A damp proof membrane (DPM) prevents moisture from rising through the concrete and ruining insulation, flooring, or finishes above. It’s a vital step in almost every concrete floor build-up — from new builds to extensions, sheds, and garages. But it’s often installed wrong: loose edges, ripped corners, or poor overlaps can make it pointless. This guide walks through the correct way to lay a DPM under concrete — clean, tight, and ready for insulation or pour.
Before rolling out your DPM, ensure the hardcore (or sand blinding layer) is compacted and flat. Remove sharp stones, debris, or high spots that could puncture the membrane. If no sand blinding has been used, consider laying a protective geotextile fleece or thin polythene sheet underneath. This isn’t just about neatness — it’s essential to protect the DPM from damage and ensure it sits flush against the ground without bridging or bunching.
Roll out your DPM across the floor area, cutting it to size with a knife or scissors. Overlap each sheet by at least 150mm where they join, and leave extra length at the perimeter to turn up against walls or shuttering. Work carefully — avoid dragging the membrane or walking on it too much. If it’s windy, use weights or temporary boards to keep it in place while you position each sheet. Keep the surface clean as you go — debris under the DPM can cause tears or bridging when the slab is poured.
Where membranes overlap, tape them with high-quality jointing tape. You can use either single-sided lap tape on the top edge or double-sided butyl tape between layers. Press firmly and smooth out air bubbles. It’s essential that the join is watertight — don’t rely on weight alone to keep sheets together. Avoid using duct tape or standard packaging tape — it won’t hold long-term. If you’re working around pipework or ducts, cut tight around them and seal with tape or proprietary collars.
DPMs should always return vertically up the perimeter — whether against concrete formwork, insulation boards, or masonry. Turn the sheet up at least 100mm above finished slab level. If you’re working inside existing walls, tape the membrane to the inner face using butyl tape or grip adhesive. This “dished” approach creates a continuous waterproof envelope, preventing moisture tracking up wall bases or escaping through slab edges. Don’t trim it flush until the concrete is poured and finished.
If any pipes, ducts, or rebar stubs pass through the slab, the DPM must be sealed tight around them. Cut a neat hole just large enough, slide it over the service, and tape around the entire edge using flexible DPM tape or specialist pipe collars. This is a weak point in many builds — even small gaps can let moisture through. Some systems use proprietary boots or grommets for drainage or electrical runs. Whatever method you use, make sure it’s sealed fully and can’t be disturbed during the pour.
Once the membrane is in place and taped, avoid unnecessary foot traffic. Lay insulation boards or temporary walking boards if needed to spread weight. Sharp tools, dropped mesh, or loose rebar can easily puncture the DPM, especially if the ground beneath isn’t uniform. If you’re not pouring straight away, cover the membrane or tape boards along key walkways. Check for holes, tears or moved joints before continuing — a quick patch now avoids major moisture issues later.
Before any concrete is poured, do a full walk-around. Are all joints taped? Are upturns secured and taped to walls or shuttering? Are penetrations sealed? Any visible rips or tears must be patched with a membrane offcut and re-taped. Once concrete’s in, it’s too late to fix anything. If you're working to spec, take photos or invite the client or building inspector to check the prep before proceeding. From here, your slab is ready to build on — dry, protected, and built to last.
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