Damp-proof membrane under a floor

Frequently asked questions

What is the difference between a DPM and a DPC?

Which plane they sit in, and they only work as a pair. A DPC is a horizontal strip built into a WALL, usually a couple of courses above external ground level, stopping moisture climbing the wall by capillary action. A DPM is a horizontal SHEET under or within the ground floor, stopping moisture rising through the slab. Each protects its own element and hands the problem to the other at the junction — so the DPM has to be lapped and sealed to the DPC around the whole perimeter, and a building where both are present but not joined has a continuous gap at the wall-floor line, which is precisely where damp appears. It is the commonest defect in a retrofitted floor: a perfect membrane over the whole slab, dressed up the wall and stopped, with nothing lapping it to the wall's own barrier.

Does it go above or below the slab?

Either, and the choice changes what else has to be right. BELOW the slab, on sand blinding over the hardcore, is the common arrangement: the membrane protects the concrete itself as well as everything above it, and the blinding exists specifically to stop the stones puncturing it. ABOVE the slab, under a screed, protects the finish but leaves the slab permanently wet from below, which matters if anything in it can corrode and matters a great deal for drying time. The version that causes most trouble is a sandwich — a membrane above and below with concrete between — because the slab then has no route to dry in either direction, and a screed or a floor covering laid on it is being laid on a slab that will never reach the moisture content the adhesive assumes.

Why do membranes fail when the material itself is waterproof?

Because a membrane is a system of laps and penetrations, and the sheet is the part least likely to fail. The classic failures are all at the edges: a lap not taped or not wide enough, a service pipe passing through with nothing sealing around it, a column base, a stanchion, a drain. The second family is mechanical: a sheet laid straight onto coarse hardcore is punctured by the stones under foot traffic and under the weight of the pour, which is why a sand blinding layer is specified beneath it and why walking a barrow across an unprotected membrane is a real risk rather than a fussy rule. And the third is thickness — a membrane specified at a gauge for the loading it will see, substituted with thinner polythene because it looks the same on the roll.