Spray foam on a flat roof

A pitched roof has a loft above the ceiling where air drifts through. A flat roof is more like a sandwich: a plasterboard ceiling, a void between timber joists, a deck of boards or panels, and a waterproof layer across the top. The joists are the horizontal timbers carrying the deck. Foam is usually sprayed into the void between them, or on to the underside of the deck itself.

That sealed arrangement leaves moisture with nowhere to go. If the membrane lets in a little rain, or humid air from the house meets the cold deck and turns to water, the timber stays damp because the foam stops it drying. RICS expects a condensation risk check before any foam is installed, and the Energy Saving Trust notes the rot hazard that follows when a deck stays wet from either cause.

Under a pitched roof, a ceiling stain can usually be traced upwards to a slipped tile. Under foam on a flat roof, water may travel along the deck or soak into a board before anything shows below. By the time a mark appears, several joists may be affected. The Energy Saving Trust does not recommend applying foam to a defective roof before the defect is mended, and flat roofs are where that advice bites hardest.

So the assessment on a flat roof usually includes moisture readings and, where possible, a look at the deck from above. The page on what a spray foam assessment checks sets out the steps. Age matters as well: MyBuilder says flat roofs typically last 15 to 30 years, so an older covering might already be close to needing replacement whatever the foam is doing.

Foam between joists can be cut and scraped away from below. The deck above it may be damp, brittle or stuck to the foam in patches. If boards are soft, they come out. Joists that have sat wet for a long time are probed for rot, and any damaged timber is repaired or replaced.

Foam sprayed straight on to the underside of the deck is harder to reach, because the ceiling often has to come down first. That raises the stakes for the covering above, so many owners decide to renew the roof in the same visit. Approved Document L treats a fresh waterproof membrane on a level roof as work on the roof's thermal element, so where most of the roof is renewed, the insulation may have to be brought up to the current standard. Our page on foam on a breathable membrane deals with a related moisture question on pitched roofs.

A flat roof is rebuilt as one of two types. A warm roof puts rigid insulation above the deck, so the joists and the void below stay inside the warm envelope. A cold roof puts insulation between the joists and keeps a ventilated gap above it. Marley, reading BS 5250, asks for a continuous 25 mm gap at the low edge when the slope is 15 degrees or flatter and the underlay will not let vapour through. The principle is that a cold deck needs air passing across it.

Which build-up suits your house depends on ceiling height, upstands at doors, the edge details and whether the roof drains to a gutter. Ask your roofer to explain which they propose and why, and to show you a sketch of the layers from the ceiling upwards. A good explanation names each layer in order, says where any vapour barrier sits, and shows how air will move through a cold build-up. Read whether the roof needs ventilation after spray foam removal for how airflow works on a pitched roof, since the same thinking applies.

The waterproof layer on top is a separate decision. MyBuilder's flat roof guide, updated on 21 August 2026, gives replacement figures per square metre: EPDM at £90 to £130, GRP at £95 to £150 and bitumen felt at £70 to £100. EPDM is one large sheet of synthetic rubber, GRP is a glass-fibre and resin coating that sets hard, and felt is bitumen sheet melted or stuck on in layers. Treat these as national guide figures that say little about your own roof.

What you pay in practice rests on the area, access, how much deck needs replacing and the insulation chosen. Our written quote gives a single total for the whole job, issued after we have surveyed the roof and checked how the water leaves it.

Edges and details deserve a word as well. A flat roof lives or dies at its upstands, the points where the covering climbs a wall, and at its outlets, where water leaves. Foam work often exposes failures at those points, such as a lifted edge or a sagging gutter board. We photograph them while the roof is open, so any extra fix is agreed with you rather than discovered afterwards.

Surveyors tend to flag foam on any roof, and RICS accepts that it can weigh on a valuation. If you are mid-purchase, our page on buying a house with spray foam covers the order to do things in.

When you ask us to look, we go up on the roof, take pictures of the covering and the structure we can see, and send you a quote in writing. Should you go ahead, the guarantee on our workmanship is given for ten years, on paper, and no rubbish is left behind. Start from spray foam insulation removal.