Seal gaps around pipes through walls and floors

Fill gaps around pipes through walls and floors with the right sealant, foam or fire-rated product for each wall type.

PG-01Gather your tools and materialsCollect everything before you start so you're not stopping mid-job with sticky hands. You'll need a dust mask, gloves, eye protection, a filling knife or scraper, and rags for cleaning. Which sealant, foam or fire-rated product you need depends on the wall type and gap size, covered in the next steps.

1 each · Dust mask

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1 pair · Disposable gloves

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1 pack · Rags or cloth

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PG-02Work out what's behind the wall or floorKnock the wall near the pipe: a hollow sound usually means a stud wall or dot-and-dab, a solid thud means solid brick or block. Use a stud detector or magnet, tracking a line of screw heads in the plasterboard, to check whether any nearby studs are timber or metal - a metal stud frame is common in flats and anything built or refitted since the 1990s. Knowing this tells you how deep the cavity is behind the pipe hole, which changes how you seal it in the next steps.

1 each · Stud detector or magnet

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PG-03Check for hidden pipes and cables, and know your isolation pointsCables and pipes can run almost anywhere near this hole, even one that already exists - never assume the space behind is empty. Run a cable and pipe detector over the area, then still treat it as live: detectors miss cables that are deep, in metal conduit, behind metal-backed plasterboard, or outside the usual safe zones, and a metal stud frame confuses cheap detectors further. Before you enlarge or scrape at the hole, find your stopcock (usually under the kitchen sink) and check it turns freely, so you can shut the water off fast if you nick a pipe. If you have any doubt, isolate the circuit at the consumer unit before drilling, and turn the water off at the stopcock before cutting into anything that could carry water.

1 each · Cable and pipe detector

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PG-04Clean out the gapRemove old crumbling sealant, dust, cobwebs and any loose bits of plaster or masonry with a stiff brush or the edge of a filling knife. Wipe the pipe and the edges of the hole with a damp cloth and let it dry fully - foam and sealant won't stick to dust or grease. If the pipe has foam pipe insulation, pull it back slightly so you can seal right up to the pipe itself.

1 each · Stiff brush

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1 each · Filling knife

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PG-05Seal a gap in solid brick, block or masonryFor gaps under 5mm, run a bead of flexible frame sealant around the pipe and smooth it with a wet finger. For wider gaps up to 50mm, use a low-expansion gap-filling foam, filling the hole about a third full since it swells as it cures. Solid masonry gives the foam something firm to push against, so there's no hidden void to worry about.

1 tube · Flexible frame sealant

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1 can · Low-expansion gap-filling foam

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PG-06Seal a gap in plasterboard on timber studsThis hole usually opens into a hollow stud cavity, so don't overfill it - a full nozzle of foam can push through the gap and bulge the board on the far side. Pack the gap first with a strip of mineral wool or a foam backer rod, then run a bead of flexible sealant over the front for a neat, paintable finish.

1 bag · Mineral wool offcut

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1 roll · Foam backer rod

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PG-07Seal a gap in plasterboard on metal studsTreat this the same as a timber stud wall - the cavity behind is just as hollow, so pack it back with mineral wool or backer rod before sealing the front. Take extra care with your fingers and the sealant gun nozzle near the edges of the metal track, which can be sharp enough to cut skin.

PG-08Seal a gap in dot-and-dab plasterboardDot-and-dab has a deeper air gap - typically 25 to 50mm - between the board and the masonry behind, so foam alone can vanish into the void before it bridges the hole. Push a length of backer rod or mineral wool back until it meets resistance, then seal the front 10 to 15mm with flexible sealant or a shallow layer of foam.

PG-09Seal a gap in lath and plasterLath and plaster is fragile - the timber laths behind can crack or pull apart if you push foam or a filling knife in too hard. Use a flexible sealant rather than expanding foam, since foam's expansion pressure can crumble the surrounding plaster. Apply it gently, supporting the area from behind with your hand if you can reach it.

PG-10Use fire-rated sealant where the wall or floor is a fire barrierIf the pipe passes through a wall or floor separating flats, above an integral garage, or into a protected escape route or loft hatch, ordinary sealant or foam is not enough. Use a proper intumescent fire-rated sealant or fire-rated foam, which expands under heat to seal the gap and stop fire and smoke spreading.

RULES

1 tube · Intumescent fire-rated sealant

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KNOW THE RULES

Building Regulations Part B requires fire-stopping where a pipe passes through an element of structure providing fire separation, such as between flats or into an integral garage or protected escape route.

PG-11Trim, finish and tidy upLet the foam or sealant cure fully - check the can or tube, usually 1 to 24 hours - then trim any excess flush with a sharp knife. Fit a plastic escutcheon or pipe collar over the pipe for a tidy finish if the gap is on show. Wipe down your tools before anything sets hard, and dispose of offcuts and empty cans responsibly.

1 pair · Plastic escutcheon or pipe collar

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