Damp Proof Membrane (DPM) Guide: 1200g, Sheet vs Liquid & How to Install

What is a damp proof membrane (DPM)?

A damp proof membrane is a large impermeable sheet — almost always heavy-duty black polythene — laid across the ground beneath or within a solid concrete floor. Its job is to stop moisture from the ground rising up through the floor and into the building.

The ground beneath any building is damp. Water moves upwards through hardcore and concrete by capillary action, and without a barrier it keeps going until it reaches the surface, where it damages floor finishes, causes musty smells, lifts vinyl and timber, and creates the conditions for mould.

Critically, the DPM in the floor has to connect to the DPC in the walls. The damp proof course is the horizontal barrier built into the masonry, usually a couple of courses above ground level. If the floor DPM and the wall DPC are not overlapped and sealed to each other, a gap is left at the perimeter and moisture simply bypasses both barriers. That junction is the most commonly botched detail in domestic floor construction.

A properly installed DPM also does a second job on modern floors: it stops water in freshly poured concrete draining away downwards, which helps the slab cure properly.

What does 1200 gauge / 300mu mean?

Polythene sheet is described two ways in the UK, and both refer to the same thing: thickness.

Gauge is the older imperial-derived measure. Micron (mu, or μm) is the metric measure. The conversion is simple — divide the gauge by four to get microns.

1200 gauge (300 micron) is the figure that matters. It is the industry standard thickness for underfloor damp proofing, and it is what UK building standards expect for that application. Anything thinner risks puncture during construction and non-compliance on inspection.

Using a thinner membrane to save money is a false economy in the clearest sense. The membrane will be walked on, have hardcore under it, and have concrete poured over it. A 1000 gauge sheet will puncture where a 1200 gauge sheet survives, and once the slab is down there is no repairing it.

Sheet DPM vs liquid DPM — which is best?

For a new floor, sheet DPM is the standard choice and the right one. It is cheap, quick to lay over a large area, and forms a reliable barrier when lapped and taped properly.

Liquid DPM comes into its own on existing floors, where you cannot get a sheet underneath the slab, and around complex detailing — pipe penetrations, awkward internal angles, junctions with existing structures — where a sheet would be difficult to form correctly. It is also a component of damp proofing systems for existing internal walls.

The two are not mutually exclusive. A common approach on a new build is a sheet DPM under the slab with liquid DPM used to seal awkward penetrations and detail junctions where the sheet is hard to work.

Best DPM for a concrete floor

For a solid concrete ground floor in a UK home, extension or garage: 1200 gauge (300mu) black polythene sheet, laid loose, lapped at least 150mm at joints, sealed with a suitable tape, and dressed up the walls to overlap the DPC.

What to look for in the product:

• 1200 gauge / 300 micron thickness — the recommended standard for underfloor use
• CE or BBA approval — evidence the product meets the relevant standard
• Good puncture and tear resistance — it will be walked on and have concrete poured on it
• Multi-fold packaging — makes a large sheet manageable to position on site
• Manufactured from recycled polyethylene where possible — same performance, better environmental outcome

Our CE-approved 1200 gauge DPM is manufactured from 100% recycled polyethylene, is waterproof and rot proof, and is designed to last the lifetime of the floor construction provided it is correctly installed and not damaged by subsequent building operations. It comes in 4m × 4m (16m²) for small projects and repairs, 4m × 15m (60m²) for extensions, and 4m × 25m (100m²) for full floors.

One important limitation: standard DPM is not a gas barrier. If you are building on a brownfield site or anywhere radon or ground gases are a concern, you need a specified gas membrane, not a standard DPM.

Where DPM goes (above or below the slab)

There are three valid positions, and the right one depends on the floor build-up.

Below the slab (oversite membrane). The DPM is laid over blinded hardcore and the concrete is poured directly onto it. This is the most common arrangement for new floors. It protects the slab itself from ground moisture as well as the finish above.

Sandwich construction. The DPM sits between the base concrete and the screed. This protects the screed and finish but leaves the base slab exposed to ground moisture, so it is used where the slab is designed for it.

Above the slab, below the finish. Used mainly on existing floors and refurbishment, where a membrane is laid over the existing concrete before a new screed or finish goes down.

Whichever position you use, the connection to the wall DPC is what makes the system work. Dress the membrane up the perimeter walls and lap it with the DPC so the two form one continuous barrier around the whole floor. Trim it back only after the screed is down and the finish is going on.

The blinding layer matters too. Hardcore has sharp edges. A blinding layer of sand or fine material over the hardcore, before the membrane goes down, prevents punctures — and punctures are the single most common cause of DPM failure.

How to install and lap DPM

• Prepare and blind the base. Compact the hardcore, then blind it with sand or fine material so there are no sharp points to puncture the membrane.
• Plan your sheet layout. Work out the runs to give the fewest joints, and lay out the roll so it unfolds in the direction of the longest dimension.
• Unfold and position the sheet. Multi-fold sheets are designed to be positioned folded and then opened out. Avoid dragging the membrane across the base.
• Dress it up the walls. Take the membrane up the perimeter walls high enough to lap generously with the DPC, and leave it long — trim later.
• Lap all joints by at least 150mm, with the sheet on the higher side overlapping the one below.
• Tape every joint with a good quality tape suitable for polythene, pressed down along its full length.
• Seal around penetrations. Pipes, service entries and columns each need to be collared and taped. Liquid DPM is useful here.
• Lap and seal to the DPC at the perimeter so floor and wall barriers form a continuous envelope.
• Inspect before pouring. Walk the whole area and look for punctures, torn laps and lifted tape. Patch anything you find with an offcut and tape.
• Protect it immediately. Cover with screed or a protective layer as soon as possible after installation, and keep barrow traffic and boot traffic off it.

Two site-practice points from the product guidance worth repeating. Do not stretch or displace the membrane when placing concrete or screed over it. And make sufficient allowance at internal angles and around edges to avoid bridging — that is, leaving areas of unsupported membrane that will tear when loaded.

DPM is fully compatible with underfloor heating and suffers no adverse effect from it.

How to join DPM correctly

This is the detail that decides whether the membrane works, and it takes five minutes to get right.

Overlap at least 150mm. Some specifications call for more at critical junctions — check your drawings. Always lap so that water would run over the joint, not into it.

Tape the full length of every lap with a tape suitable for polythene. Ordinary parcel tape and general-purpose duct tape lose adhesion in damp, cold conditions, which is exactly the environment they will spend their life in. Press the tape down firmly along its whole length — a lightly applied tape lifts as soon as anything is dragged over it.

Keep the surfaces clean where you are taping. Dust and moisture on the polythene stop the tape bonding.

At the perimeter, tape or seal the membrane to the DPC. This is the joint that a surprising number of otherwise good floors fail at.

Frequently asked questions

What gauge DPM do I need?

1200 gauge (300 micron) for damp proofing under a concrete floor. That is the UK industry standard for the application and what building standards expect.

Is 1200 gauge DPM enough?

Yes, for standard domestic and commercial floor construction. Heavier 2000 gauge is available for specialist and heavy-duty applications. Note that standard DPM is not a gas barrier — brownfield or radon sites need a specified gas membrane.

Sheet or liquid DPM — which is better?

Sheet for new floors, where you can lay it under or within the slab. Liquid for existing floors, complex detailing and awkward penetrations. Many projects use both — sheet across the area, liquid to seal the difficult details.

Does DPM go above or below the slab?

Most commonly below, laid over blinded hardcore with the concrete poured onto it. It can also go in a sandwich between base concrete and screed, or above the slab on refurbishment work. In all cases it must lap and seal to the wall DPC.

How much overlap for DPM joints?

At least 150mm, taped along the full length of the lap, with the upper sheet overlapping the lower so water runs over the joint rather than into it.

Products mentioned in this guide

• 16m² Heavy Duty DPM 1200 Gauge 4m × 4m
• 60m² Heavy Duty DPM 1200 Gauge 4m × 15m
• 100m² DPM Black 1200g 300mu 25m × 4m
• General Building Supplies category

Related guides

• What Is a Vapour Control Layer (VCL)? (different membrane, different job)