Radon Barrier Membrane: The Gas-Tight Sheet You Have to Get Right Before the Slab Pour
A UK guide to 1600 gauge radon barrier membrane for extension floors: basic versus full protection, BR 211:2023, sealed laps and pipe collars, the building control hold point, and 2026 prices.
You order standard 1200 gauge polythene floor membrane, same as every other extension. The groundworker lays it, tapes it up the walls, calls it done. Then building control turns up for the oversite inspection, checks the postcode against the radon map, and refuses to sign off the pour. The address sits in a radon-affected band: the floor needs a gas-tight 1600 gauge barrier with sealed laps and collared pipe penetrations. None was ordered. The slab is now delayed while the right membrane gets sourced, with the groundworker standing around on a day you are paying for.
That conversation is entirely avoidable. It comes down to one check you do before you order anything, and one membrane that costs only marginally more than the sheet you would have bought anyway.
What it is
A radon barrier membrane is a heavy polythene sheet laid across the extension floor to stop radon gas seeping up from the ground into the building. Radon is a naturally occurring radioactive gas. It is colourless, odourless, and produced by uranium decaying in certain rocks and soils. It is the UK's second-leading cause of lung cancer after smoking. The barrier does two jobs at once: it blocks radon and it blocks ground moisture, replacing the standard 1200 gauge damp proof membrane (DPM) you would otherwise lay. You do not fit both. A radon barrier is the moisture barrier in a radon-affected area.
Whether you need one is decided by postcode, not county. Radon risk varies street by street. The single most important step is to enter the postcode at ukradon.org before you order any membrane. The UK Health Security Agency runs the map and test service. Most UK postcodes need no radon measures at all: a standard DPM is fine and no further action is needed. Get this check wrong in either direction and you are either delayed at the pour or you spent money you did not need to.
External resource
UKHSA radon postcode checker
Enter your extension's postcode to find your radon probability band. Do this before you order floor membrane, not after the groundworker has laid the wrong sheet.
ukradon.org
The legal route runs through Approved Document C (site preparation, resistance to contaminants and moisture), which treats radon as a ground contaminant and points to a BRE technical document for detail. That document is BR 211. The current edition is BR 211:2023, the sixth edition, published May 2023. It superseded and withdrew BR 211:2015, so any older guidance or merchant datasheet still citing the 2015 edition is out of date. BR 211:2023 also formally extended its scope to cover extensions, conversions and refurbishment, making it the document that governs your build.
Basic protection versus full protection
| Tier | When it applies | What you fit | Components |
|---|---|---|---|
| No measures | Below 3% probability on the UKHSA map | Standard 1200 gauge DPM only | DPM + standard jointing tape |
| Basic protection | Roughly 1–3% (lower affected bands) | Sealed radon barrier across the whole floor, continuous with the wall DPC | Radon barrier + double-sided jointing tape + single-sided lap tape + pipe collars |
| Full protection | Higher bands (above roughly 10% probability) | Sealed radon barrier PLUS a sub-floor sump and capped standpipe | Everything in basic, plus a sump kit and capped depressurisation pipe |
Basic protection is a sealed membrane on its own. Full protection adds a radon sump: a sealed void formed under the slab, connected to a pipe that runs up and is capped off. If a test after completion comes back above the action level, a fan gets added to the existing pipe to actively draw gas out from under the floor. The working principle is called sub-floor depressurisation.
The floor-area trigger catches people out. For an extension to an existing house with no radon protection of its own, full protection is generally only required when the extension's ground-floor area exceeds 30. Below that, basic measures may still be sensible but the statutory full-protection requirement may not apply. Interpretation varies between authorities, so this is a confirm-with-building-control point, not a number to assume.
200
The UK radon Action Level. If a three-month test in your finished home averages at or above this, you activate the standby sump with a fan. The Target Level (the ideal outcome) is 100.
The whole reason to build in protection now is cost. A sump kit and membrane uplift are cheap at the build stage when the floor is open. A full retrofit after completion, breaking out the finished floor to form a sump or installing positive-input ventilation, runs from £800 – £3,000 with a specialist. Activating a standby sump that is already in place is a fraction of that, just a fan and an electrical connection at £150 – £400. You are building cheap insurance.
Why 1600 gauge, and what the standards actually test
The required membrane is 1600 gauge, 400 micron thick. That is heavier than the standard 1200 gauge (300 micron) DPM, but, and this surprises people, not because a thicker sheet blocks gas better.
1600 gauge is the minimum, and the reason is robustness, not permeability. The thinking, traced back to the BS 8485 drafting panel (the standard for methane and carbon dioxide ground gas protection), is that a reinforced polythene membrane below 400 micron is unlikely to survive the construction process. It has to take the abuse of a slab pour and trades working over it without tearing or puncturing. A torn gas barrier stops nothing. The durability minimum is there because it is what delivers gas protection in practice.
This is also why you should not go looking for something thicker thinking it is a higher grade. A 600 micron sheet is not "more protection". Every BR 211-compliant radon barrier on the UK market sits at 1600 gauge, 400 micron. Specify that and stop.
Now for the part no merchant datasheet explains properly. You will see radon barriers marketed as compliant with BS EN 13967. That is a product standard for flexible waterproofing and damp-proof sheets. What EN 13967 actually covers is this: dimensions, thickness, watertightness, impact resistance, and water vapour transmission. It does not include a radon gas transmission test. EN 13967 compliance confirms the sheet works as a DPM. The gas-barrier function is tested separately, under a gas-permeability method, and confirmed through a BBA certificate, which requires gas testing. So a credible radon barrier carries both: EN 13967 for the moisture function (its CE/UKCA marking) and a BBA certificate for the radon function. Visqueen R400, for example, holds BBA certificate 13/5069. Look for the BBA number, not just the EN 13967 line.
One more naming trap. Do not confuse a radon barrier with a methane or CO2 gas barrier. They overlap (the 400 micron threshold is common to both gas-barrier worlds) but they are specified differently. If a ground investigation has flagged landfill gas or made ground, that is a BS 8485 design problem with its own membrane and collar specification, not a radon question.
The system: four parts, not one
Here is where radon work diverges hard from laying a normal DPM. A standard DPM is laid, lapped, taped, and that is largely it. A radon barrier is laid the same way, but treating it as just a coloured DPM does not stop gas. Discipline at every joint, penetration and perimeter is the entire point. Across installer forums, airtightness discipline is the most-repeated theme. You run double-sided butyl jointing tape inside the overlap to bond two sheets, then cover the seam with single-sided lap tape on top. The double-sided tape makes the gas seal. The single-sided tape protects it and holds the lap flat. Do both. A radon DPC strip ties the floor barrier up into the wall course (more on this below), so on a fully detailed job you are often handling five distinct products.
Radon DPC strip is a gas-resistant damp proof course that goes in the mortar bed at the wall, replacing standard polythene DPC in affected areas, so the floor barrier and wall barrier link up into one continuous system.
How it goes in
The barrier itself handles like a heavy DPM. A roll weighs around 29 kg and stores fine in a dry corner of site. Lay it across smooth, compacted sand blinding, never straight onto sharp hardcore. A stone protruding through the blinding will be pressed through the membrane by the weight of wet concrete during the pour, and you will never see the puncture once the slab is in.
The technique that makes it a gas barrier rather than just a moisture barrier is in three details: laps, penetrations, and perimeter.
Laps. The minimum overlap at every joint is 150, sealed with the double-sided-then-single-sided tape method above. Overlapping without taping is the cardinal error and the fastest way to fail an inspection.
Pipe penetrations. This is the hardest part of the whole job, and three out of four installer forum threads say so. Every soil pipe, waste pipe or cable that pierces the floor breaks the barrier, and each one needs a radon collar, a proprietary moulded "top hat" unit. You cut the membrane close to the pipe, bond the collar's flat brim to the membrane with double-sided tape, seal the joint with single-sided tape, and clamp the collar's upstand to the pipe with a jubilee clip. Collars come sized to the external pipe diameter: 110mm, 135mm, 160mm. One per pipe. Where you have a cluster of small conduits coming up together (say, six 63mm pipes through one spot), a standard collar will not fit and you are into two-part pourable sealant instead, so flag grouped penetrations to the groundworker early, and ideally space them out during first-fix planning so each can get its own collar.
Perimeter. Turn the membrane up the wall and seal it to the radon DPC. Two things people miss here. First, in cavity wall construction the membrane has to bridge the cavity: run it up over the inner leaf and drop it a couple of courses into the outer leaf, rather than just turning it up the inner skin. Stopping at the inner leaf leaves the cavity open as a path for gas, and it is one of the most commonly missed details on site. Second, leave deliberate slack at the upstand. Do not pull the membrane drum-tight at exactly the point where you need the seal to hold.
Tip
Photograph everything before the pour. The sealed laps, every collar, the cavity bridge, the perimeter upstand, the sump if you've got one. Once the concrete is in, none of it can be seen, checked, or fixed. Your own photos are your evidence if a question comes up at completion or on resale.
The building control hold point
This is non-negotiable, and it is the part that separates a smooth job from an expensive one. Building control inspects the radon work at the oversite visit, before the slab is poured, because the membrane, collars and sump cannot be retrospectively verified once they are under concrete.
Warning
The radon barrier is a building control hold point. The BCO must inspect the sealed membrane, the pipe collars, and the sump (if fitted) before the pour. There is no going back after the concrete is in. Pour without sign-off and you risk being required to break out the slab. Book the inspection at least 24 hours ahead and don't let the concrete lorry arrive before it's signed off.
The inspector is checking the same points you photographed: the correct 1600 gauge membrane, laps at 150 sealed both ways, a collar on every penetration, the cavity bridged at the perimeter, the upstand connected to the radon DPC, no tears, and the sump correctly formed and connected on a full-protection job. In affected areas they will check your postcode band against the work, and a standard DPM gets caught in that moment.
How much do you need
A single 4m x 20m roll covers 80m² of membrane. That is enough for most single-storey extensions once you have accounted for perimeter upstands, which eat into coverage. A 4m x 6m extension (24m² floor) uses roughly 32-35m² once upstands on all four sides are added in, so one roll with margin. You would only reach a second roll above about 65-70m² of floor area, where the floor shape forces several longitudinal joints.
For consumables on a typical single-storey extension: 2-3 rolls of double-sided jointing tape, one roll of single-sided lap tape, one 20m roll of radon DPC (which covers 20m of wall perimeter, enough for most single-storey jobs), and one collar for every pipe coming through the floor. Count penetrations off the drainage and services layout before you order, because a missing collar on pour day stops the job.
Cost and where to buy
The membrane is the big-ticket item, but it is still cheap in the scheme of the build. A 1600 gauge roll runs £120 – £175. Visqueen R400 (red) and Novia BR211 (green) 1600 gauge barriers are the two products you see most. Both are BBA-certified, EN 13967-marked and BR 211:2023-compliant. Buy on price and availability. There is no meaningful performance difference. Treat any roll priced significantly below the market range with suspicion until you have confirmed it is genuinely 400 micron and BBA-certified. Watch for ex-VAT pricing dressed up as a bargain.
The consumables are small money individually but you need all of them:
| Item | Typical price (inc VAT) | Allow |
|---|---|---|
| Radon barrier roll, 4m x 20m / 80m² | £120 – £175 | 1 roll (most single-storey extensions) |
| Top hat pipe collar (110mm + jubilee clip) | £20 – £25 | 1 per pipe penetration |
| Double-sided butyl jointing tape (50mm x 10m) | £9 – £12 | 2–3 rolls |
| Radon DPC strip (100mm x 20m) | £10 – £13 | 1 roll per ~20m perimeter |
| Radon sump kit (full protection only) | £40 – £80 | 1 (full-protection jobs only) |
The whole radon package for a typical extension, the membrane uplift plus tapes, collars, DPC strip and sump kit, lands at £100 – £250 in materials. Labour folds into the groundworker's oversite price; it is the same operation as laying a DPM but with more care at the joints.
Buy from specialist merchants. Permagard, Build Supplies, Insulation Superstore and manufacturers' own stockists (Visqueen, Novia) carry the membrane plus the matching collars and tapes as a system. The advantage of buying a lot from one specialist is compatibility: the tape and collars are designed to bond to the membrane. General DIY sheds stock standard DPM but rarely a full radon system, so do not assume the nearest Wickes or Screwfix will have collars on the shelf. Order the complete kit ahead of the pour date.
Common mistakes
Using standard 1200 gauge DPM in a radon area. It fails BR 211 outright and it will be caught at the oversite inspection. The fix is sourcing the right membrane and relaying on a delayed pour. Check the postcode and order 1600 gauge before the groundworker starts.
Treating laps and penetrations as optional sealing. A lapped-but-untaped radon barrier is just a coloured DPM. The double-sided-then-single-sided tape at every lap and a collar on every pipe are the membrane's whole reason for existing. This is the difference inspectors look for.
Stopping the membrane at the inner cavity leaf. This is one of the most commonly missed details on site. The membrane must bridge the cavity to the outer leaf or the cavity becomes an open path for gas.
Pulling the perimeter tight. Leave deliberate slack at the upstand. A taut membrane tears as the slab settles, opening the seal at exactly the point where it matters.
Pouring before the BCO has signed off. This is the single most expensive mistake here. Once concrete covers the membrane, nothing can be inspected or corrected. Book the inspection, get sign-off, and then pour.
Where you will need this
- Foundations and Footings - the radon barrier is laid across the oversite as part of the floor build-up, before the slab pour
- Damp Proof Course - the radon DPC strip ties the floor barrier up into the wall course in affected areas
These details apply at the groundwork stage of any extension or renovation where the postcode falls in a radon-affected band. The membrane goes in once, gets one inspection, and can never be revisited.