What is a damp proof membrane?

A damp proof membrane (DPM) is a continuous sheet of impermeable material - typically heavy-duty polythene - laid beneath or within a ground-bearing concrete floor slab. Its purpose is to prevent moisture from the ground rising up through the floor into the building above. It works alongside the wall DPC to form a complete moisture barrier around the base of the building.

What is the difference between a DPM and a DPC?

A DPC (damp proof course) is a horizontal barrier built into the base of a wall to stop moisture rising up through the masonry. A DPM (damp proof membrane) is a sheet membrane laid in a floor to stop moisture rising through the ground into the floor slab. The two must lap together at the wall-floor junction to form a continuous barrier around the building.

What thickness of DPM is needed?

The standard minimum for a DPM beneath a floor slab is 1200 gauge (300 micron) polythene. Where ground conditions are more aggressive - high water tables, contaminated ground, or radon risk areas - thicker or specialist membranes may be required. Radon-resistant membranes must meet BS 8485 and are typically 2500 gauge or specialist multi-layer products.

Should the DPM go above or below the concrete slab?

The DPM can be placed either below or above the concrete slab. Below the slab (under the blinding) protects the slab itself from moisture but the membrane is harder to protect from puncture during construction. Above the slab (between slab and insulation) is easier to protect but leaves the slab exposed to ground moisture. Both positions are acceptable under Approved Document C provided all laps and upstands are correctly formed.

Damp Proof Membrane (DPM) - Construction Glossary

Definition

A damp proof membrane (DPM) is a continuous sheet of impermeable material - typically heavy-duty polythene - laid beneath or within a ground-bearing concrete floor slab. It prevents moisture from the ground migrating upward through the slab into the floor structure and building above. The DPM must lap with the wall DPC at the perimeter to form a complete and continuous moisture barrier at the base of the building envelope.

In practice

The standard DPM for domestic floor construction is 1200 gauge (300 micron) polythene sheet, laid either below the concrete slab (on top of the blinding layer) or above it (between the slab and any insulation or screed). All laps in the membrane must be a minimum of 300mm and taped or welded. The membrane must be turned up at the edges and lapped behind the wall DPC to prevent water bypassing the joint.

In areas of elevated radon risk, a radon-resistant membrane (typically 2500 gauge or a specialist multi-layer product meeting BS 8485) is required in place of standard polythene. Liquid-applied DPM products are available for use as a retrofit treatment on existing slabs, though surface preparation is critical to their adhesion and performance.

Building Regulations

Relevant PartPart C - Site preparation and resistance to moisture

Minimum thickness1200 gauge (300 micron) polythene

Minimum lap300 mm - taped or welded

Radon areas2500 gauge or BS 8485 radon membrane required

Approved Document C requires ground floors to resist the passage of moisture from the ground. The DPM and its junction with the wall DPC must form a continuous barrier. Approved Document C Appendix A provides a radon risk map of England - areas with elevated risk require upgraded membranes and may also require a sub-slab depressurisation system.

Full Building Regulations guidance

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