Ground floor penetrations: sealing soil pipes and incoming services against air, damp and radon
The ground floor is where the airtight layer, the damp-proof membrane and, in affected areas, the radon barrier all meet the services coming into the house: the soil pipe, the water main, the electricity and data ducts. Each pipe through the floor has to seal against three things at once: air, moisture and ground gas.
Why ground floor penetrations matter
- Air. Pipes through a suspended timber floor open straight into a ventilated void, which is effectively outside. Holes round waste and soil pipes in older homes, often hidden behind the bath or the WC, are classic leaks.
- Radon. In radon-affected areas, the ground floor barrier has to be gas-tight, and BRE guidance (BR 211) treats service penetrations as part of that barrier. Sealing them is not optional.
- Moisture. The same penetration is a route for damp and vapour if the membrane is not continuous round it.
New build: ground-bearing slabs
Under a slab, services pass through the damp-proof or radon membrane before the concrete is poured. Usual practice is a preformed top hat unit pushed over the pipe and sealed to the membrane with compatible jointing tape. It is fiddly work in a muddy excavation, which is why it so often goes wrong.
- Cut the membrane as close to the pipe as possible.
- Seal the top hat base to clean, dry membrane, and clamp the sleeve to the pipe.
- Protect it from the concrete pour and from boots.
- Photograph it before it disappears. Once the slab is poured, it cannot be inspected or fixed.
Above the slab, where the pipe passes up through the floor finish and the airtight layer, it needs sealing again. A sized grommet or a liquid grommet bonded into the airtight layer at floor level closes the air path into the wall and floor build-up.
A liquid membrane on the slab
Passive Purple is certified as a radon and methane barrier as well as an airtight layer and vapour control. On a slab or upstand, it can be brought up and around the penetration so the barrier and the pipe seal are one continuous film. That is far easier to get right than a taped top hat on a site membrane. See our radon and methane page, and radon, airtightness tape and service penetrations.
Suspended timber floors (mostly retrofit)
- Find them from below if you can. With floorboards up, or from the void, every pipe and cable through the boards is visible and reachable.
- Seal at the airtight layer. In an insulated suspended floor, that is usually a membrane or board on the warm side of the insulation. The pipe seal belongs there, not just at the floorboard.
- Existing pipes cannot take a standard slide-on grommet. Use a split grommet or a liquid grommet. See sealing existing pipes in retrofit.
- Keep the void ventilated. Sealing the floor is about stopping the void's air getting into the house, not about closing off the void.
Soil stacks
A soil and vent pipe usually crosses the airtight layer twice: once at the ground floor, and again at the top-floor ceiling into the loft. Both need sealing, and both are big diameters that tape handles badly. A correctly sized grommet at each crossing, bonded into the airtight layer, is the reliable detail.
Incoming water, power and data
Bring incoming services in through ducts below the slab where you can, and seal inside the duct at the house end as well as round the outside. Otherwise air, and in some areas ground gas, can travel straight up the duct. Allow a sealed spare duct for future services so nobody drills through the floor later.
Fire and drainage notes
Where pipes pass through fire-separating floors, a tested fire-stopping product is needed alongside the airtight seal. Any seal on a soil pipe must not interfere with the pipe's own joints or supports.
Our own airtight pipe grommet, TED (Tight Entry Detail), arrives in December 2026, in sizes to suit soil pipes and service ducts. Ask us about it, or about detailing a radon-affected site today. Start with airtight pipe grommets explained for the basics.