Damp Proof Membrane (DPM): The Sheet Under Your Floor That Prevents Rising Damp

A three-year-old extension develops damp patches along the base of the walls. The surveyor opens up the skirting boards and finds the floor membrane was never connected to the wall damp course. Moisture has been wicking up through that gap since the day the slab was poured. The fix requires lifting the floor finish, removing perimeter insulation, and retrofitting new membrane sections with butyl tape. Cost: several thousand pounds. The original membrane cost about £60.

That's what a damp proof membrane failure looks like. Not a dramatic flood, just a slow, invisible moisture path that reveals itself years later. Getting the DPM right during groundwork is straightforward, but the details matter enormously.

What it is and what it's for

A damp proof membrane (DPM) is a heavy-duty polythene sheet laid under your extension's floor slab to stop ground moisture rising into the building. The ground beneath any UK building is permanently damp. Without a DPM, that moisture migrates upward through the concrete by capillary action, saturating insulation, rotting timber, and creating the conditions for mould.

Building regulations require it. Approved Document C, Requirement C2 (Resistance to Moisture), mandates that ground-supported floors "resist the passage of ground moisture" via a DPM that is "continuous with the damp proof courses in walls." The minimum specification is 1200 gauge (300 micron) polythene to BS EN 13967.

The DPM also doubles as a radon barrier in most domestic ground floors. If your extension is in a radon-affected area (parts of Cornwall, Devon, Somerset, Derbyshire, Northamptonshire, and mid-Wales), the membrane is doing two jobs at once. Any gap or poor junction isn't just a moisture problem, it's a radon compliance failure too.

DPC vs DPM: two different things

These terms get confused constantly, even by some builders. A damp proof course (DPC) is a thin horizontal strip embedded in the mortar bed of a wall, typically 150mm above external ground level. It stops moisture rising through the blockwork. A DPM is a broad sheet laid across the entire floor area. It stops moisture rising through the slab.

The critical detail: the DPM must connect to the DPC. The membrane turns up at the perimeter wall and laps onto the DPC strip, creating a continuous moisture barrier from floor to wall. Break that connection and moisture bypasses both barriers at the junction. This is the single most common DPM failure mode, mentioned in six out of eight community forum threads on the topic.

Types and specifications

For a new extension floor slab, you need sheet DPM. Liquid-applied DPM exists but it's for retrofit situations (existing floors in older buildings) where a sheet can't be laid.

The most important row in that table is the fourth one. A typical underfloor heating (UFH) floor build-up has TWO separate polythene layers: the actual DPM at 1200 gauge below the slab, and a thin 500 gauge slip layer above the insulation to stop PIR foil reacting with wet screed. These are not the same thing. Forum threads regularly show homeowners panicking that UFH clips have "pierced the DPM." They haven't. The clips pierce only the thin upper slip layer. The real DPM is safely buried under 150mm of concrete.

Standard roll sizes

The standard roll for domestic work is 4m wide by 25m long, giving 100m² of coverage. This is the most common size stocked by builders' merchants and DIY retailers. Smaller rolls (4m x 15m = 60m²) exist but are less common and worse value per square metre.

The 4m width is deliberate. Most room dimensions fall within 4m in at least one direction, so a single sheet width often covers the narrower dimension of an extension with no longitudinal joints needed.

The correct floor build-up

This is the sequence your groundworker should follow, and the order building control expects to see. From bottom to top:

1. Compacted hardcore (MOT Type 1 or similar) - the structural sub-base
2. 50mm sand blinding - a smooth layer protecting the DPM from sharp aggregate
3. 1200 gauge DPM - laid flat with 150mm overlaps taped and 150mm+ upstands at all perimeter walls
4. Concrete slab (typically 150mm of C25/30)
5. PIR insulation (Celotex, Kingspan, or equivalent - thickness per building regs)
6. 500 gauge polythene slip layer - prevents PIR foil reacting with screed
7. Screed (sand-and-cement or liquid) - the finished floor surface

Click to enlarge

The DPM goes below the slab and below the insulation. Approved Document C technically permits the DPM above the slab as an alternative, but the industry consensus is strongly against it for heated floors. If your DPM were above the concrete, your underfloor heating would spend energy warming a large damp lump of concrete underneath. PIR insulation must sit above the DPM because moisture degrades PIR performance over time.

How to work with it

DPM is straightforward to handle but unforgiving of shortcuts. It arrives as a heavy roll (a 4m x 25m roll weighs roughly 25-30kg) and stores easily in a dry area on site.

Laying the membrane

The sand blinding must be compacted and smooth before any membrane goes down. Sharp stones protruding through the blinding will puncture the DPM during the concrete pour when tonnes of wet concrete press down on it. Run your hand across the blinding surface. If you can feel anything sharp, add more sand and compact again.

Unroll the DPM across the blinded area. Where sheets overlap (if your extension is wider than 4m in both directions), maintain a minimum 150mm overlap. Tape every overlap with butyl DPM jointing tape, not duct tape, not packaging tape, not gaffer tape. Butyl tape is a permanently adhesive, waterproof tape specifically designed for this. Standard tapes degrade underground within months. Your building control inspector will check what tape was used.

At all perimeter walls, turn the DPM upward by at least 150mm. Experienced builders leave 200mm because the regulatory minimum frequently gets damaged during the pour. Tape the upstand to the wall DPC using the same butyl tape. This creates the continuous moisture barrier that Approved Document C requires.

What building control look for

The pre-pour inspection is one of the mandatory hold points in the building control sequence. The inspector checks:

• Membrane gauge (1200 minimum for residential)
• Joint overlaps (150mm minimum, taped)
• Perimeter upstands (150mm minimum, connected to DPC)
• No punctures or tears
• Blinding layer smooth and adequate
• Insulation position and type (if being placed before the pour)

The LABC confirms this as a mandatory inspection point. The membrane cannot be retrospectively verified after it's been covered by concrete. This is why building control insist on seeing it before the pour, and why skipping the notification is such a serious error.

Protecting the membrane during construction

The period between laying the DPM and pouring the concrete is when damage happens. Wheelbarrows, reinforcement mesh being dragged across the surface, scaffold boots, dropped tools. If the pour isn't happening the same day the membrane goes down, cover it with spare polythene or plywood sheets to prevent punctures.

When placing mesh reinforcement, use proprietary plastic spacers (chairs) to hold the mesh off the DPM. Don't drag mesh sheets across the membrane. Lift and place them.

How much do you need

Worked example: 4m x 6m single-storey rear extension

Floor area: 24m². A single 4m x 25m roll (100m²) covers this with plenty to spare. You'll use roughly 30-32m² of membrane once you account for the 150mm perimeter upstands on all four sides.

For a larger extension (say 5m x 8m = 40m²), you still need only one roll but you'll have a longitudinal joint where two sheet widths overlap. That overlap consumes an extra 0.6-0.8m² of membrane.

One roll of 100m² covers most single-storey domestic extensions with waste left over. Two rolls is overkill unless your extension exceeds about 70m² of floor area.

DPM tape: You need enough to tape all sheet joints plus the full perimeter where the DPM meets the wall. For a 4m x 6m extension with no longitudinal joint, that's roughly 20m of perimeter taping. A 10m roll of butyl tape covers half of that. Budget for 2-3 rolls to be safe, especially since wasting tape on a bad run is common the first time you use it.

Cost and where to buy

The standard product is a 1200 gauge (300 micron) black polythene DPM, 4m x 25m, BBA certified. Prices vary considerably between DIY retailers and trade suppliers.

Screwfix, Toolstation, and Wickes all stock 1200 gauge DPM at DIY retail prices. Trade online suppliers like Stone Builders Merchants, FastBuildSupplies, and Permagard sell the same specification at the lower end of the range. The price difference between DIY retail and trade is substantial enough to justify a trip to your local builders' merchant.

The Visqueen brand (now part of the BMI Group) is the name most builders use generically, much like "Hoover" for vacuum cleaners. There's no meaningful performance difference between Visqueen and unbranded BBA-certified rolls at the same gauge. Both meet BS EN 13967.

DPM tape is an essential consumable that gets forgotten. Budget for 2-3 rolls per extension floor at £10-15 each. Using the wrong tape (duct tape, packaging tape) is a building control failure and a long-term moisture risk.

Total DPM cost for a typical extension

For a 4m x 6m extension: one roll of DPM plus 2-3 rolls of butyl tape. Total material cost is typically under £120, even buying from a DIY retailer. Even at the expensive end, that's trivial compared to the cost of remedial damp work.

Buy from a builders' merchant if you can. The trade price for the same product is 30-40% cheaper than DIY retailers. Travis Perkins, Jewson, and independent merchants all carry 1200 gauge DPM. You don't need a trade account for a one-off purchase; most merchants will sell to anyone at the counter.

Radon areas: when 1200 gauge isn't enough

In parts of England and Wales with elevated radon risk, the standard 1200 gauge DPM isn't sufficient. Radon is a naturally occurring radioactive gas that seeps from certain rock types and can accumulate in buildings.

The UK Health Security Agency publishes radon probability maps by postcode. Check yours before your build starts.

• Less than 3% probability: No additional radon protection needed. Standard 1200 gauge DPM is sufficient.
• 3-10% probability: Basic radon protection. 1200 gauge DPM with all joints sealed to DPC. This is what you'd do anyway for moisture protection, but the sealing must be airtight, not just waterproof.
• Over 10% probability: Full radon protection. Minimum 2400 gauge (600 micron) membrane, sealed at all joints and service penetrations, plus provision for sub-floor depressurisation (a sump and pipe run to the roof that can be connected to a fan if post-build testing shows elevated radon). Your structural engineer or BCO will specify this.

The highest-risk counties are Cornwall, Devon, Somerset, Derbyshire, and Northamptonshire. If your extension is in one of these areas, expect building control to scrutinise the DPM and its connections more closely.

Alternatives

Liquid-applied DPM is the main alternative to sheet DPM. It's painted or rolled onto an existing concrete floor in two or three coats. For new extension work, sheet DPM is the right choice: it's cheaper, faster, more reliable, and building control prefer it. Liquid DPM comes into its own for retrofit work on existing floors, garage conversions where you're working with an existing slab, and complex floor shapes where cutting and taping sheet becomes impractical.

Self-adhesive DPM exists as a premium product. It has a peel-and-stick backing that eliminates the need for separate jointing tape at overlaps. It costs roughly double the price of standard sheet DPM. The convenience rarely justifies the cost for a straightforward extension floor where standard sheet with butyl tape works perfectly well.

Where you'll need this

• Foundations and Footings - 1200 gauge DPM laid under or within the floor slab, joints overlapped and taped
• Damp Proof Course - DPC strip in walls connects to floor DPM at the wall/floor junction
• Windows and Doors - DPC tray and cavity closers around openings to prevent moisture bridging
• Skip Hire and Site Setup - heavy-duty polythene sheeting doubles as ground protection and temporary material covers

Common mistakes

Not connecting the DPM to the wall DPC. This is the number one failure. The membrane turns up at the wall and must be sealed to the DPC with butyl tape. Without this connection, ground moisture bypasses the DPM entirely at the wall/floor junction. Building defect investigators consistently identify this gap as the root cause of rising damp in new extensions. One real case: a three-year-old extension with full building control sign-off developed rising damp because the DPM was never lapped to the DPC. Calcium carbide testing confirmed moisture readings above 20%.

Cutting the DPM flush with the top of the slab. If the membrane is trimmed level with the concrete instead of being left with a 150mm upstand, there's nothing to lap onto the DPC later. The fix involves removing perimeter insulation to expose the membrane edge and bonding new DPM sections with double-sided butyl tape. Expensive and disruptive.

Using the wrong tape. Duct tape, gaffer tape, and packaging tape are not waterproof long-term. They deteriorate underground within months. The BCO will check for butyl DPM jointing tape (or manufacturer-specified equivalent). Using the wrong tape is a failed inspection and a rework.

Skipping the sand blinding. Laying the DPM directly onto compacted hardcore means sharp aggregate is in direct contact with the membrane. During the concrete pour, the weight of wet concrete (2,400kg per cubic metre) presses the membrane down onto those sharp edges. Punctures are invisible once the concrete is poured and unrepairable without demolition.

Confusing the DPM with the slip layer. If your builder says "the DPM goes above the insulation," they may be talking about the thin 500 gauge slip layer, not the actual moisture barrier. The real DPM (1200 gauge minimum) goes below the slab. The slip layer above the insulation is there to separate materials, not to stop moisture. Getting this distinction wrong means your floor has no effective moisture protection.