aj mccormack and son
Pavingexpert Search

Threshold Drainage
On This Page...

threshold drainage
Related Pages


Level threshold interceptor drainage solutions

Traditionally, paving is laid at a level that is at least 150mm below DPC level, which is usually equivalent to internal floor level. However, Document M of the Building Regulations requires level threshold access to be provided to all new buildings, and a growing number of homeowners like the idea of having a step-free level walkthrough from the lounge or conservatory onto the patio. This can present a problem at thresholds: how to provide a flat access without running the risk of surface water finding its way into the property?

While not always necessary, the use of threshold interceptor drainage is a simple solution that will protect property without compromising the aesthetics. The following photos and diagrams illustrate just a few of the more popular solutions....

under sill
Paving under sill
level paving
Paving laid with extra fall away from doorway
linear channel at threshold
Using linear channel option
The use of a threshold drain is usually a 'failsafe' measure, installed to prevent any accidental flooding of the external paved area spreading into the property.

Ideally, the paving should fall away from the property, as per normal, as it is never a good idea to rely on a threshold drain as the 'main' drainage for an area of paving. However, there are the inevitable instances where use of a threshold drain as the only drainage outlet on a paved area is unavoidable. In many such cases, it is often possible to engineer an alternative, albeit at additional cost and inconcvenience, but such solutions are always to be preferred to relying solely on a threshold drain.

slot drain
A typical slot drain
slot drain at threshold
Slot Drain used as a threshold interceptor
slot drain
Cross-section of slot drain installation at threshold


aco step drain
Using Aco Step Drain
Aco StepDrain
StepDrain installed (ignore crappy blockwork)
Aco Step drain
StepDrain showing grating and outlet connection


hepworth threshold drain
Using Hepworth Threshold Drain
hepworth threshold drain
Hepworth Threshold Drain