How to Install Damp Proof Membrane: Standards, Overlaps and Wall Upstands

A damp proof membrane (DPM) blocks ground moisture from penetrating concrete floor slabs and basement structures. Unlike a damp proof course (DPC) installed in walls, a DPM is a horizontal or vertical barrier—either sheet-based or liquid-applied—designed to protect floors and below-ground elements. It has been a requirement under UK Building Regulations since the Public Health Act 1936 and is now governed by Approved Document C and BS 8102:2009.

The primary function of a DPM is to prevent moisture transfer from the ground. By acting as an impermeable barrier, it reduces dampness, protects structural elements from water damage, and improves indoor air quality by limiting mold and mildew growth. A properly installed DPM must connect continuously to the wall DPC at all perimeters, creating a complete sealed envelope that prevents any moisture path into the building.

What Are the UK Regulatory Requirements for DPM Installation?

Approved Document C, Section 4 requires that ground floors resist the passage of ground moisture to the upper surface of the floor. A minimum 1200 gauge (300 micron) polythene sheet DPM is the most commonly specified solution for domestic ground floors. This standard ensures adequate resistance to capillary rise and ground vapour.

According to BS 8102, the DPM must be continuous and sealed at all joints and laps. Installation requires a minimum overlap of 150mm at joints, with the option of 200mm for heavy-duty or basement applications, and 300mm for high ground water risk environments. The membrane must also include a minimum 150mm upstand at walls, linking directly to the DPC above to form a continuous barrier.

Which DPM Types Are Available and When Should You Use Them?

Choosing the correct DPM depends on ground moisture risk, floor construction type, structural loading, and whether the application is new build or remedial. The following types are commonly specified in UK construction:

Materials such as polyethylene sheets are cheap, flexible, and widely used. Bituminous membranes are robust and often specified for harsh conditions, while liquid-applied membranes offer seamless coverage for complex shapes.

How Do You Install a Polythene Sheet DPM Correctly?

Unless the application surface is smooth, it should be blinded with compacted soft sand to guarantee a soft bed, free from any objects that may puncture the membrane during installation or when concrete is applied. This preparation step is critical to avoid perforation and ensure long-term performance.

When joining two sheets, provide a minimum overlap of 150mm between each sheet. For the best waterproof protection when making a join, form a double welted fold between the joining sheets with an overlap of 300mm from both sheets. Alternatively, seal the 150mm overlap with 100mm wide jointing tape. Taped joints should be self-adhesive or DPM tape compliant with BS 8102.

Any punctures in the membrane should be patched with sheets of identical thickness, lapped at least 150mm away from the perforation edge and sealed with double-sided pressure-sensitive tape. It is recommended that the DPM be covered with a protective layer or screed as soon as possible after installation to avoid stretching or displacement.

What Are the Key Installation Checks for Wall Upstands and Continuity?

When installed within structures, the DPM should be linked to the DPC within the structure walls to form a continuous damp proof barrier throughout the building. The minimum upstand at walls is 150mm, which joins to the DPC above. This continuity is essential to prevent moisture ingress at the junction between floor and wall.

Care should be taken when applying concrete or screed on top of the DPM to avoid stretching or the DPM being displaced. For waterproofing in basement applications, consider using cavity drain membranes in combination with appropriate drainage systems, especially where ground water risk is high.

When Is a DPM Not Suitable?

Standard polythene DPMs are not recommended for use as a general gas-proof barrier system on brownfield sites or in applications where gases such as radon are present. In these environments, a gas-resistant membrane meeting BRE 211 specifications must be specified instead.

Proper sealing at joints and laps cannot be overemphasized. Installation must be continuous, with thickness typically ranging from 0.2 to 1.0 mm depending on exposure. Compliance with BS 6515 (UK) or local building codes is mandatory to ensure the membrane performs as intended over the life of the structure.

Practical take-away: Always verify the ground moisture risk and application type before specifying your DPM. Use the minimum 1200 gauge for standard domestic floors, increase to 2500 gauge for commercial or high-risk sites, and ensure all overlaps and upstands meet the 150mm minimum. Protect the membrane immediately after laying to avoid damage during concrete or screed placement.