Damp Proof Membrane Installation: Ground Floors and Substructure

Quick Answer: A damp proof membrane (DPM) must be installed in all new ground-bearing concrete floors under Approved Document C. The minimum specification is 1200-gauge (300 micron) polythene, lapped minimum 150mm at joints and sealed. The DPM must link to the damp proof course (DPC) in adjacent walls — a DPM isolated from the wall DPC leaves a path for moisture ingress at the wall/floor junction.

Summary

A damp proof membrane does one thing: stops ground moisture rising through a concrete floor slab. Without it, water vapour from the ground migrates upward through the concrete by capillary action, appearing as dampness, efflorescence, or floor covering failure. In older houses without a DPM, damp floors are one of the most common and expensive problems homeowners encounter.

The physical installation is straightforward — polythene sheet under a concrete slab. The complications arise at edges (linking to wall DPC), at penetrations (pipes, cable ducts), and when specifying for aggressive ground conditions (radon gas zones, contaminated land, high groundwater). A domestic extension or new build with a correctly installed DPM costs virtually nothing extra to get right; retrofit DPM installation to an existing slab can cost thousands.

For tradespeople, understanding DPM specification is particularly important because it sits at the interface between groundworks, brickwork, and insulation. The DPM is installed by the groundworker but must coordinate with the bricklayer's DPC and the insulation specification. If the coordination fails, moisture enters the building.

Key Facts

Quick Reference Table — DPM Specification by Application

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Application Minimum Spec Notes
Standard domestic ground floor 1200-gauge (300 micron) BS 6515; laid below slab or above slab below screed
Areas with heavy floor traffic (commercial) 1200–2500 gauge Higher gauge resists mechanical damage during pour
Radon affected area (Level 1) 1200-gauge + sealed joints Check UKHSA radon map for area classification
Radon affected area (Level 2+) 2500-gauge (625 micron) or above Gas-barrier grade; all joints heat-welded or tape-sealed
Retrofit / overlay on existing slab Liquid DPM or sheet DPM + screed Check for structural adequacy of existing slab
Contaminated land Specialist barrier membrane Hydrocarbon/chemical resistant; consult specialist
Below timber ground floor DPM on oversite concrete Different from suspended floor DPM — see notes below

Detailed Guidance

Position of the DPM in the Floor Build-Up

Two main positions:

Below the structural concrete slab: Build-up (top to bottom):

In this position, the DPM is placed on the compacted sub-base before the concrete pour. Lay blinding (50mm of lean mix or sand) over the hardcore first to protect the DPM from puncture by sharp aggregate. Take care not to puncture the DPM during the concrete pour — avoid scraping rake handles across it.

Above the structural concrete slab (between slab and insulation or screed): Build-up (top to bottom):

In this position, the slab cures without the DPM present, which can improve strength slightly. The DPM is protected from mechanical damage during the pour. However, the slab itself remains damp — this is acceptable for structural purposes but ensure the concrete specification accounts for the damp environment (minimum GEN3).

The position affects the insulation strategy. With DPM below slab, insulation goes above the DPM and below the screed (warm insulation). With DPM above slab, insulation is sandwiched between two sheets of concrete/screed and must be compressive-load-rated PIR (not mineral wool batts).

Linking the DPM to the Wall DPC

This is the critical junction. The DPM in the floor must be continuous with the DPC in the perimeter wall — otherwise moisture can rise up through the wall/floor joint and enter the building at the perimeter, showing as damp patches at the base of walls or under skirting boards.

Method 1 — turn-up and lap: Turn the DPM up the inside face of the masonry wall by a minimum of 150mm. The bricklayer laps the bitumen felt DPC in the wall over the upturned DPM edge. If the DPC is at a lower level than the DPM height up the wall, the lap can be bitumen tape rather than mechanical.

Method 2 — cavity tray: In cavity wall construction, the cavity tray installed above the floor level links to the DPM. The bricklayer installs the tray; the groundworker tapes the DPM to the vertical face of the inner leaf and laps to the cavity tray position. Confirm the sequence with the bricklayer before pouring the slab.

Key rule: The DPM and DPC must form a continuous impermeable barrier. No gap, no matter how small. Any discontinuity allows moisture ingress.

Installation Step by Step

  1. Prepare the sub-base — compact 150mm hardcore (MOT Type 1 or clean crushed concrete), remove any sharp protrusions; blind with 50mm sand or lean mix concrete
  2. Lay the DPM — unroll on the blinded surface with minimum 150mm overlaps; ensure sufficient material to turn up all walls by at least 150mm; avoid punctures
  3. Seal all laps — use DPM jointing tape (self-adhesive butyl tape minimum 75mm wide); press firmly; check every joint
  4. Seal all penetrations — where pipes, ducts, or cables pass through, use proprietary pipe collar seals; wrap tape tightly around the pipe and seal to the DPM surface on both sides
  5. Turn up at walls — fold the membrane up against the masonry by 150mm minimum; temporarily fix with tape or push against block; do not puncture with nails
  6. Place reinforcement and pour concrete — keep heavy boots and machinery clear of the DPM; use walkboards to distribute load during pour; check the DPM is not displaced before final pour
  7. After pour — trim the upstand to finished floor level height; ensure the bricklayer links the wall DPC to the trimmed DPM before first fix begins

Liquid DPM for Retrofit

Where an existing concrete slab has no DPM (common in pre-1970s houses), liquid DPM can be applied to the top surface:

  1. The slab surface must be sound, clean, and dust-free
  2. Prime with the product-specific primer coat
  3. Apply liquid DPM in two coats at manufacturer's specified coverage rates
  4. Allow to cure per manufacturer's data sheet before applying screed or floor finish
  5. Joints in the substrate are bridged only by flexible liquid DPM — if the slab has active cracks, these must be routed, filled with flexible filler, and reinforced with glass scrim tape before applying the DPM

Liquid DPMs are not suitable for free-standing water or positive water pressure. For basements with hydrostatic pressure, a cavity drain membrane system (Type C waterproofing) is required — see basement tanking and waterproofing systems.

Radon Barrier Specification

Radon is a naturally occurring radioactive gas produced by uranium decay in rock and soil. It enters buildings through the floor. The UK Health Security Agency (UKHSA) publishes a radon map — check the site postcode to determine the radon protective measures class.

In radon areas above 10% probability (Level 2), the DPM becomes a critical component of the building's health strategy. Do not cut corners.

Frequently Asked Questions

What's the difference between DPC and DPM?

A damp proof course (DPC) is a horizontal barrier in the wall (typically at 150mm above finished external ground level) that prevents moisture rising through the brickwork. A damp proof membrane (DPM) is the barrier in the floor preventing moisture rising through the concrete slab. They are different products in different locations, but they must be connected to form a continuous barrier around the building perimeter.

Can I use the same polythene for the DPM and as temporary sheet protection?

No — construction polythene (e.g. 250-gauge skip covers) is too thin and will be punctured during the concrete pour. Use purpose-made 1200-gauge DPM polythene (sold in rolls, typically 4m wide). In an emergency, 1000-gauge is acceptable for non-radon applications. Never accept anything thinner on a building subject to Building Regulations inspection.

How do I stop the concrete damaging the DPM during the pour?

Use blinding concrete (50mm GEN0 lean mix) or sand blinding over the hardcore before laying the DPM. During the pour, lay scaffold boards or polystyrene offcuts across the DPM as walkways; avoid dragging rakes and vibrators across the surface; pour concrete from the far end and work backwards to avoid foot traffic on the DPM.

Do I need a DPM under a garage floor?

Building Regulations require a DPM in any habitable floor. A garage is not typically habitable, but if it is to be used as a workshop, home gym, or utility space (and particularly if it is attached to the dwelling), a DPM is advisable. Building Control will advise in each case. If there is any chance of future conversion to habitable use, install the DPM now — retrofitting later is expensive.

Regulations & Standards