DPM Installation Under Concrete Floors: BS 8102:2022 Guide

Quick Answer: A Damp Proof Membrane (DPM) is required under all new ground-bearing concrete floors to prevent ground moisture and water vapour rising into the building. Approved Document C2 / Section 4 of the Building Regulations requires resistance to moisture, met by 1200 gauge (300 micron) polyethylene sheet to BS 8102:2022 and BS EN 13967, lapped 150mm minimum at joints, with all joints sealed using DPM tape or welt-folded. The DPM must lap into the wall DPC to form a continuous moisture barrier around the building footprint.

Summary

Ground moisture moves upward by capillary action and as vapour. Without a barrier it migrates through concrete (concrete is permeable to vapour), into floor finishes, and ultimately into the room atmosphere. The result is rotted timber floor finishes, lifted vinyl, mould on the back of carpets, and rising humidity that condenses on walls.

A DPM is a continuous sheet that blocks both liquid water and water vapour. The standard UK material is 1200 gauge (300μm) polyethylene, supplied in rolls 4m wide × 25m long. Heavier grades (1500 / 2000 gauge) and specialist materials (radon barrier, gas-resistant) are used where ground conditions demand.

The membrane sits either above the concrete slab (sandwich construction, traditional) or below the slab (modern preference). Both are acceptable; below-slab is more durable, isolates the slab from ground moisture, and is the NHBC default for new build.

Key Facts

Quick Reference Table

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Application Membrane Type Thickness Notes
Standard new build ground floor Polyethylene DPM 1200 gauge (300μm) Below slab, lap up to wall DPC
Sandwich construction (old) Polyethylene DPM 1200 gauge Above slab, below screed
Radon protection (Class 1 area) Gas-resistant DPM 400μm gas barrier BS 8485:2015 spec
Methane / hydrocarbon ground Multi-layer gas barrier Manufacturer spec NHBC Standards Chapter 4.1
Domestic garage floor Polyethylene DPM 1000 gauge minimum Less critical; still required
Heated screed (UFH) Polyethylene DPM 1200 gauge Below insulation; above slab if sandwich
Existing slab retrofit Liquid epoxy DPM 2 coats per spec Where sheet cannot be installed
Tanking — below ground Bituminous / cementitious tanking Multi-coat BS 8102 Grade 3
Cavity wall DPC interface DPC 150 gauge polyethylene Minimum 100mm wide Lapped with floor DPM

Detailed Guidance

Build-up sequence — below-slab DPM (modern)

Typical Ground Floor Build-up (Below-Slab DPM)
1. Subsoil — compacted to bearing strata
2. Hardcore — 100-150mm Type 1 sub-base, compacted in layers
3. Sand blinding — 50mm clean sharp sand or pea gravel
4. DPM — 1200 gauge polyethylene; lap 150mm at joints; tape sealed
5. Insulation — PIR/EPS to Part L requirement (typical 100-150mm)
6. Slip layer — separating sheet (often integral to DPM detail)
7. Concrete slab — RC25/30 designed mix, typically 100-150mm
8. Screed (if separate) — typically 65-75mm sand/cement; or anhydrite
9. Floor finish — tile, vinyl, timber, carpet

Build-up sequence — above-slab DPM (sandwich)

Sandwich Build-up (Older Spec / Retrofit)
1. Subsoil compacted
2. Hardcore + sand blinding (as above)
3. Concrete slab — direct on blinding (no DPM yet)
4. Cure slab 28 days
5. DPM — 1200 gauge polyethylene laid on slab
6. Insulation — XPS or PIR
7. Screed — bonded or unbonded
8. Floor finish

Lap and seal

Every joint must be sealed. Two standard methods:

  1. Welted joint (dry) — fold both edges 150mm together, then fold over once more (z-fold). Provides mechanical seal but not vapour-tight.
  2. Taped joint (preferred) — 150mm overlap, both surfaces clean and dry, double-sided DPM tape applied, second sheet pressed firmly onto tape. Roll with a sealing roller to expel air.

Where the DPM is to act as a radon/gas barrier, taped joints are mandatory and the tape must be from the same manufacturer as the membrane for compatibility.

Wall DPC interface

The DPM must connect to the horizontal wall DPC to form a continuous barrier. Three common details:

The interface detail is shown on the architect's drawing and inspected by Building Control.

Penetrations

Every penetration through the DPM is a potential leak path. Common ones:

The treatment:

  1. Cut DPM in a cross pattern around the pipe
  2. Fold the flaps up the pipe
  3. Tape or use a proprietary collar/flange to seal
  4. For gas barrier installations, use top-hat or boot-shaped pre-formed seals (e.g. Visqueen)

Insulation position

Building Regulations Part L requires floor U-value of ≤0.18 W/m²K for new build (refer to current ADL). This is typically achieved with 100–150mm PIR insulation. The position of the DPM relative to insulation matters:

Manufacturer instructions for the specific insulation product (e.g. Celotex GA4000, Kingspan TF70) state acceptable interface details. Some PIR insulations require a separation layer between DPM and insulation to prevent thermal damage.

Common errors

Error Consequence Fix
DPM punctured by sharp hardcore Localised damp patches at finish stage Sand blinding course before DPM; tape patch any damage
Joints not lapped 150mm Damp tracking through joint Re-lay; never less than 150mm
Joints not taped Vapour migration; failure under UFH Always tape; specify tape on the order
No wall lap Damp rising at floor/wall junction Strip back DPM at perimeter, dress up wall, tape to DPC
DPM torn by reinforcement Localised failure under load Patch with tape; place chairs on flat plates
DPM laid in folds/wrinkles Trapped air → blistering of finish above Lay flat; smooth with brush before slab
Wrong-side-up DPM Some printed membranes have a directional grain Check manufacturer print; usually "this side up" arrow

Liquid-applied epoxy DPM (retrofit)

Where a sheet DPM cannot be installed (refurbishment, existing slab without membrane), a liquid epoxy DPM is brushed or rolled onto the slab surface. Typical products: F. Ball F76, ARDIT P51 PRO. Applied in two coats at right angles, 0.4–0.6 kg/m² per coat. Cure 24h between coats, 72h before tiling/screeding. The slab must be clean, dry to ≤6% moisture content, and dust-free.

Radon and ground gas

UK areas with significant radon (parts of Cornwall, Devon, Derbyshire, Northamptonshire, Northern Scotland) require gas-resistant membranes plus a sub-floor sump system or passive vent. Determination is via UKHSA Indicative Atlas of Radon and the regional building regs maps. Two basic levels:

BS 8485:2015 covers ground gas protection for hydrocarbon (former petrol station, landfill) and methane (organic ground) sites.

Frequently Asked Questions

Can I use builder's polythene as a DPM?

No — general-purpose polythene is too thin and contains recycled material with pinholes. Always specify 1200 gauge (300μm) DPM polyethylene to BS 8102 / BS EN 13967, or thicker for radon / gas barrier. The membrane should be marked with its gauge and standard reference.

What about a power-floated concrete slab — does it still need a DPM?

Yes. Power-floating reduces surface porosity but does not eliminate moisture migration. A power-floated slab without DPM will still transmit ground moisture into floor finishes. Some power-trowel finishes incorporate sodium silicate hardeners which slightly reduce permeability, but they are not a DPM substitute.

Can I skip the DPM if the ground is bone-dry?

No. Ground that is dry at the time of construction may not stay so — water table changes, leaking drains, surface runoff during the life of the building all introduce moisture. Building Regulations require the DPM regardless of ground condition at the time of construction.

Do I need a DPM with insulated foundation systems like Kingspan TF70?

The insulation product itself does not replace a DPM. The standard build-up still includes a polyethylene DPM either below or above the insulation. Manufacturer technical guidance shows the correct layering — follow it.

Can I tile directly onto the DPM?

No — never. DPM is a vapour barrier, not a substrate. Tiles, screed, or slab go on top of the DPM in the build-up. Bonding tile adhesive to polyethylene does not work and the tiles will lift.

Regulations & Standards