Spray foam in a garage or outbuilding roof

A detached garage, a workshop or a studio at the bottom of the garden is usually uninsulated, so a spray applicator offers an instant fix: foam straight onto the underside of the roof covering, no battens and no boards. It also seals gaps. For a building used as a gym, an office or a hobby room, that appeals.

The trouble is that the roof underneath is rarely built like a house roof. Corrugated sheets, felt on boarding, or a sheet profile on light purlins all behave differently, and the foam sits tight against the back of the covering. Any water that gets in, or condenses on the cold side, has nowhere to go.

RICS warns that foam fitted badly can leave a building damp, with moisture forming where nobody planned for it, and timber damage after that. In a garage or workshop that is easy to picture: a metal or fibre sheet is cold on its outer face, warm air meets it, and moisture forms against the foam where it cannot be seen. Timber purlins and boarding behind the foam can stay wet for long periods.

Fixings are a second concern. Screws and bolts through sheets sit under a layer of foam, so rust or a loosening fixing is hidden until a sheet moves. We would rather see the back of the sheets than guess. Our guide to inspecting roof timbers under spray foam explains the moisture readings and sample removal we use.

Many older garages carry corrugated cement sheets. Some of those contain asbestos. Under HSE guidance, lifting asbestos cement sheets off a roof counts as non-licensed work when they come down intact, and nothing is smashed on the way. Weathered sheets that are otherwise sound are included. If taking a roof off would crush the sheets and throw up a lot of dust, HSE puts it in the notifiable non-licensed category, and notification must be made before work starts.

This matters because foam stuck to a sheet cannot always be scraped without abrading the surface. If your garage roof is cement sheet of unknown age, tell us before you disturb it, and do not start scraping yourself. We sample or assess first and plan the sequence so the sheet comes off whole where it can.

The method follows the roof. On timber boarding, foam can often be cut and peeled away in sections, with the board checked as it is revealed. On metal sheets, a gentler approach is needed so that coatings and fixings are not damaged. Where the foam has stuck hard, we may recommend taking the sheets off and recovering the building, since a worn garage roof rarely justifies prolonged hand scraping.

That is a decision worth costing both ways. MyBuilder's 2026 garage roof guide gives typical ranges of £700 to £1,000 for a single flat garage roof, £900 to £1,400 for a double flat roof and £2,500 to £4,500 for a pitched one. Foam removal, quoted nationally at £20 to £50 per m² by the same site, adds to the cost of keeping the existing sheets; a new covering may turn out to be the cheaper route. Your written quote covers both.

Our process guide, how spray foam is removed from a Stroud roof, describes the house version of the work.

Outbuildings collect things. Cables run along purlins, a light hangs from a bracket, and stored tools or bikes sit directly under the roof. Foam sprayed over a cable can hide its route, so we ask who installed the circuit and look for a consumer unit before cutting near anything electrical. If we are unsure, we ask an electrician to isolate or confirm the run.

Plan to empty the space, or at least cover it. Foam offcuts are light and spread widely, and a garage with a car or a motorbike in it needs sheeting down first. We lay protection, cut the foam away in sections and collect it as we go.

Once the underside is bare, you can decide how warm the building needs to be. A store for tools may need nothing. A workshop used year round may need a ventilated build-up with a vapour control layer, or a warm roof laid over the top with a new covering. Either way the structure gets inspected first, and any damaged purlins or boards are dealt with before insulation goes back.

Season matters less for a small building than for a house, but it still affects how long a bare roof can be left open. Our answer on removing spray foam in winter covers that, and whether the building will feel colder covers comfort.

We carry the waste off, leave the plot tidy, and back the work with a written 10-year workmanship guarantee.

Photographs of the underside from inside, a close-up of any sheet laps or fixings you can see, and a note of whether the building has a mains electricity supply and any wiring tucked behind the foam. Tell us whether the roof is flat or pitched, and what it is covered in. From those we can say whether a survey visit should focus on removal, on recovering, or on both, and the spray foam insulation removal overview shows where this sits among our other answers.