Damp Proof Membranes: A Complete Guide

What is a Damp Proof Membrane?

A Damp Proof Membrane (DPM) is a sheet material used to prevent moisture from passing through floors, walls, or other building elements. It acts as a barrier between damp ground and the living space above, protecting your home from rising damp and moisture ingress.

DPMs are an essential component in construction, particularly for ground floors and below-ground structures like basements.

Types of Damp Proof Membranes

Polyethylene Sheet (Polythene DPM)

The most common type, typically 1200 gauge (300 microns) thick:

• Cost-effective and widely available

• Easy to install

• Suitable for most residential applications

• Must be laid with adequate overlaps (minimum 150mm)

Self-Adhesive Membranes

Used where a more robust solution is needed:

• Bonds directly to the substrate

• Reduces risk of punctures and displacement

• Often used in commercial applications

• More expensive but more reliable

Liquid Applied DPMs

Painted or sprayed onto surfaces:

• Ideal for irregular surfaces

• No joints or overlaps to fail

• Can be applied to walls and floors

• Often bitumen or epoxy-based

Where Are DPMs Used?

Ground Floors

In modern construction, a DPM is installed beneath concrete floor slabs. It should be continuous with the damp proof course (DPC) in the walls to create a complete moisture barrier.

Basements and Cellars

Below-ground spaces require comprehensive waterproofing. DPMs are used in conjunction with tanking systems and drainage solutions.

Retrofitting

When renovating older properties without adequate damp proofing, DPMs can be installed over existing floors before laying new flooring.

Installation Best Practices

• Surface preparation: The substrate must be clean, dry, and free from sharp protrusions

• Overlaps: Minimum 150mm overlap on all joints, sealed with appropriate tape

• Wall connection: Turn up walls to connect with DPC level

• Protection: Avoid puncturing during construction

• Testing: Conduct moisture tests before and after installation

Signs You May Need a New DPM

Indicators that your existing damp proofing has failed include:

• Damp patches on ground floor walls

• Musty smells

• Tide marks or salt deposits (efflorescence)

• Lifting or bubbling floor coverings

• Mould growth at floor level

• High moisture readings on walls

Professional Assessment

Diagnosing damp issues correctly is crucial. What appears to be rising damp could be condensation, penetrating damp, or plumbing leaks—each requiring different solutions.

Mould Detect provides expert damp assessments using calibrated moisture meters and thermal imaging to accurately identify the source of moisture problems. Our reports provide clear recommendations, and our partner LWR can carry out any necessary remedial work.