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In some parts of the Peak fringe, the ground gives off radon — and building regulations ask the floor slab to keep it out. What that means for the concrete floor you pour.
Radon is a natural, colourless, odourless radioactive gas that seeps up out of the ground — and in a handful of areas on the eastern edge of our patch, there’s enough of it that a new building’s ground floor has to be built to hold it back. Because that defence lives inside the floor slab, it’s worth knowing about before you pour.
Radon is produced by the slow decay of uranium in certain rocks and soils; it rises through the ground and can build up inside buildings, where prolonged exposure is a health risk (this is UK Health Security Agency territory). The UK Radon Action Level for homes is 200 Bq/m³, and UKHSA designates a “Radon Affected Area” as anywhere that 1% or more of properties are at or above that level.
It’s very much a local, geology-driven thing. Across the lowland Manchester and Cheshire plain — glacial clay over sandstone and mudstone — radon is generally low. It’s the Carboniferous limestone and gritstone of the Peak/Pennine fringe that carries the higher potential. Radon is mapped by the square kilometre, so it varies even within a town.
Under the Building Regulations (Approved Document C, resistance to contaminants and moisture), a new building in a Radon Affected Area must include reasonable measures to keep radon out. The recognised specification is the BRE guide BR 211, “Radon: guidance on protective measures for new buildings.” How much protection depends on the site’s radon potential band:
| Radon potential (chance of being over the action level) | What’s required |
|---|---|
| Below 3% | No radon measures needed |
| 3% up to 10% | Basic protection — a radon barrier (membrane) |
| 10% or more | Full protection — a radon barrier plus a sump or ventilated subfloor void |
Basic protection is a radon-proof membrane across the whole footprint of the building, continuous with the floor’s damp-proof membrane and the wall damp-proof courses, with every lap and every service penetration sealed. Full protection adds a way to draw radon out from under the floor — a radon sump (or a ventilated void beneath a suspended floor) — left ready to be made active with a fan if a later test shows it’s needed.
The radon detail sits right inside the floor build, so it interacts directly with the slab and its membranes:
| Element | What it means on site |
|---|---|
| Radon membrane & the DPM | The damp-proof membrane is upgraded to (or paired with) a gas-resistant radon membrane running across the entire ground-floor footprint — continuous, correctly lapped and sealed, with every pipe and cable penetration sealed. An unbroken barrier is the whole point; a puncture or an unsealed lap defeats it. |
| Perimeter sealing | The membrane is dressed up the edges and tied into the wall damp-proof course / cavity tray, so gas can’t bypass it at the slab edge. |
| Radon sump (full protection) | A small collector void or perforated pipe formed in the granular sub-base beneath or beside the slab, vented to outside air — passive by default, upgradeable with a fan. It’s built as part of the sub-base and slab, so the detail has to allow for it. |
| Ground-bearing vs suspended slab | A ground-bearing slab carries the sealed barrier across as an unbroken sheet; a suspended (beam-and-block or cast) floor leans on a well-ventilated subfloor void as well, which can double as the extraction path for full protection. |
On our patch, the higher-radon ground is the Peak/Pennine fringe. These towns fall within or partly within a Radon Affected Area — a new floor here may need radon protection, so it’s worth checking the specific address:
Parts of the eastern/upland edge of nearby areas — Disley, Marple, High Lane, Bollington and the eastern edges of Macclesfield, Oldham, Rochdale and Tameside — can also fall within an affected area even where the lower town doesn’t. Check the specific address on the UKHSA map. Across the lowland Manchester and Cheshire plain, radon potential is generally low.
• Ready-mix for foundations & floor slabs
• The Pennine-fringe ground guide
• Back to the Ground Conditions Atlas
Check the address on the UKHSA radon map at ukradon.org, or get a radon report for the postcode. Radon potential is mapped per 1 km square and varies over short distances, so a town-level answer isn’t enough — the address-level check is what building control will expect.
It’s set by the site’s radon potential. From 3% up to 10% you need basic protection — a sealed radon barrier membrane across the floor. At 10% or more you need full protection — the barrier plus a radon sump or ventilated void that can be made active with a fan later. Below 3%, no measures are needed. Your radon report and designer confirm which applies.
No — the radon assessment, membrane specification and sump design belong to your radon report, designer and building control. Our job is to supply the right concrete and pour the floor slab to that specified detail, working around the membrane and sump so the barrier stays continuous and sealed.
Not usually — radon protection is about the membrane, sealing and sub-floor detail rather than the concrete grade. What matters on the pour is protecting the gas-resistant membrane and any sump so they aren’t punctured or bridged. Tell us the floor detail when you order and we’ll pour to suit it.
Sources: UK Health Security Agency / UKradon (radon maps, Radon Affected Area definition, 200 Bq/m³ action level); British Geological Survey radon mapping; GOV.UK Building Regulations Approved Document C (resistance to contaminants and moisture); BRE report BR 211, Radon: guidance on protective measures for new buildings; LABC radon guidance; High Peak Borough Council and Staffordshire Moorlands District Council radon information. General guidance only — radon protection for any site must come from an address-specific radon report and the project designer / building control.
Ready-mix and volumetric concrete, poured to your floor detail — radon membrane and sump or not.