aj mccormack and son

Flags & Slabs - Page 07
The Brew Cabin
flags and slabs


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Forum Question Splash strips and DPCs - Andrew - Apr 26th 2003

I have two and a bit questions on setting out a new patio adjacent to the house.

1) I plan to put a layer of pea gravel or shale between the edge of the patio and the house. Are there any regulations regarding how wide this should be? I was planning on 4"?

2) The old 'imperial' house has been extended recently with new metric bricks. The dpc is therefore lower on the original part (about 1") than on the new extension. Should I work to below the old dpc level rather than the new although it will leave a gap below the blues which I will have to cover with the gravel edging?

3) Are there any rules or building regs that I need to consider for the these two questions or a site where I can look them up?

This is my first question so I hope it is clear enough! Thanks in anticipation


forum answer Tony McCormack - Apr 26th
Here we go with two and a bit answers, then....

Q1 - The gravel filled gap between hard paving and the brickwork of a house is known as a 'Splash Strip' and is a bit of a nonsense, really. If the paving is laid properly in the first place, the splash strip is redundant, but that doesn't seem to stop some architects and designers from using them on every project. Anyway, as they are a bit of nonsense, there are no regulations governing them, they do not appear in the Building Regs and they are left to the whim of the contractor and/or designer. A typical splash strip will be somewhere between 150mm and 300mm in width, and is filled with a gravel chosen for its looks or its cost rather than any particular hydraulic properties.

Q2 - work to the lower of the two DPCs. It's better to be more than 150mm below dpc than slightly less than 150mm.

Q2b or is it Q3? - Nope. All the info you need is here.   smile

Forum Question Rigid or flexible sub-base? - Pete Hughes - Apr 28th 2003
I will be laying a new patio using concrete slabs which will be mortar pointed (i.e. not butted). I want to lay the slabs on a mortar bed (I've always found it easier to bed the slabs down to level on wet mortar) but am not sure which type of subbase to use. 100mm of Type 1 or similar would be easier/cheaper but I've always understood that you shouldn't mix a flexible subbase with an inflexible bedding course - is this right? If I'm right then I would presumably need to lay a concrete base, say 100mm thick?

I will also be laying the slabs on the driveway - this is currently tarmac. I haven't checked depths on this yet but was hoping to take up the wearing and base courses and possibly re-use the existing subbase. Then lay the slabs on a mortar bed as above. Is this OK or should I lay a concrete base for extra strength (the slabs are only about 30-40mm thick)?

Thanks for any advive you are able to give.

forum answer Tony McCormack - Apr 29th 2003
Laying flags/slabs on wet mortar is a bloody nightmare (for me anyway), and, in theory, you're right that a rigid bed on a flexible sub-base is less than ideal, but it is done on quite a lot of footways for reasons that I've never fully understood, despite having it explained to me dozens of times. It's supposedly a 'gradual stiffening' of the individual layers, from the loose and unbound sub-grade, a slightly stiff sub-base, and then a stiffish bedding layer and, finally, the stiff flags.

The joints between the individual flags act as a sort of 'control joint', and so allow the rigid bedding to crack in a pre-determined spot, which is all well and good, but then why not just rely on a flexible bed that can accommodate slight settlement of the sub-base? In my experience, when rigidly-bedded flags start to rock, they continue to rock indefinitely, and the only remedy is a lift and relay. With flexible bedding, the flags have a chance of re-establishing a non-rocking equilibrium, noot that it always works that way!

On public streetworks, the 'mortar bedding' is often a lime-based mortar that remains slightly more flexible than a cement-based mortar. This is reckoned to accommodate slight movement more successfully than rigid bedding and it does seem to work. However, getting a lime-based bedding mortar for a patio is not really feasible, as the manufacturers like to deliver in cubic metres, not in barrowfuls!

If you use a weak mortar (or the 10:1 bedding mix that I advocate), then you should be ok for a patio. For the driveway, I'd elect to re-use the existing sub-base if at all possible and then choose the bedding according to the type of flags being laid. If they were BS pressed concrete flags or 50mm+ stone, then a sand bed or a weak bedding mix would be my choice, but for cast flags (patio stuff) or the Indian sandstone wafers that are very popular just now, then you really, really, really need a full concrete bed, 50-75mm thick, to give then the necessary strength.

If you must use a cement mortar bed, then use a full bed, not spot bedding, especially for the driveway work. Spot bedding causes more bloody problems in the long term than any other method of bedding!

Pete Hughes
Apr 29th 2003
Thanks for that.

I will use type 1 subbase and lay the slabs on a screeded 10:1 mix for the patio (i.e., all flexible); for the driveway I will lay a mass concrete sub-base and lay the slabs on a full bed wet mortar (i.e., all rigid). Does this sound OK?

Do I need to use an edging at the edge of the patio where it meets the grass or will a concrete haunch be OK (I will cover this with topsoil)?

Tony McCormack
May 1st 2003
Your spec sounds OK, Pete, but have you considered laying the driveway flags directly onto a bed of semi-dry concrete? It's a lot less work than laying a concrete base and then laying the flags.

An edging for your patio is not essential - it's a decorative touch that might look nice, but it does nowt for the actual structure.   smile

Forum Question Old roots - Andy Seal - May 1st 2003
I'm laying a patio at the top of my garden, on an area that used to be grassed. One edge of what will be the new patio was planted with leylandii. I've cut these down but the roots remain. Will they unsettle my patio or will they die off ?
forum answer Tony McCormack - May 2nd 2003
Roots - small fibrous roots aren't a serious problem, but any roots more than 5mm or so in diameter can lead to minor settlement if not removed before laying the paving. They can rot away, leaving voids which, in time, collapse, resulting in settlement of the paving. If you have any larger roots visible, it's best to hoick them out before laying your paving, and the same goes for those 'mats' of fibrous, interwoven roots that are often found with conifers.
Forum Question How tight a radius? - Chrissy K - May 3rd 2003
Can you please answer 3 questions before I start out on bumpy road of patio design...

1) my patio is going to be an "L" shape. I do not want there to be any right angles on it so I want the internal angle to be curved, as if someone has taken a bite out of it. Can you tell me what the minimum radius would be of such a cut?

2) Can you please tell me how to do it? You mention in your site to use a "power cut-off saw". These are about £35 to hire for the weekend. Would it be possible to use an angle grinder from argos, or would I be wasting my time, and how long would it take to cut each flag?

3) I like the look of Marshalls Heritage range (Calder Brown) but this is about £25 from my local builders yard. Am I being ripped off, or is that a fair price? I am wanting about 40m², and if it is a fair price can you recommend any other manufacgoundturers that maybe slightly cheaper? I know that Wickes offers something for about £17 /m², but just doesn't look quite right!!!

Thanks in advance

forum answer Tony McCormack - May 5th 2003
1 - I'm not sure I follow - how can you use rectangular flags on a rectangular, L-shaped patio and not have any right angles?

D'yer mean summat like....

patio image

...if so, then there is no minimum radius, theoretically. However, a radius of less than 600mm is very, very tight and awkward to cut.

2 - If I knew what you were doing, I might be able to tell you how to do it, but, as I'm not sure, I'll wait for clarification first.

You could use a Nangle Grinder from Argos (or anywhere else, for that matter!), but you will find it difficult because of the small blade size, which means you'll not always be able to achieve full-depth cuts, and the blades don't last very long when cutting concrete or stone. On t'other hand, the small size of the Nangle Grinder makes it easier to handle, especially for intricate cuts or tight radii. All-in-all, it's probably worth investing in a Nangle Grinder, as you can use it for other DIY jobs around the home, whereas a Power Saw is far more expensive and less useful for the average DIYer, even if it does better cuts.

However, you MUST get a pair of goggles, a decent dust mask and a pair of gloves. The dust that is spit out from the cutting is nasty, nasty stuff, and, because of the small size of the Grinder, you tend to be working even closer to the cutting edge than you would be with a Power Saw.

3 - Heritage Paving - that price is about right. There are umpteen different wet-cast riven reproduction flags on the market, and it's a matter of finding one you like. To be honest, you don't get anything decent for much less than 20 quid per square metre, and, for the top-of-the-range concrete copies, you're looking at 30-50 quid per square metre. Meanwhile, you can get genuine stone flags for around 20-25 quid per square metre - admittedly, they're very thin imported Indian sandstone, but, for a patio, it's hard to beat them for aesthetics at that price.

Chrissy K
May 5th 2003
The radius will be about 1.2m. I will be doing a random pattern, but your drawing was spot on!

I have seen advertised "Stonemarket York riven" for about £17 m² - what is your opinion on this please?

My other question is how long would it take to cut 600mm length of say 40mm concrete flag?

Thanks again

Tony McCormack
May 6th 2003
I must be psychic if I can really mind-read what the patio will look like! Flippin' eck!   wink

Stonemarket York Riven - have you seen it 'in the flesh'? The surface is quite heavily textured, which is not to everyone's taste, as it can be impossible to get the patio table to stop rocking, but from a quality point of view, it's a good product. There's not a lot of variation in the colour, but there's a good range of sizes and, with a contrasting mortar joint, it can look quite effective.

To cut a 600mm long flag, in a simple straight line would take around a minute, I suppose. A bit quicker with a diamond-bladed power saw; a bit slower with your Argos Nangle Grinder. It depends on how deep you cut. You don't need to do a full depth cut - a 10-15mm cut on the face and a 10mm deep cut on the back will usually be enough for the flag to 'snap' and save time, effort and blade.

For a curved cut, you may well be forced into doing a full depth cut, so a 600mm long cut might take 3 or 4 minutes.

Forum Question Soft sub-grade - Rusty - May 6th 2003
I read that a sub base is not really required for patios, however, what is your opinion if the current soil condition is quite soft when soaking wet? Should I give it a good plate compressing down then add a small layer of hardcore just to thicken it up? I think I will also use the method of adding a bit of cement to the bedding layer to add a bit of security.


forum answer Tony McCormack - May 7th 2003
If there's a concern over the state of the sub-grade, then I'd suggest you scratch out the soft spots and fill them with sub-base material. If the sub-grade is a real mess, then perhaps a Geo-membrane will be the answer as it will help prevent the sub-base material disappearing into the mire.

Remember that a sub-base needs to be at least 75mm thick to have any inherent strength. Owt less than that is just a fill layer, which isn't a problem, but shouldn't be thought of as being load-spreading.

And the actual material used to form the sub-base is critical. Relying on brickbats and bits or rough 'hardcore' can cause more problems than it solves. It really is worth splashing out on a tonne or so of granular sub-base material, such as DTp1 and using that to form the sub-base, as that will make a dramatic improvement to the sub-grade, with or without a geo-membrane.

May 7th 2003
OK, I'm going to sound stupid now but what's geo-membrane, is it just the really heavy duty weed membrane you put down???
Tony McCormack
May 8th 2003
Geo-membranes and weed membranes are part of the same family known as geo-textiles, but a geo-membrane is far, far stronger, and is used to improve the strength of a pavement or other structure, whereas a weed membrane is intended to stop dandelions growing in your decorative slate mulch.

Have a read of the Geo-Fabrics page.

Forum Question Thin Slate Patio - A Bancroft - May 5th 2003
I plan on laying a patio in my back garden. My wife & I both like the idea of slate as the surface & we have a friend who works in the stone industry. However, I'm concerned that the slate won't be thick enough - it's only 10-15mm thick. Is that too thin for a patio?

I'm also confused over whether or not I need a sub-base. Some web sites say it's necessary, others don't. It's a new house, so the back garden is currently covered in a layer of sand. We get lots of rain here, so I'm currently thinking that digging up the sand & then laying & compacting a sub-base would be a good idea (as a Brit living in Texas, when I say "lot's of rain" I mean massive downpours - we had 8in one evening last Nov!). So, is a sub-base a good idea?


forum answer Tony McCormack - May 6th 2003
That slate is a bit too thin for paving work, where we don't like to use owt less than 30mm in thickness, but, if you really want to use it, then it will have to be laid on a full bed of mortar. Even then, I'd be worried about it!

If you use a mortar bed, then you need a base of some form - a sub-base is ok for a patio, but it needs to be properly prepared and thoroughly compacted. A base would be better, given the problem with the thickness of the slate, and for that, I suppose the best material to use would be a concrete, something around 20 Newtons strength (whatever that is in the strange, nay incomprehensible, 'bags' that seem to be used to define concrete strength in the US).

If the ground is reasonably stable, then you could lay the slate directly onto a concrete bed-cum-base and miss out the mortar bedding, but the skill level required for this is quite high and you need to work at a good pace to cover the concrete before it starts to set.

Given the rain you have to contend with, I'd suggest a minimal sub-base, just enough to prepare the ground, then a base of 100-150mm of concrete, and finally, the slate laid on a 20-35mm thick layer of Class II mortar.

How does that sound?

A Bancroft
May 6th 2003
Looking at the slate, it looks more like tile: it's only 12"x12" and the bottom has been machined into ridges. So your answer is what I was expecting, but not what I was hoping for. sulk

I was hoping to lay the slate on a bed of sand so I could easily raise it an inch or so at a later date (there is a concrete patio right next to it which we'd hoped to cover in slate in the future). And a mortar bed would make raising the slate difficult.

Reading between the lines, I think you're saying that this slate isn't really suitable for paving work. Since we haven't bought any of it, maybe it's time to consider some other form of paving.....

Thx for the advice

Tony McCormack
May 7th 2003
You can lay on a mortar bed without actually bonding the paving unit to the mortar. If you use a semi-dry mix, it will give you the rigidity you need to support the slate, but, when you come to lift them at some later date, they'll come away clean (ish).

If the slate is machined tiles with a ridged base, that really does suggest that they were intended for flooring, and therefore would be laid on a mortar screed over a concrete base, as discussed earlier.

Forum Question Crazy Paving Repairs - Rockrat - May 7th 2003
I need to repair my patio and garden path, both of which are crazy paving. The patio (160 sq ft) appears to be laid on fine-ish sand and the occasional area of what looks like red cement(?). Most of it is in good nick but there are small collapses here and there, and voids under some of the paving stones.

The garden path (70 cm x 14 m) was laid straight onto the soil (heavy-ish clay) and is 50% 'buggered' - mortar gone, slabs sunk, water pools. The paving is natural stone but I've no idea what sort - pale grey, fine grained and very smooth on top and completely rough underneath. I would rather not rip up the whole patio unless I really, *really* have to but would prefer a temporary solution until cash and time is available for the big job.

However, the mortar used is 'weird'. It is white with small (3-5 mm) stone chips and glassy smooth.

Any ideas about the best approach, the mortar? For the path, I'm happy to lose the crazy paving - I just need a quick and not too expensive replacement that is kid resistant (which with my two is equivalent to heavy traffic by stampeding tyrannosaurs) and which enjoys major flooding by new neighbour with his hours of  #'*!*@ incessant lawn watering.

forum answer Tony McCormack - May 7th 2003
Red cement? I guess you mean 'mortar' rather than cement, and a red mortar is not that unusual, if it's prepared using a reddish building sand, as is common here in the North-west of England.

The 'voids' sound worrying - that's something normally seen with the hated spot-bedding,. Does that sound right?

Anyway, to rectify the path - I can't say what would be best unless I could see the paving for myself. You could make a temporary repair to the lost or sunken paving by overlaying with a fine-grained concrete, but I'm not sure how feasible that would be.

If you were looking for a cheap, temporary surfacing as a replacement, then gravel of some form would be the answer, but it doesn't respond well to regular inundation. Still, when you come to do the permanent job, the gravel can be reclaimed and used a coarse aggregate for concrete.

Next cheapest solution would be to use the nasty little 99p flags from the DIY sheds and bed them onto a semi-dry mix, but, if you're going to that sort of time and expense, it's debatable whether you're not better off doing the 'permanent' fix now, and saving all that effort.

Forum Question Jointing Slate Tiles - John Cousens - May 8th 2003
I have just had a new patio laid, using 300x300 slate tiles. The supplier of the slate recommended using a thick-bed adhesive on a concrete base, which I have done. They further recommended a waterproof grey tile grout, which I purchased but not yet used because of concerns expressed by my builder.

The concerns are that because of the depth of the joints(up to 10mm + thickness of tile) together with the width of the joints (approx 7mm), 2 things could happen:

1. The grout could take a long time to go off, thus making the task of cleaning up the tiles after smoothing the joints difficult and lengthy.

2. The amount of grout could cause the grout to crack under its own weight.

The builder has therefore suggested that he uses a strong dryish mortar mix instead. He acknowledges that mortar is not waterproof but believes that a couple of coats of sealant will do the trick.

Are his concerns reasonable? And is his solution appropriate?

Many thanks,

forum answer Tony McCormack - May 8th 2003
The grout could take forever and a day to cure, in theory, but it depends on the type. Do you have a brand name and/or manufacturer?

Even if the grout does take a while to cure, that really shouldn't affect cleaning, as the microscopically thin smear of grout on the surface of the slate will not be affected by joint width or depth. However, you have to offset that against the obvious reluctance of your builder who seems to want to find problems with this method.

As for the grout cracking - I'm not having that! Most of the waterproof grouts have a damned good flexural strength and are far less likely to crack than mortar.

Using a mortar is ok, and there are additives that can be used to make a standard cement mortar as waterproof as any tile grout, which may be a better solution than using a sealant, which has to be re-done every couple of years or so, anyway!

If your builder is dead set against using the grout, then maybe you could consider summat like Ultraflow, a rapid setting, hard-as-bloody-iron pumpable mortar from Instarmac. That'd do the job, and, because it can be applied via mastic gun, there's little or no spillage and no snots to clean!

Forum Question Confused on flag choice - Chrissy K - May 8th 2003
1) I have to pave 40m² and would like a very distinguished looking flag. I like the look of the Stonemarket Riven in their brochure, but you said that it is a very uneven surface, which is not very good for tables (I had not thought of that), can you recommend a few more types that may possibly fit my criteria of...

  • being aesthetically pleasing (ie looks like old quarry stone)
  • being colour fast
  • comes in various sizes to do a random pattern
  • In the £15-20 /m² price bracket

I hope I am not asking for the world in my criteria....

My 2nd question is....
Why did my builders, when laying flags around my house, leave a 4inch gap between the flags and the walls filled in with decorative gravel. Is this for regulation purposes or could they not be fagged to cut the flags to size?

My 3rd question is.....
I hope to lay them on 50-60mm of compressed grit sand. Why do I need to add 10% cement to this, is it to bind the sand together, because would the coarseness of the sand not do that itself?

4th and final question (for now)
How bad would it look if lay all the patio in one go, and then filled in the gaps with either dry cement/sand or actual mortar?

5th question (I lied)
Would it be possible to make a web page up with numerous flag types displayed, the price per square metre, and your general opinion on the product?

Please be gentle with your answers


forum answer Tony McCormack - May 9th 2003
Q1 - 15-20 quid per square metre...'re not going to get much for that, and you certainly won't get the Stonemarket 'top of the range' stuff. There's some very nice imported Indian Sandstone doing the rounds for that sort of money. Whereabouts are you? I might be able to tell you where to go and look.

Q2 - Because they're eejits, possibly. This is the notorious 'Splash Strip' that is supposed to reduce the risk of damp but is a bit of a con, as it just reduces the amount of expensive paving that has to be laid. There is no Building Reg. that insists on a Splash Strip and I have never, ever heard a single convincing argument for its use on a residential patio.

Q3 - 50-60mm of sand is too much. 50mm maximum, otherwise it can lead to problems with settlement.

The cement binds the sand grains and makes them resistant to groundwater movement and ants. If you have a really good coarse sand, such as the stuff we get from the Pennine quarries around Horwich, that can generate sufficient interlock to more or less bind itself, but many of the river and marine grit sands need a peppering with cement to encourage them to stay put.

Q4 - using a dry sand/cement mix for the jointing on a flagged patio with 8-12mmm joints is a waste of time and materials. It won't last more than a couple of years, if that! If you must use dry mix, then it must be swept into a joint that's been freshly buttered with mortar, so that it can properly bond to the mortar and to the flag. Best of all is a proper mortared joint.

Q5 - it would be, as soon as I've found a way of getting 30 hours into the day! Ackshirley, this is another of those pages in my 'development' folder, along with a similar page for Block Pavers and Clay Pavers, but the choice is so-o-o-o HUGE that the Block Paving version, which goes under the codename "Wotblok", has been in development for the last 2 years!

Chrissy K
May 9th 2003
I am based in North Manchester, a place called Bury. It has been suggested to go to Travis Perkins.
Tony McCormack
May 9th 2003
<Choke!!> Travis Bloody Perkins?!?!?!? When it used to be John Kay's in Bury, it was a decent yard with a good range of stock, but since the global domination of TP and Jewsons, you need to find a specialist stockist of paving materials or a small independent BM, or all you'll be offered is the run-of-the-mill over-priced stuff from a very small number of large, national manufacturers.

There's a list of fairly local suppliers of stone paving on the Stone Links page. Stone Essentials in Rossendale is the nearest I can think of to you - tel: 01706 211120 and tell them I referred you.  smile

If you ever get down as far south as Stockport, Ernest Axons at Cheadle Heath has a good selection of paving materials, too - 0161 428 0314.

Forum Question Is a sub-base necessary - Rusty - May 11th 2003
I have read that it is not always necessary to have a subbase layer if the soil it quite established. Would it be possible for you to define how well compacted and established it should be to not need a sub layer?

I can give you a brief description of the soil now if it helps. When dry, the soil seems quite solid under the feet, even if "jumped" on. However, when the great rains came down the other week it became quite soft (which I suppose is common sense!!!). I had to build the small walls through out the garden and with walking on it quite a bit it was noticed a bit of sinking, but not by loads mind.

Would this not matter once flagged, because I can't imagine much rain water getting to the soil once down.

If the soil if sufficient for loading, then would it be a good idea to put down some strong membrane and then the sharp sand layer on top to strengthen it all up, or would this cause problems?

The flags size by the way is 450 x 450 x 32mm and to be laid with gaps between and filled with a filler.

forum answer Tony McCormack - May 13th 2003
Defining the competence of a sub-grade is fraught with difficulties, as the ground varies so much from place to place and any description-based definition is almost certainly doomed to fail. That's why a standard test, the CBR, is used to determine the suitability or otherwise of a sub-grade.

However, for a patio, we needn't get too concerned, as we're not anticipating massive loads. If the ground is firm and stable when dry, then it's probably ok - if, when you jump on it, it wobbles around a metre or so away from where you are, then that's more of a problem.

If you have any concerns, then using a geo-membrane would help, but, for 95% of all patios, laying the flags on a cement-bound bed, such as the 10:1 mix of grit sand and cement that I advocate, is more than adequate.

May 13th 2003
The ground is pretty much stable when dry, but I will give it a further good compacting with a whacker before I put the 10:1 mix on top. I was thinking of using a Geo-Membrane as well to ensure there is no mixing of the 10:1 with the sub layer. I've looked on the geo-fabrics page but a bit confused on what to actually get. I was thinking of just using the woven fabric you can get in most stores now-a-days, but would this be stable enough? I looked at the thicker more durable woven stuff you can get (think is a polyester type material, seems pretty stong) but I was wondering if the bed might slip about on the smooth surface. Any recommendation would help a lot.

Also, what thickness sharp sand would you recommend as a bed without a sub-base?

One more thing!!!! Reading your jointing page, if there is the slightest of movement in the flags, does the joint not crack???? Is there not a flexible jointing material?

Tony McCormack
May 14th 2003
You can get a roll or two of Patio Partner from the Terram mini-website which is ideal for your sort of project, or you can get mini-packs (50m²) of Terram 1000 (same product, but different colour and different name) from better BMs for around 75 quid.

Lay your paving on a bed not exceeding 50mm. I like to add a sprinkling of cement to sand bedding beneath patio flags, as it helps stabilise everything.

And you're right, if there is any movement in the flags once they're laid, then the jointing will crack. This shouldn't be too much of a problem as flags are supposed to be firm and stable on the bed and not rely on the jointing for their strength, unlike, say, blocks or setts.

However, in practice, especially with smaller patio flags, the jointing can contribute hell of a lot to the stability of the flags, and so, if there is any movement, a rigid jointing medium, such as mortar, will crack. But if you use a flexible medium, you either pay a small fortune for a sealant such as Nitoflor, or you use an unbound medium, such as sand or splitt, which allow vegetation to establish itself on the pavement.

You have to decide which is best for you. With stone or riven-effect flags, I prefer mortar jointing, but with some of the polished or shot-blasted flags, a 3mm flexible sand-filled joint is fine.

May 14th 2003
Just wondering Tony, does the woven fabric decrease the structural integrity of the base??? I only ask because they currently have a huge roll (god knows where they got it from cause the roll width is about 6 metre!!!!) of a woven (I think) grey fabrics, it is stronger than the cheap thin stuff you get from Wilkinsons?

I'm pretty sure its woven (never come across non-woven!!!!), it kinda looks like the fibres are just mixed randomly (rubbish explanation!!!!).

And as they have loads, I was wondering if I could make use of it save them spending £50 - £75.

Cheers for help Tony

Tony McCormack
May 14th 2003
If it's a permeable geo-membrane, then it should be ok. Even if it's a weed barrier, it won't do any harm, but it probably won't last more than a few years as the sub-base will pierce it.

If it's an impermeable membrane, such as a dpm, then it could cause problems.

A non-woven permeable membrane looks like the stuff on the Geo-sheets page, a random orientation of fibres that are thermally bonded (techno-speak for hot glued).

May 14th 2003
If it's the top picture on the geo-fabrics pages then I think that's the one. It's pretty thick (could kind of define the thickness as you can crease it). So that should be ok to use???

You say no more than 50mm of sharp sand as the bed, with some cement....10:1. Am I right in saying the thinner the bed, the more solid (as long as it's not too thin) or don't it really matter?

Tony McCormack
May 15th 2003
The membrane you have sounds fine, Russ.   smile

50mm is the maximum thickness for a bedding layer, and 30mm is the minimum, although you can go down to 12mm if it's a mortar. Like a lot of other things in life, you don't want too much or too little. 40mm is ideal.

May 16th 2003
I will initially set it out a 50mm then, then whacker it down and finally screed off to 40mm..........sound a plan???

Thanks for your help, Wish me luck this weekend, saying that if the rain keeps on it looks like I wont be laying the flags as I presume its best not to do it in the rain!!!! sad


Tony McCormack
May 16th 2003
To create a 40mm screed, you need around 70mm of sand, that's compacted with the plate to around 45-50mm, and then you have an excess that can be screeded to final level.

Working in the rain is not a problem, unless there's mortar involved, but if it's chucking it down, then it's best left. Fine drizzle is not too bad, and showers are tolerable, but steady, non-stop, soak-you-to-the-bones rain, such as we have today, is awful and you're better off seeking shelter in the a local alehouse while revising your strategy!   smile

May 20th 2003
Well, what a weekend, chucked it down every day and with the ground as water logged as it is I didn't even think about attempting to prepare the ground for flagging. So carried on building the next level walls.

You say use 75mm of sand and then plate down. I have 3 tonne of sharp sand, do you think I should order some gravel as well or will the sharp sand alone (with a 10:1 cement mix) be ok??? I have loads of 20mm gravel (a wee bigger than the recommended 6mm!!!!)) left over but I can imagine that is a bit too big.


Tony McCormack
May 20th 2003
The sand alone will be fine. Adding gravel is fine if you're using individual bedding, but when you're planning to create a screed, the inclusion of gravel can 'drag' on the surface of the screed,  creating 'furrows', which aren't the end of the world, but can sometimes be a problem when laying smaller elements, such as 300x300 'flags' or block pavers.

Obviously, a 6mm gravel wouldn't be too bad, as it will mix in with the coarser particles of the sand, but a 20mm gravel would  cause much more of a problem and is best left as a coarse aggregate for knocking-up concrete.

If you're using the 10:1 bedding mix, don't make up more than you will cover with the paving in around 3 hours, as the cement starts to go off as soon as it comes into contact with any moisture.

Forum Question Northwest supplier of Indian stone - Chrissy K - May 13th 2003
Can any-one recommend a good supplier of Indian stone? Tony - you mentioned Stone Essentials, of Rossendale. I phoned them and they suggested Bridge Street Stone of Colne.

They can supply stone for £16.50/m² (+vat), which to me is quite reasonable, but my question to you and the board is..................
Is there anything better or is that a good price, for hopefully a good product

Thanks in advance

forum answer Tony McCormack - May 14th 2003
There's Stonescape at Wiggin - 01942 866666 - ask for Julian and tell him I referred you. They have a very good selection and a nice display area where the paving has been laid out in panels so you can see it 'in the flesh'.

Their yard is at the back of the Cemetery in Ince, on the Warrington side of Wigan and worth a visit if you're really keen on the Indian stone.

There's also Hardscape in Bolton, but I'm not sure how keen they are on "diy'ers" visiting their yard. Still, you can call Matthew Haslam on 01204 590666

Let me know how you get on, as I'd be keen to hear your thoughts on what these suppliers have to offer, and how helpful they are when it's a non-trade enquiry.

May 27th 2003
We recently (April 03) bought our riven Indian sansdtone from Beeston Reclamation, in Beeston, Cheshire. A bargain at £18 per sqyd (ask for trade price!)

I've lost their number but they will be in the book and it was a pleasure to deal with them.

good luck

Chrissy K
Jun 16th 2003
you quoted me 2 suppliers, Stonescape and Hardscape.

Stonescape have offered £'15+vat (£40 delivery), Hardscape were either expensive, or useless I can't remember which.

Needless to say I'm going with the Stonescape and have no started digging out the foundations.....oh what fun.

Thanks for all your advice so far it has been much appreciated.


Tony McCormack
Jun 18th 2003
Let me know how you get on, Chrissy. Take photos as the work proceeds and make notes of what you find to be easier or harder than anticipated - sharing your experiences will help others following the same path.   smile
Chrissy K
Jul 15th 2003
The foundations have now been dug and most of it skipped and carried off, the rest will be done at the end.

The stone arrived from "Stonescape" and has worked out at just under £19 (which includes vat & del).

I'm very happy with the service. Although they said it would arrive at 10am Saturday morning. It actually arrived at 9am and my paracetamol hadn't kicked in by then.

The colour chosen was "raj Green" under the advice of Julian, as it is a more consistent colour then other stones.

The depth of the stone varies from about 25mm to 50mm. I have sorted the stone into 2 piles. Smooth and rough. The smooth are going in the centre of the patio to try and reduce the risk of patio furniture rocking, the rough stuff is going where it will do the least damage (for want of a better word).

I will keep you posted how it is going but it maybe some time. I'm laying at about 6m² per day.

Give me a golf course any day !!!!!!

Tony McCormack
Jul 18th 2003
If I had a golf course cluttering up me garden, you'd be welcome to it!   wink

The Raj Green usually works out quite well, but I'd be wary about separating the rougher flags from the smoother ones - you could end up with a patio that looks as though it's been constructed in two stages from different materials. It's best to mix the textures and allow the paving to have a more random look.

Forum Question Flagstones over existing concrete - joejoe - May 16th 2003
I know that you are primarily U.K. focused but I was hoping you could provide some advice for someone living in the frost belt of the US (Minnesota).

We normally have 4 to 5 months of winter where I live with the ground freezing many inches (frost line of 1+ meters). The climate is extreme here with winter temps as low as -30 degrees Celsius and summer temps as high as +40 degrees.

Now to my quandary:
I have an existing concrete patio, 30 years old and 100mm thick. It is reinforced, and also has movement joints (expansion). It also has one 5 mm or so crack across the width of one section. It is about 9 meters by 5 meters. Other than the crack, it is in decent shape, with little surface deterioration, etc.  Between the sections (areas separated by the expansion joints) there is little difference in height.

We would like to install a stone patio, something more attractive than the raw concrete.  We have tentatively selected a type of flagstone quarried hear in Minnesota that is sawn on two sides to different thickness, but for the patio flags would be 25 mm thick.  It is a type of limestone called dolomite, very fine grained, resistant to weathering, and used widely as a building material in this area.

This flagstone is an even thickness, since it is sawn on 2 sides, but would be irregular in shape (crazy paving).  Ideally, I would like to install this new patio over the existing concrete, yet have it last for a number of years (20+).  But if this does not make sense (to use the existing concrete as a subbase), I would take the existing concrete out and start new.

If I keep the existing concrete subbase, should I...

1)bed in the 10:1 grit sand/cement bedding at 40 mm thick, and then grout with type II mortar? Since my flags would be regular thickness, I could use the screed technique.
2)bed in type II mortar 20 to 25 mm thick (with the same mortar pointing between the flags)?
3)Or should I treat the flags almost as a thick stone tile veneer and use a premixed tile mortar made for thick tile called medium bed thinset tile mortar?

From what I gather, you spread this special type of thinset on with a notched spreading trowel. It is formulated to retain its strength even when spread thick (hence the name medium bed).  Since my existing concrete has the proper fall, and the flags are even thickness, I think this may be the easiest to lay and possibly the strongest.

With any of the above methods, I don't want to have major cracking and I also don't really want to cut through the stone at the existing concrete expansion joints.  Is this the recommended way if I lay across existing concrete with movement joints?  Would the 10:1 bedding prove more tolerant of sub base movement than the solid mortar bed or medium thickness thinset?

4) Or finally, would you recommend removing the existing concrete, and starting over with laying 100mm of dpt1 (I think class 5 crushed limestone here) subbase, then the 40 mm bedding?

Your advice would be greatly appreciated.

forum answer Tony McCormack - May 16th 2003
I'm usually very cautious about advising on paving in the USA as the methods and materials used are totally alien to us in Britain and what works for us is not necessarily the best thing for you, and vice-versa.

Anyway, for what it's worth, I'd offer the following advice...

Bedding - the existing concrete base sounds fine, so there should be no problem re-using it, provided that the extra height created when you lay the new paving will not compromise floor levels or any damp proof measures you may have. I can't see any point in breaking out the existing base and replacing it with a flexible sub-base - it's a poorer construction, it involves a lot of extra work and it's unnecessary, as far as I can see.

For the actual bedding, the tile adhesive sounds ok. In UK parlance, anything less than 30mm thickness is classed as a tile rather than a flagstone, but that, in itself, is not sufficient reason to use a tile adhesive for a job such as this. The thin nature of your chosen stone means that it must be laid on a full bed. I'd be worried about using my usual 10:1 bedding mix, as it's not intended to be subjected to repeated freeze-thaw cycles. A mortar would be ok, I suppose, and would be my choice if there were any deviation in thickness of 5mm or more, or an uneveness in the base, but, as all that seems to be fairly regular, then a proprietary adhesive would be a better choice, as long as it is approved for external use in the climatic conditions you have to endure.

But is this bedding material suitable for use as a jointing material?

Movement joints - you cannot bridge the expansion/movement joints with your stone paving. If you do, it will probably crack. Ideally, the joint is carried through to the surface of the paving, so you'd have to align your random-shaped stone in such a way that the underlying joint could be extended upwards, preferably with a flexible jointing compound (I can't suggest one as I don't know the US-ian market, but in the UK, we'd probably use Thioflex or Nitoseal).

Although you might not notice it, there will be some movement of the slabs either side of the joint, and, if you use a rigid medium, such as stone, to bridge the joint, then the shear forces will be transferred from each slab into that fragile, 25mm thick sliver of stone, and, as stone is absolutely useless when in tension, it will simply crack. If you had used a mortar or a low-strength bedding medium, it might be that the 'crack' would develop along the interface between the flag and the bedding, but with a decent adhesive, such as we described above, then the stone itself is the "weakest link" and the crack will appear as a fracture in the stone.

However, there's no need for you to have a perfectly straight line 'joint' in the stone paving, you can create a gap over the line of the joint that varies in width from, say, 5mm to 40mm, and fill that with the flexible joint sealant.....

expansion joint

I hope that answers most of your immediate questions, but if you've any more, I'll try to help, and I'd definitely be interested to hear how you get on with it all.

Forum Question Restoring crazy paving - Duncan Millar - May 31st 2003
My patio is crazy-paved with york flagstone and similar natural stone, but I would like to do it up a bit. First there are mortar and cement smears and spots here and there, and while it looks very attractive when wet, it looks dusty and colourless when dry. This might be due to a film of mortar that got over the whole thing when the paving was first (not very well) pointed.

I would like to use a product to remove any cement and mortar, then a sealant to bring out the stone colours. Finally, I would like to re-point the whole area, as there is a fair bit of mortar that has broken up and disappeared. I have read about various cement-stain removers and sealants on your very comprehensive and informative pages, but, to avoid any mistakes, could you recommend the most suitable?

Also is there any suitable ready made mortar?

Many thanks.

forum answer Tony McCormack - Jun 1st 2003
To clean up the cement staining, any decent brick cleaning acid as sold at a Builder's Merchant will be fine. Avoid the so called 'Patio Cleaners' which tend to contain a detergent and a bit of perfume to make you think they work better - those in the trade will always use a plain, simple brick-cleaning acid. There are many different makes on the market, but Feb, PLA or Sealotone are all good brands.

However, you have your sequence of operations arse-about. You need to do the repointing first, then the cleaning and finally the sealing.

To do the re-pointing, mix your own mortar - a Class II mortar (4:1) with a bit of added colour if you like, and repair all the joints, not just some of them, or you'll end up with a patchy-looking effort.

I have a deep mistrust of the ready-made mortars sold in 25kg or 40kg bags at DIY stores. They're only sand, cement and plasticiser, measured and bagged, and then you're charged 4 times the going-rate. They are no better than self-prepared mortars.

Give the freshly pointed paving at least 3 days before washing down with the acid. And once you've got it all nice and clean, with properly filled joints, the choice of sealant depends on what finished effect you want. There's a long posting I made in this forum entitled Sealants for Slabs that lists the pros and cons of various products. You have to choose the one you think will give you the finish you want.

Forum Question Oil and Wax stains - Emily Sedgwick - Jun 2nd 2003
any ideas on how to remove olive oil stains from my slate patio? Have tried patio cleaner but it doesn't seem to work.

Many thanks.

forum answer Tony McCormack - Jun 3rd 2003
Patio cleaner won't work on oil and I do wish the manufacturers would make that clear on the packaging. far too many of these products are misleading in that they give the impression that they are stain removers, when they are actually concrete/cement cleaners and can only be effective at cleaning cement stains or rejuvenating the appearance of old, dirty concrete paving.

Cat litter is claimed to be good at drawing out oil from stone paving, but I can't say if it would work with slate - still, there's no harm in trying! Failing that, an oil remover, such as Jizer, might be what you need.

Emily Sedgwick
Jun 4th 2003
Thanks a lot, I'll give that a try. Any idea how to remove candle wax from slate?

Thanks again.

Tony McCormack
Jun 4th 2003
Olive oil - candles - just what have you been doing on that patio, Emily??? wink

Candle wax will melt if you apply gentle heat, and can be absorbed into a paper towel, then treat the residue with a degreaser, as mentioned for the olive oil.

Forum Question Sub-base: Granite or Limestone? - A Bancroft - Jun 2nd 2003
I'm putting a patio in my back garden ('yard' in American English) and plan to lay a sub-base over the sub-grade.

After phoning round a few places here in Texas, it seems the material choice is crushed concrete, crushed granite or 3/8" limestone (funnily enough, no-one mentioned DTp1!)

The crushed concrete supplier won't deliver less than 15 tons which is a tad more than I require (by about 300%!). Which leaves either crushed granite or 3/8" limestone.

Is the granite worth the extra $ over limestone?


forum answer Tony McCormack - Jun 3rd 2003
For a typical residential patio, even one in Texas-shire, any of those fill materials will be fine for a sub-base. I reckon it would be difficult to source DTp1 around your neck of the woods because there is no DTp (English English for Department of Transport)   smile

We get both crushed granite and crushed limestone in this part of the world, and there's nowt to choose between them. Granite is hard and inert while limestone has a tendency to self-bind, making it a popular choice for northern road builders. For a patio, it really doesn't make that much difference - it's the 'grading envelope' ie, the distribution of lumps and fines, that is more important than the type of rock. As long as there's a good distribution of the lumps and fines, then it should be "reet gradely" as they say in Lancastrian English.   smile

So - how much does a ton(ne) of crushed rock cost in Texas-shire? British buyers have to pay around 25 quid (40 dollars) for a single tonne.

A Bancroft
Jun 4th 2003
Sounds like I can make my decision based on cost & availability, thx.

The limestone is going for $40 yd³ and the cheapest granite I could get is $46 yd³.

Exactly what the density of the rock is, I have no idea. But this web page reckons limestone varies from 110 to 160lb/ft². So 1 cubic yard will be between 2970 and 4320 lbs. i.e between 1.3 and 1.9 metric tons.

Tony McCormack
Jun 4th 2003
I can't work in cubic yards, pounds, cubits and all those other ancient measures; they make me brain hurt, but we reckon limestone has a density of around 2.2-2.4 tonnes per m³ and granite 2.4-2.9 T/m³ (these being the new-fangled metric tonnes, which are 1,000 kg, not 2,240 (??) pounds)

$40 for a cubic yard is around $48 per cubic metre, or $20-24 per tonne, which is around 15-18 quid per tonne, a good bit cheaper than we can get it in the UK in small quantities.

Forum Question Paving against the wall - Tom Charnock - Jun 3rd 2003
I'm building a patio next to my house and so far I've put in the sub-base layer and I've allowed for a 1:60 fall off (a figure that I found in a DIY book before I found this site). Anyway I have a couple of questions...

The sub-base layer didn't compress as much as I expected and it looks like the top of my flags will be about 130-140 mm below the damp course. Is this enough? I know the advice is to be about 150mm but will 130mm do any damage and is a surveyor likely to notice this?

I could make my bedding layer a little thinner to bring it down a bit, is this a sensible approach? Alternatively I could choose not to flag right up to the house and just leave the sub-base exposed with maybe some gravel on top. It would be very difficult to lower the subbase further as my garden slopes towards the house and the other end of the patio will already be several inches below the lawn and I don't think it would be sensible to make the fall-off less than 1:60.

My second question is when it comes to laying the flags up to the house do a just abut them to the wall or do I butter the wall first and when they are laid do I point the gap between the flags and the wall?

Thanks in advance for your answers.

Cheers Tom

forum answer Tony McCormack - Jun 3rd 2003
If you can get 130-140mm between the top of the paving and the dpc, that will be fine - don't lose any sleep over it! Although 150mm is always quoted as the minimum, for a project such as yours, no-one is going to complain about 10-20mm difference, and it's highly unlikely to be detrimental to the house itself, given that you have adequate fall away from the walls.

For your second point, I like to butter the wall and then place the flag as that gives a more certain joint than laying the flag 'dry' and then trying to poke in the pointing mortar to fill the joint.

Forum Question Laying a perfectly flat patio - when the slabs aren't flat - Barry - Jun 3rd 2003
I'm presently laying a courtyard patio for a nursing home. It will be trafficked by wheelchairs and pedestrians only. They requested a cheap but stable surface, and opted for 900x600x50mm precast concrete slabs.

I've set up a sub-base of 100mm compacted DoT Type I and am laying on a 30mm wet base of 5parts sharp to 1 of OPC. This has given me a very strong patio indeed.

However, despite my very best efforts, I have noticed in a couple of places that there are 'lips' of 2-3mm at parts of the interfaces between the stones. Looking more closely with a steel straight-edge, I've found that the slabs are actually 'bowed' slightly along their length, making a snooker table finish impossible. They are Marshalls slabs and I hadn't expected this.

Normally I wouldn't worry, but the residents are elderly and tend to shuffle, so 2mm really could be a problem. The Client seems happy enough, but I wonder if it may be flagged up during a later inspection.

Should I try to shave down these problem patches with some sort of grinder (and if so, what), or is there another solution?

Any advice would be very much appreciated.


forum answer Tony McCormack - Jun 4th 2003
In your situation, Barry, I'd take up the bowed flags and get replacements from Marshalls. If these are the A50 flags, they really should not be bowed.

You don't mention what type of jointing you're using. If you're butt-jointing, then any deviation from true will be highlighted, but by using a 10-12mm mortar joint small deviations can be accommodated quite easily and the transition between flags 'smoothed', if you know what I mean.

However, now that the flags are in and on that bedding mix they'll be damned hard to extract, then you might be better off getting a diamond-bladed power saw to "grind" off any lips. It's not a pretty effect, but it's better than having one of the residents trip.

Just how much of a bow are we talking about over the length of the flag? 5mm? 10mm? More??

Forum Question Imported sandstone flags - ALS - Jun 5th 2003
Has anyone used or know of anyone who has used WICKES natural riven sandstone flags? Price-wise they're not bad, but they look thin and I would want to use them for steps.

What way do you lay these flags i.e. with the tapered edge facing up or down?

forum answer Tony McCormack - Jun 7th 2003
I haven't seen the Wickes ones, but I'm guessing they'll be similar to a lot of the other imported stone that's doing the rounds just now. A lot of them are only 25mm thick and although they should be ok for patio use, I wouldn't be happy with them on a driveway or for commercial work.

And, to answer your 2nd Q, taper edge down. so that the joint is narrower at the surface than at the base. See the Laying Riven Paving page.

Jun 10th 2003
Hi Tony, thanks for the answers, but just out of curiosity, why are they tapered when other flags etc are square edged?
Tony McCormack
Jun 14th 2003
They are tapered because that's the way they 'break' when cut with hand tools. To achieve a neat edge on the top surface, the underside has to be undercut, sloping backwards. It's actually easier to achieve that type of finish than to have a flagstone with 'square' edges.

The other benefit is that, when laid with a mortar joint, as they should be, the mortar is 'locked-in' by the shape of the undercut.

Forum Question Flags supplied from Different Batches - David W - Jun 6th 2003
I ordered 300 Marshall's Perfecta flags from my local stockist (Keyline). In 'phoning for a quote, I had made it clear that I wanted 5 packs of 60 as supplied by Marshalls, the idea being to make sure that they were from the same batch. I was told that they had just had a delivery from Marshalls and could supply straight away, so I placed my order.

It turned out that they had only 3 full packs in stock, which they delivered a week ago, and they said they would make up the balance and deliver it within a few days. They said that this would mean having flags from two different batches but from the same factory, so should be OK. I agreed but emphasised that they had to be two full packs from the same batch, not odds and ends, and they assured me that this would be so.

Despite their assurances, they later made up the shortage of 120 flags from broken packs and left these in piles on my drive whilst I was out today, stacked flat with some face up and some face down. Looking at a few of these, there are scratches on some of the faces which seem quite deep and may or may not weather out. Also, the patterning (stone chipping size) on some is much finer than on others so they don't look like they will be a good match. Until they have weathered I won't be able to judge the actual colour match.

Do you think I should insist that the stockist takes them back and supplies two unopened packs from the same batch? Or am I expecting too much?

My wife has just told me that she has counted the flags in the stacks and there are about 30 extra (i.e. 150). It's too late (6.35 pm) to 'phone Keyline now but I assume that they couldn't locate full packs and have left the extras so that I can pick and choose suitable ones. This is not easy with flags that have efflorescence as I'm sure you'll agree.

Not sure what to do now. Any advice welcomed, please!

forum answer Tony McCormack - Jun 7th 2003
Did you give a written order to Keyline or was it all done on the 'phone?

Basically, they should supply you with flags from the same batch, as that is what you ordered and you did make it quite clear to them. If a contractor had been treated in this way, they would take them back and ensure the order was filled as requested, and so, IMHO, they should do the same for you.

Contact Keyline first thing Monday and speak to the depot manager, not one of the underlings. Don't have a rant, but explain the situation calmly, and tell him that you've had to postpone a contractor as you couldn't risk having them lay the flags as supplied by Keyline, and that you've had to pay the contractor for a wasted Saturday - that should get you a bit of sympathy.

If you get no joy, then contact Keyline's Head Office and contact Marshall's themselves. Even though it is not their problem, directly, they can exert far more pressure on Keyline than you or I can, and they are usually quite keen to help out with this sort of 'customer relations' problem.   smile

Don't give up!

David W
Jun 9th 2003
Tony, thanks very much for your advice -- excellent as always. Unfortunately my impatience to get the job started has led to me starting to use the suspect flags so I can't now return them which, as you will see, I should have done.

The situation was that I had already persuaded my son to swap his weekend shifts to help me lay the flags, so I decided to inspect the flags one-by-one to see if I could get enough good ones out of them. I found that most of the scratches rubbed out fairly easily so I simply put to one side those that were chipped and/or had obviously bad facings. I was left with enough apparently good ones to do the job and I managed to segregate them so that those with like-finishes went together (this took HOURS!) because some had a 'finer' aggregate content than others. The idea was that I could use one lot on the patio and the other on a part of the path that was visually separate.

Having now laid what looked like the best ones in the most prominent positions on the patio, I've found that a number of them have a flaw which can only be seen when they are damp, either from rain or rising moisture. The effect of the flaw is similar to having a 75mm crack from one edge through which moisture creeps and locally migrates, though I can't actually see a crack, and it is the same length and position on all.

Looks like I'm stuck with a patio/pathway that will always look flawed unless the surface of the flags (Perfecta buff) stabilises over time. ;I hope that others learn from my mistake -- and thanks again for your advice which I should have taken.

Forum Question Flag and Gravel drive - Gardendesigner - Jun 8th 2003
I am looking to lay a drive made up of randomly laid flags, with a variety of joint thicknesses, surrounded by gravel.

Looking through the previous posts it looks like I would need to lay stone flags or 50mm BS pressed concrete flags in a weak bedding mix over a 100mm mot class 1 sub-base or alternatively if I use patio flags I would need to lay them directly onto a 75mm full concrete bed.

Can you please confirm that my assumptions are correct and advise me on how to incorporate the gravel? Do I just pour it on to fill the gaps? If I leave the joints too wide will this cause the flags to lift when a car drives over them?

Any advice would be gratefully received, thanks in anticipation,


forum answer Tony McCormack - Jun 8th 2003
If you're having unnaturally wide joints, then it's best to lay the flags on a full mortar/concrete bed. I'd go for 100mm of C7.5 or equivalent concrete, and bed the flags directly onto that, but, if you're working more slowly, then use a concrete base, 100mm thick, then bed the flags onto a Class IV mortar over the top of it.

You could use a flexible sub-base (100mm DTp1) if you prefer, but the full concrete base is best for patio flags, which have very little flexural strength.

Leaving big wide gaps can, and probably will, cause problems with flags being 'flicked' upwards when trafficked, which is why I'm so keen to push you towards a full concrete bed, that will hold the flags firmly in place. The concrete would, ideally, come up the side of the flags, to within 25mm of the surface, and then fill the gap with your chosen gravel, pushing it into the still-plastic concrete before it sets completely.

A weak bedding mix (10:1) won't be strong enough to restrain the flags in this sort of design. It's fine for patios and flags laid with 'normal' joints, but for this type of project, it's best to think of it as a series of stepping stones, rather than as a composite pavement.

Jun 10th 2003
Hi Tony,

thanks for the prompt reply. If I go for the slow method you describe and bed my flags into a full bed of class IV mortar on top of a concrete base, what depth of mortar should I use?

Thanks Gardendesigner.

Tony McCormack
Jun 14th 2003
20-40mm will be plenty.

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