover up the pit when not needed. That way it can blend in with the overall slab pattern. Do I need to incorporate some drain facility into the pit underneath, or do these covers give a decent seal?

I'm intending to construct about a 300mm deep well underneath, and to make this from blue bricks for heat resistance.

Add a fire grate for the coals or logs and away we go!

Does anyone have any thoughts on how I could approach this any better, or even have any experience of such a feature?

forum answer Tony McCormack - 9th Jul 2003

the recessed covers do allow some water to penetrate between the frame and the cover, unless you splash out extra bobs to buy one of the double-sealed units that are normally used on internal applications. Therefore, if you use a standard tray and cover, you'll need to make some allowance for drainage of your fire pit.

I've no experience with fire pits. I know they are popular in America, because I keep getting emails from there asking about them, but I've never educated myself about their possibilities within the patios of these soggy islands as I assumed we had far more sense. I know that intense heat and concrete/porous stone are not a good mix, so I think some form of refractory or heat-resistant lining would need to be used, but, as I said, I've never really considered them to be a worthwhile feature, so haven't any real advice to pass on.

Barnsley Jon
Jul 10th 2003
Tony - thnks for your views.

I'll check out the availability and costs of sealed vs. unsealed covers and take it from there on drianage of the pit.

I'll let you all know how it turns out, completion is expected in mid-August, and whether it all works fine or if we end up cracking (or worse!!! ) any of the lining materials when exposed to heat.

Barnsley Jon
Aug 11th 2003
I thought I'd give an update on how things are going.

The fire pit is now built. I've used engineering brick for both bottom and sides. The bottom is a concrete bed, then bricks laid on their sides and mortared in (sort of a wall laid horizontally rather than vertical). I've built in a drain at one corner (with base sloped slightly toward it) using 100mm ceramic pipe - that was the smallest diameter I could get. The drain runs out under the rest of the patio and ends up coming out above ground level hidden under a deck. We have about a 1:10 sloping site, so this was easy enough to arrange. The whole thing is topped off with a 600x600 recessed paver tray. I get the feeling this is going to be bloody heavy to lift off when filled, so I'm getting 4 lifting keys to apply maximum amount of hands to the job! smile

The pit itself is around 650x650, and about 350mm deep from the rim with the cover removed. Should be plenty of space to get a decent grate in there and a good blaze going.

Next step is filling in the rest of the patio base between raised edge walling with bedding layer. I'm not looking forward to barrowing about 3 tonnes of sub base round the house, but it's got to be done. Finally, compact it and lay the slabs on a screeded bed.

I'll post some pictures soon. I've just ordered a digital camera and Amazon have shipped it today. Seems like a good topic to try out the new toy on.

By the way, thanks for all the advice on the site. I keep on looking up the next step as I move forward, and there is always useful and clear information to help.

Tony McCormack
Aug 12th 2003
3 tonnes of sub-base? That's nowt! You can get that shifted in less than an hour if you shape yerself, Jon!   wink

I look forward to the pictures from your new toy!

Forum Question Best Laying Method - A Dillon - Jul 9th 2003
I am a bit stuck and would really appreciate some help

I have three questions I would like your advice on:

1) The old patio, crazy paving, is 165mm below the DPc, but I have built a pergola and there is no water coming down onto the patio area. So do I still need to worry about the DPC? The new slabs are 32mm thick and I am intending to lay them onto a 50mm bedding layer, ie the new patio will be 83mm below the DPC - is this ok under the pergola?

2) Which is the better method of construction, rigid or flexible? I seem to prefer rigid but want to do the best job possible, which would you say is the better method and why?

3) If you tell me to remove the old patio, should I then lay a 75mm layer of hardcore, ie 6(Ballast):1(cement). Will this be enough? Or as the old patio has hardcore beneath it, should I remove this as well or just the crazy paving and therefore use the old sub-base layer?? Also how do I prevent the new hardcore from spilling out around the edges?

Please help, I am unsure as to which way I should proceed.

Kind Regards - Ajay

forum answer Tony McCormack - 9th Jul 2003
1 - yes, even though you think there is no water penetrating beneath the pergola, you should still aim to comply with the 150mm below dpc rule. 83mm below dpc is less than ideal and, as I've said a thousand times, it's just the sort of thing a surveyor notices when valuing a property and it can lead to a reduced valuation, or an insistance for remedial work. It's always best to do the job properly.

2 - Laying method for a patio? Depends on the materials being used, but for flags/slabs, I'd nearly always go with a rigid construction because it offers better stability for larger paving units that are bed-dependent for their load-bearing capability.

3 - If the old patio comes out, then you can make up the levels using a cement-bound material (your 6:1 ballast mix) or you could just use granular sub-base material. No need to remove existing sub-base material, if it's in good condition.

I'm not sure how the 'hardcore' (as you call it) can 'spill out' unless this is an elevated pavement, in which case you need some form of retainer at the edges. Under normal circumstances, the sub-base is buried, so there's nowhere for the sub-base material to spill.

A Dillon
Jul 9th 2003
Thanks a lot for your reply to my question.

I didn't think that the sub base would be below ground level because I was thinking of laying over the existing patio, but from the sound of it, you are suggesting that I remove the old patio before starting work on the new one. The crazy paving sems to be bedded directly onto the sub base, there appears to be no bedding layer, could it just be really thin, ie 10-15 mm? Hence it is not immediatley visible.

In your experience how easy would it be to remove the crazy paving, all types of shapes and sizes, and leave the sub grade intact? The sub grade is definitley sound as it was very hard to penetrate, although I am worried as to why the low spots, where puddles form, occur? Is this because the sub grade is not good enough and has sunk?

PS "Hardcore" is a term I have picked up from people in the trade who seem to think of hardcore as a mixture of 6:1 ballast:cement.

For the bedding layer, do I use sharp sand (3): cement(1) with small quantites of water? Or do I use Ballast( mix of sharp sand and gravel)3: cement 1.

Is 50mm layer for bedding ok?

Tony McCormack
Jul 10th 2003
Taking up crazy paving is a doddle, usually, especially if, as you believe, it is on a very minimal bed. A pick will break it up in next to no time and then it's just a matter of dumping it into a skip.

Once it's out, the sub-grade can be re-graded in a few minutes, by raking over it with the back of a spade or a rake, and then re-compacting the lot with the vibrating plate.

The low spots are probably of long standing and occurred when there was some minor settlement of the sub-base or inaccurate laying of the crazy paving.

Hardcore is a term I prefer to avoid because it means different things to different folk. Quite a few one-handed surfers end up on this site, looking for hardcore, and are somewhat deflated when offered pictures of crushed rock or broken bricks. Strange men!   wink

Hardcore can be anything from half-bricks, to slate-waste, to gravel, or, as you believe, a lean-mix ballast-based concrete. On this site, I try to use accurate terminology that refers to specific materials, so that there is less chance of confusion amongst the site-users. Obviously, I can't (and don't) expect everyone else to be as specific as I try to be, but if I slip into generic or loose terms, then the accuracy of my information is reduced.

For your bedding, I would suggest that you use my standard recipe for patio paving, which is a 10:1 mix of grit sand and cement, with no added water, as is described in some detail on the Laying Flags page, which I heartily recommend to you as essential reading.   smile

A Dillon
Jul 24th 2003
Thanks for all your help. I am a bit confused however, you suggested that you would opt for a rigid construction when answering my first post but in your last post you say that I should use the dry 10:1 mix?? Which should I use?
Tony McCormack
Jul 28th 2003
The 10:1 mix is a rigid bedding system. The cement content, although minimal, is sufficient to bind the sand, making it hard and therefore it qualifies as a 'rigid bedding medium'.

Hope that's cleared it up for you.   smile

A Dillon
Jul 31st 2003
Just one more thing, since my patio is 8 metres by 2.8 metres do I need to have an expansion joint?? Or can I leave this out? The problem is that the concrete sub base has already been laid with only construction joints at 3m widths. The Expansion joint would just sit in the mortar bedding layer. The mortar layer is approx 40mm. So my question is whether it is worth placing it now in the bedding layer.
Tony McCormack
Jul 31st 2003
If there's no expansion joint in the existing base, then no need to incorporate one in the patio. The usual rule is that expansion joints are continued upwards, if overlain, so, if there was one present in the existing slab, you'd be best making a joint in the patio directly over it.

Normally, any slab longer than 6m would have a movement joint of some description, even if it were only to act as a crack control joint. If you have concerns abouit the existing slab, then you could saw a control joint into the surface before laying the paving. This would act as the slab's weak point and then, if it was going to crack, it would crack along that line. However, even if you did this, I wouldn't worry about incorporating a complementary joint in the patio paving - if the slab cracks, the crack will be reflected through the bedding but it would then find a route along the jointing between the flags, which can easily be re-pointed.

I think you're worrying unduly about things that might never happen, and, even if they did, are very easily rectified.   smile

A Dillon
Aug 11th 2003
Hi Tony,

Many thanks for all your help on this site. I have now finished the patio and it looks brilliant, I can't believe how satisfying it is to look at it. I had to use an angle grinder to make cuts in awkward shapes, ie around the drain and stuff. I probably cut around ten slabs this way, they are standard wet cast concrete slabs. I have a question about dust from grinding. I wore a decent filter mask through all the cutting and no dust could be seen in my nose afterwards but I still felt a bit clogged up and irritated, breathing wise, is this normal as I have not done this before. Have you ever suffered anything like this??

Many thanks - Aj

Tony McCormack
Aug 12th 2003
I suspect that, even though you had the dust mask on, you still breathe in some of the dust through your mouth, and there's still dust in the air for several minutes after cutting has finished (assuming there's no breeze).

I think we are going to see a lot of silicosis-like problems with construction workers in the next few years. We had many years from when power saws first became popular when wearing a dust mask was seen as being 'girly' or 'soft'. Even now, I can go onto a site and see pavings or roof tiles or kerbs being cut by an operative with no dust mask, because they seem to think a few gobfulls of dust won't do them any harm, and the mask is buried under a pile of wet-weather gear at the back of the van, and by the time they've trudged across the site, they could have had the cut done....but they don't seem to realise that it's not just a few gobfulls: it's a few gobfulls on a regular, daily basis, and it's nasty, abrasive dust that will irritate their lungs and trachea.

In these litigious times, when employers are being sued for all sorts of minor transgressions, such as shouting at numpties, making the tea too hot or not providing moisturiser, I find it surprising that many employers and site supervisors do not insist on full protective gear when operatives are using a cut-off saw. Even for occasional use DIYers, I would compel them to wear a mask and goggles when using a saw - it's just not worth the risk!

Forum Question Laying slabs on a mortar bed - A Dillon - Jul 17th 2003
Thanks very much for all your help. I would like to ask you a bit more though.

I have so far removed my old patio and sub base entirely. It was ok in some parts but not in t'other parts. I have now layed a new sub base of approx 65mm, this was 5:1 Ballast to cement. I am now faced with the tough question of going for a mortar bedding or a sand bedding layer?? My slabs are 450mm x 450mm. I dont know which to go for.

Also I would like to know whether I could screed the mortar in the same way that sand would be screeded, obviously only one length at a time. Is it worth the extra hassle. I am a bit cautious about a sand bedding layer because I know someone who's sand bedding has failed.

Which is the preferred and more durable method? How thick in mm should the mortar bed be? The slabs are 32mm thick. Any tips are gratefully appreciated.

forum answer Tony McCormack - 18th Jul 2003
Use a 10:1 bedding mix of grit sand with cement, as described on the Mortars & Concretes page. It can be screeded, if that's how you want to work, and it should be 35-50mm thick when consolidated.

Jul 22nd 2003

Before I ask any more, I think your site is really helpful and informative, thanks for all the good work.

I am building some steps and as they are quite large, I have used concrete blocks and bedded them down onto the sub base with sharp sand and cement mortar. Is this the right way? You advise the use of bricks for steps, are concrete blocks ok , breeze blocks, for use in the building of a step?

Thanks for all your help.

Tony McCormack
Jul 23rd 2003
CMUs, or Breeze Blocks as they are still referred to in some parts, are fine for constructing the risers for steps - they're not the prettiest of materials, but there's nowt wrong with them, structurally.

I'd normally bed them onto a 100mm layer of concrete, but laying them on mortar is fine, as long as the sub-base is sound.

A Dillon
Jul 23rd 2003
Many thanks Tony, I will do that and let you know how I get on.


Forum Question Paint on the Patio - Ian Cridland - Jul 21st 2003
I dropped a paint can from a ladder and the paint went all over the patio paving slabs. Altogether the paint affects about a dozen 2 ft square slabs. I've tried to remove with white spirit and now (now it has dried good and proper) with paint remover (Nitromors, etc).

Any ideas what I could use? Ive heard of using hydrochloric acid - is this likely to work?  

forum answer Tony McCormack - 21st Jul 2003
Hydrochloric acid won't work on paint - it's used for cleaning cement, mortar or concrete stains, and for cleaning up mosses and whathaveyou. Besides, acid has a nasty habit of changing the colouring of concrete flags, so it's best avoided.

Paint Stripper is the way to go, combined with a power washer, perhaps, or a wire brush. Depending on the type of paint and area of spillage, you might be better off ripping out the stained flags and replacing them with new.

Forum Question Laying a riven (mock) stone patio - Steve Mackerel - Jul 22nd 2003

I recently attempted to lay my first ever patio, out of Bowland riven mock stone flags, mainly 500 x 250 x 45

I dug out an old, compacted lawn and, as advised by the builder's merchants, spread out some concreting sand into which I raked some cement and roughly leveled it.

I then had a first attempt at mixing mortar and laying the first flag. Disaster! Despite what I had seen on the telly, my mortar mix seemed too thick and dry to work with. After several unsuccessful attempts, I gave up and scraped off the mortar from the sand.

I then discovered articles on your web site that suggested a 10/1 mix for bedding instead of mortar - obviously similar to what I had been advised to lay for the base before mortaring.

I now wish to use your 10/1 mix method for bedding, but have several questions:

1) It has rained since weekend, and I now have a patio bed full of a concreting sand/cement mixture, some areas are mainly sandy and therefore soft, some hard where it's mainly cement, and bits in between. Will I need to take this up and start again, or can I put your 10/1 mix on top (I'm worried about settlement in the semi soft areas)?

2) I'm confused about the type of sand to use. The builder's merchant where I got the stone spoke of grit sand, but delivered bags marked concreting sand. The merchant where I now wish to buy the remaining sand I need is talking about sharp sand as they do not stock grit sand or concreting sand! Does it matter which of these I use for the 10/1 mix I wish to bed the flags on?

3)Final question (sorry). What is the best method for pointing on the 10/1 mix. I bought some Geofix to brush in dry, but that was when I was going to mortar and attempt to 'butter' the flags?

forum answer Tony McCormack - 23rd Jul 2003
1 - I'd throw out any really hard bits, dampen the rest lightly with a hose, then sprinkle cement over the rest, rake it through and run the plate compactor over the lot. If you don't have a plate compactor, then stamping on it so that you compact the entire area will be almost as good.

Once that's done, you can use 20-25mm of the 10:1 mix for bedding.

2 - Oh the joys of terminology in the building trade!! To all intents and purposes, grit sand is concreting sand is sharp sand. There are minor differences between them, but, for the type of project you're undertaking, those differences are totally irrelevant and you can use grit sand OR concreting sand OR sharp sand, or even a mix of all three. What you DON'T want to use is building sand, which may be known as soft sand or plastering sand. Confusing, innit?

3 - You could use Geo-Fix but I still have reservations about its use on the wide joints you'll end up with using the Bowland flags. I still firmly believe that you can't beat a mortar joint with that type of flag. Butter the receiving edges with a Class II mortar as you lay, point up the joint asap to keep out other crap, and then, after an hour or so, strike the joints to finish them. Leave them for about 4 hours and then you can brush off.   smile

There's a new page dealing with pointing 'in production'. I took the photies a couple of weeks back, and drafted the text over the last few evenings, so it's just a matter of coding it all into html now, and slotting it in to the rest of the site.

Steve Mackerel
Jul 24th 2003
Thanks Tony.

When you say 'Butter the receiving edges with a Class II mortar as you lay', do you mean use the 10:1 sand/cement mix to butter the edges, or lay the flags on a 10:1 sand/cement mix and then use some 3:1 mixed mortar to butter the edges?

The Bowland stones seem to be designed to touch at the bottom while leaving a gap for poiting at the top. If I butter the edges, do I still force them to touch at the bottom, which will presumably force the buttered part of the bedding mix upwards?

Finally, does using the 10:1 mix to lay the flags mean I can stand on or use the patio immediately, or should I wait a couple of days until the cement has gone off?

Tony McCormack
Jul 24th 2003
Use a Class II mortar for buttering and pointing (that's roughly a 3:1 mix), and use the 10:1 mix of grit sand and cement for bedding.

Bowland are not the only manufacturers to have these weird moulds that make the flags have splayed-outwards edges, contrary to natural stone, which always has the edges splayed inwards. However, you should endeavour to keep the flags far enough apart to ensure they are not quite touching, but that the top of the joint is not excessively wide.

Using a 10:1 semi-dry mix means you can walk on the patio a mere 24 hours later - in fact, if you're careful, and the flags are securely bedded with no movement or rocking, you can walk on it immediately. At this time of year, the bedding mix will be reasonably 'hard' after about 6 hours, but, if you're pointing as you proceed, best to leave them for 24 hours, just to be sure.

The new page on pointing stone flags has now been uploaded and you might find it to be a useful read.

Steve Mackerel
Aug 5th 2003
Thanks. The patio is down (at last!).

The pointing was a lot more difficult than I thought it would be from seeing people do it. Maybe it comes with practice?

Anyway, I need further advice. I have laid some 500x500 riven flags as a path near the patio. I laid them on a 10:1 mix, but decided to try and point them using the dry grouting method as a 3:1 mix. I decided not to use a Class 2 mortar to butter, but simply brushed the dry mix into the gaps, wet it with a fine spray of water and covered it so it could dry out slowly. Problem is, I forgot to strike the joints with a trowel.

It's been a couple of days now, and the mortar seems to have loose sandy grains on top and is not fully hard. Is it a write off, or am I being impatient, as I was expecting ther mortar to have gone rock hard by now.

wink Cheers.

Tony McCormack
Aug 5th 2003
You're right: real skill at pointing does come with practice. My Dad's had me doing it since I was 3 years old, as he hates doing it, and has no qualms about using child labour to save him the trouble! I'm getting to be quite good at it now!  

The dry-grout method never gives a really solid joint unless it is brushed into freshly-buttered joints and tooled to a firm finish, but I wouldn't worry too much about it just yet. The "mortar" is not going to get noticeably harder (it does, actually, but you'd need a laboratory full of equipment to prove it) but it may well be hard enough for your purposes. After all, it's only a way of filling the gaps between the flags, so, if it's hard enough to stay put and resist colonisation by weeds, it'll do, but if you find it starts flicking out, then a re-point may be necessary.


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