DPM Jointing Tape: How to Seal Damp Proof Membrane Joints Properly

The bricklayers have laid your DPM, the building control officer is booked for tomorrow's pre-slab inspection, and your builder has just used a roll of silver duct tape from the back of his van to seal the joints. The BCO will reject this. The slab pour gets pushed back, the screed lorry arrives the day after for an empty site, you pay for the wasted load, and the screeder reschedules three weeks out because they're booked solid. All of this happens because the wrong tape went on a five-pound joint detail. Get the tape right and none of it does.

What it is and what it's for

DPM jointing tape is a self-adhesive tape designed to seal the overlapping joints between sheets of polythene damp proof membrane (DPM) so that the moisture barrier under your floor slab is continuous. The tape sits on the lap where two sheets meet. Without it, water vapour and liquid water can travel through the unsealed seam and rise into the floor build-up, eventually causing damp problems in the screed, the floor finish, and anything timber sitting on top.

A DPM is a sheet of heavy polythene (1200 gauge / 300 micron is the UK standard for ground floors per Approved Document C) laid over the sub-base before the slab is poured. Sheets come in 4m x 25m rolls and any extension floor wider than 4m needs at least one longitudinal joint. Joints must be lapped a minimum of 150mm and sealed. The tape is what does the sealing.

The tape is not optional. Approved Document C, the building regulations document covering moisture resistance, requires the floor membrane to be polyethylene of at least 300 micron with sealed joints. Untaped joints are a documented building control hold point. Your BCO will inspect the DPM before the slab is poured, and if joints aren't taped, the pour doesn't happen until they are.

There's a second job the tape does that's worth knowing. Where your property sits in a radon-affected postcode, the same DPM acts as a radon barrier as well as a moisture barrier. Radon is a colourless radioactive gas that seeps up through the ground in parts of the UK (predominantly the South West, the Midlands, and Northamptonshire). For radon, the tape requirement is stricter: 300mm overlaps and double taping, both inside the lap and over the seam edge.

The two-tape system

Most homeowners assume DPM tape is one product. It isn't. There are two distinct types, and competent installations use both:

Single-sided polythene tape. This is what most people picture when they hear "DPM tape." A roll of black polythene-faced tape with a strong rubber or acrylic adhesive on one side, typically 75mm wide and 33m long. It goes on top of the joint, covering the visible edge of the upper sheet and sticking it down to the lower sheet. Sometimes called "lap tape" or "seam tape."

Double-sided butyl tape. A short roll (50mm wide, usually 10m long) of thick black or blue butyl mastic with a release paper on each side. It goes inside the lap, between the two sheets of DPM, before the upper sheet is pressed down. Butyl is the same chemistry used in window glazing seals. It stays soft, conforms to surface irregularities, and gives a positive long-term bond between the sheets.

For standard domestic floor DPM with a 150mm overlap, single-sided tape alone over the seam edge is the routine spec and is accepted by most BCOs. For radon barriers, basements, or anywhere the DPM also acts as a gas barrier, the belt-and-braces approach is double-sided butyl inside the lap plus single-sided over the visible edge. This is what the manufacturers (Visqueen, Permagard, IKO) all specify in their approved jointing systems.

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Why aluminium foil tape is not a substitute

Walk into a builders merchant and you'll see two visually similar tape rolls on adjacent shelves: black polythene DPM tape and silver aluminium foil tape. They look interchangeable. They aren't. Using foil tape to seal DPM joints is a recurring mistake that fails inspection and produces compromised barriers.

Aluminium foil tape is designed for a completely different job: sealing the joints between PIR insulation boards (the foil-faced rigid insulation used above the DPM and below the screed). The adhesive is formulated to bond to the metallised foil facing on PIR, not to polyethylene sheet. The foil substrate also corrodes when in prolonged contact with moisture, which is exactly the condition your DPM tape needs to survive in. Buried under a slab, foil tape will degrade within years. A proper polythene DPM tape, in the same conditions, lasts the lifetime of the building.

The other tapes that get reached for in a panic and shouldn't be used: duct tape (cloth-backed, water-soluble adhesive, fails within months under a slab), masking tape (paper backing dissolves in damp), and packaging tape (the adhesive lets go below 10°C). BCOs have explicitly rejected duct tape on inspection. If you see any of these on your DPM, get the proper product before the BCO arrives.

For PIR board joints above the DPM, aluminium foil tape is correct. The two tapes do completely different jobs at different layers in the floor build-up. Buy both, label both, and tell your installer which one is for which.

How to apply it properly

Surface preparation determines whether the tape holds. Lime dust, plaster dust, sand grit, and standing water are the four enemies of tape adhesion. Even a freshly laid DPM picks up dust through static electricity within minutes of being unrolled. Tape laid on contaminated surfaces lifts within days.

The application sequence for a standard floor DPM lap:

1. Lay the lower sheet flat. Roll out the first DPM sheet so it sits flat on the sub-base with no creases or folds along what will become the lap edge. Creases create pinhole leaks once the tape goes over them.

2. Position the upper sheet. Roll out the second sheet with a minimum 150mm overlap (300mm in radon zones). Mark the lap edge with chalk or a marker so you can track the overlap line through the lap.

3. Clean both surfaces. Wipe a 200mm strip on each side of the joint with a clean dry rag. For a guaranteed bond, especially where the surface has been walked on, use isopropyl alcohol wipes. Let any moisture flash off. Do not tape over a wet or damp surface.

4. For the two-tape system, apply double-sided butyl first. Roll out the double-sided tape inside the lap, around 50mm back from the leading edge of the upper sheet. Press it down onto the lower sheet. Peel the release paper. Lay the upper sheet over the exposed butyl and press down firmly with a roller or your hand along the full length of the tape.

5. Apply single-sided tape over the seam edge. Roll the single-sided polythene tape along the visible edge of the upper sheet so it covers the seam line and bridges onto the lower sheet. Aim to centre the tape on the seam: roughly 30-40mm of tape on the upper sheet, 30-40mm on the lower sheet (with 75mm tape).

6. Burnish the tape. Run a hand roller or the heel of your hand firmly along the full length of the tape. Adhesion is pressure-sensitive: under-pressed tape fails. The whole length needs to be properly bonded, not just the start and the middle.

7. Inspect. Walk the line. Look for lifted edges, creases under the tape, or sections where the tape has folded back on itself. Re-press anywhere that looks loose.

For pipe penetrations and corners, cut shorter strips and lay them in overlapping patches rather than trying to wrap a single piece around a complex shape. Each patch should overlap the previous patch by 50mm.

Cold weather kills adhesion

This is the single most common practical failure with DPM tape and it comes up in five out of nine UK forum threads on the subject. Below 5°C the adhesive on both single-sided polythene tape and double-sided butyl tape hardens to the point that it stops bonding. Tape applied in cold conditions either doesn't stick at all or appears to stick and then peels off within hours.

Visqueen, the dominant UK brand, specifies a minimum application temperature of 5°C for all their DPM products and tapes. Permagard and IKO state the same. The number is not negotiable: it's the threshold below which the adhesive chemistry fails.

Cold weather rules:

• Don't apply DPM tape below 5°C. If your forecast for the morning is 3°C, schedule the DPM and tape work for the afternoon.
• Store the rolls overnight in a heated site cabin, in your van cab, or indoors. A roll left in an unheated garage in February reaches the same temperature as the air. A roll stored at 18°C overnight is workable for an hour or two outside before it cools.
• Warm the surface if you can. If the membrane and sub-base are below 5°C but the air is above, the tape will still fail because it bonds to the surface, not the air. Direct sun on the membrane for an hour helps.
• Don't use a heat gun. Hot adhesive on cold polythene melts the polythene and creates a permanent surface defect.

In practice this means avoiding DPM tape work between November and March on days that don't get above 5°C. If the schedule forces a winter pour, allow extra time for warming the materials and check every metre of tape afterwards.

Radon zones change the spec

Roughly a fifth of UK postcodes sit in radon-affected areas, with the highest probabilities in the South West (Devon, Cornwall, parts of Somerset), the Midlands, Northamptonshire, Derbyshire, and parts of Wales. The UK Health Security Agency publishes a radon map online showing the probability for any postcode in England, Wales, and Scotland. Your architect or building control officer will tell you whether your site needs radon protection.

In radon-affected zones, the DPM also functions as a radon barrier. The tape spec changes:

• Overlap minimum: 300mm (not 150mm).
• Tape system: double-sided butyl inside the lap AND single-sided polythene over the seam edge. Both, not one or the other.
• Membrane gauge: 2400 gauge (600 micron) where radon probability exceeds 10% per the UKHSA map. 1200 gauge is acceptable in the 3-10% probability band with sealed joints. Below 3% probability, no additional radon measures are required.
• Service penetrations: dedicated radon collars, not improvised tape wraps.

The BCO will inspect to a higher standard in radon zones. Untaped joints, single-tape installations, or under-lapped seams will be rejected outright. The cost difference between standard and radon spec is small (an extra roll of butyl tape per joint, perhaps 50% more tape overall) but the installation discipline is stricter.

How much do you need

Tape consumption is proportional to the total length of joints in your DPM, not the floor area. For a typical single-storey extension:

• A 4m x 25m DPM roll covers 100m². Most extensions under 16m² floor area need only one roll with no longitudinal joint, just a perimeter upstand on each wall.
• Extensions over 4m wide in either direction need at least one longitudinal joint. A 5m x 6m floor needs one full-length joint of around 6m.
• Multi-storey or larger plans may have two or more joints.

For each metre of joint, a single-sided 75mm tape roll (33m long) covers around 30 metres of joint after allowing for cut waste and patching. One roll handles most domestic extensions with one or two longitudinal joints.

For radon installations using both tapes:

• One 75mm x 33m single-sided roll covers around 30m of joint over the seam.
• One 50mm x 10m double-sided butyl roll covers around 9m of joint inside the lap.
• A typical 30m² extension with two longitudinal joints needs one roll of each.

Always have a spare roll on site for repairs and pipe penetration patches. The trade rule of thumb: never start a DPM install with the exact roll count, always carry one extra of each.

Cost and where to buy

Single-sided polythene jointing tape (75mm x 33m) costs £10 – £13 per roll. The 50mm version (cheaper, around six pounds per roll for Toolstation Ultratape) is fine for narrower laps but 75mm is the recommended width for full lap coverage and is what most professional installs use.

Double-sided butyl tape (50mm x 10m) costs £10 – £15 per roll. The wide range reflects the difference between standard tape (Permagard PermaSEAL, Visqueen blue, around nine to ten pounds) and radon-rated tape (Damplas Radon, Wickes around sixteen pounds). For a standard floor DPM use the cheaper end. For radon barriers buy the radon-rated product.

Where to buy:

• Screwfix and Toolstation: stock single-sided polythene tape (Everbuild, Ultratape brands). Click-and-collect within an hour. The simplest route for a homeowner needing one roll.
• Wickes and B&Q: stock both single-sided and double-sided. Damplas Radon is on Wickes shelves. Pricing is slightly higher than Screwfix/Toolstation.
• Travis Perkins, Jewson, Bradfords: carry the trade brands (Visqueen, Capital Valley Plastics). Best for ordering with a DPM roll on the same delivery.
• Insulation Merchant, Permagard, Rawlins (online): specialist suppliers carrying full Visqueen and PermaSEAL ranges. Best for radon-rated and butyl tapes.

Visqueen is the dominant UK brand and has BBA approval for the full DPM-and-tape system (membrane, single-sided lap tape, double-sided butyl, gas resistant lap tape). Where the BCO is strict (radon zones, gas-affected sites), buying the matched system is worth the small premium because the BBA certificate documents the joint specification. For standard domestic floor DPM, any of the major brands (Everbuild, IKO, Capital Valley, PermaSEAL, Visqueen) work fine.

Alternatives

There aren't many. The recurring confusion in the trade is whether something else can do the job, and the answer is consistently no for anything serving as a permanent moisture barrier under a slab.

Aluminium foil tape: for PIR insulation joints above the DPM, not for the DPM itself. Different adhesive, different substrate, corrodes if buried.

Render tape: mentioned on builder forums as "cheap and sticks like glue." It does stick aggressively but isn't moisture-rated for permanent buried use. Acceptable for temporary patching during build but not for the final taped joint.

Self-amalgamating tape: sometimes used on industrial DPM. Bonds to itself and conforms to complex shapes. Expensive and overkill for domestic floor work.

Painted-on liquid DPM at the joint: an accepted alternative on some specialist installations. Needs the proper product (not bitumen paint) and a longer cure time. Outside the budget and skill envelope of most domestic builds.

For a standard domestic floor DPM, the right answer is the right tape. The product is cheap, the application is simple once you know the rules, and there's no good reason to substitute.

Repair and patching

Punctures happen. The DPM gets unrolled, blockwork debris falls on it, a wheelbarrow runs over it, a follow-on trade walks across it with stiletto heels (literally, there's a forum thread). Punctures are normal and BCOs accept properly executed repairs.

The repair sequence:

1. Identify the puncture. A torch held low across the membrane in low light shows tears and pinholes that aren't visible from above.
2. Cut a patch of the same DPM material, sized to overlap the puncture by at least 150mm in every direction.
3. Clean the surface around the puncture with a dry rag (or alcohol wipe if there's any contamination).
4. Lay double-sided butyl tape in a square around the puncture, well within the patch outline.
5. Press the patch down firmly onto the butyl, working from the centre outwards to avoid trapping air.
6. Tape the edge of the patch with single-sided polythene tape on all four sides, bridging the patch onto the surrounding DPM.

This is the same two-tape detail as a sheet joint, just on all four sides of a patch. Done before the slab is poured and inspected by the BCO, this is fully accepted as a permanent repair. Done after the slab pour, you have a problem.

Where you'll need this

DPM jointing tape comes out at one specific stage of the build: the floor slab preparation, after the DPM is laid and before insulation goes down.

• Foundations and footings - sealing DPM laps over the oversite before the slab is poured
• Damp proof course - taping the DPM-to-DPC junction at the floor-wall interface
• Insulation - sealing DPM around perimeter upstands and at service penetrations before screed insulation goes down

These are the primary touch points on a typical kitchen extension build. The same tape and the same techniques apply across any extension or renovation project where a polythene DPM is laid: garage conversions with new floor slabs, ground-floor extensions of any room type, and basement tanking systems where the DPM doubles as a vapour barrier.

Common mistakes

Treating tape as optional. The Approved Document C requirement for sealed joints is not a recommendation. Some experienced contractors report never taping with no problems, but those installations either had generous overlaps (300-600mm) or weren't inspected to the current spec. Tape your joints. The cost is trivial and it removes a category of failure mode permanently.

Using duct tape, gaffer tape, or masking tape. All three appear on real building sites and all three fail within weeks under a slab. The adhesive is wrong, the substrate is wrong, and the moisture rating is wrong. BCOs reject these on inspection and the rejection delays the slab pour.

Confusing foil tape with DPM tape. They sit on adjacent shelves at builders merchants and look interchangeable. Foil tape is for PIR insulation board joints (different layer, different job, different adhesive). Buying foil tape for DPM joints is a category error that will be caught at inspection or, worse, will appear as a damp problem two years after completion.

Taping over creases or folds. Once the tape goes down, any wrinkle or fold underneath becomes a permanent pinhole leak. Lay the membrane flat, smooth out wrinkles before the upper sheet goes down, and inspect the lap for creases before applying tape.

Skimping on overlap. 150mm minimum, 300mm in radon zones. A joint with 80mm of overlap and tape over the top is not a compliant joint regardless of how well the tape is applied.

Applying tape in cold weather. Below 5°C the adhesive doesn't bond. The tape sticks visually and peels off invisibly. Either reschedule for warmer conditions or warm the materials properly before application.

Not pressing the tape down hard enough. Pressure-sensitive adhesives need pressure. A tape laid down loose with no roller pass holds at perhaps 30% of its rated bond strength. Roll every metre of tape, both ways, before walking away.

Buying one roll without a spare. Punctures and patches need tape. Running out of tape mid-install with the BCO arriving the next morning is a familiar trade story. Always carry one extra roll of each type.