Is Spray Foam OK in a Garage or Outbuilding?

Why outbuildings are different

A house loft sits above heated rooms and takes in humid air from cooking, washing and breathing. A garage is usually unheated, and a typical garden building is used intermittently. That lowers the moisture load on the roof, so some of the condensation worries that apply to houses are smaller. A detached garage with a pitched tile roof, a dry interior and a car or tools inside is a less demanding setting than a bedroom ceiling.

It is not a free pass. Outbuilding roofs are often built more lightly. Rafters can be slim, the roof felt may be a basic bitumen type, and tile fixings or flashings are often neglected because nobody goes up there. Foam applied to a roof that already leaks seals the problem in, exactly as in a house. The RICS consumer guide describes dampness being held against timber out of sight, and a timber garage roof is no different.

Flat roofs and garden rooms

The hardest case is the flat-roofed garden room or garage. These are commonly built with a timber deck, often plywood or OSB, on joists, covered with felt, EPDM or GRP. Foam sprayed under the deck turns it into a warm roof with no ventilation and no way for the timber to dry, so any leak or condensation sits against the deck. Boards of that kind swell and rot, and the failure is hidden until the roof sags or leaks. Our page on spray foam over roof felt covers a similar question for felt-covered pitched roofs.

If you want to insulate a flat roof, there are safer arrangements: rigid boards above the deck under a new membrane, or a ventilated cold-roof build-up with insulation between joists and an air gap above. They cost more at the outset, though they can be done when the covering is renewed. If the garage roof is made of asbestos cement sheets, do not disturb it, and speak to us before any work, since the HSE sets rules for handling that material.

Pitched garages and workshops

A pitched roof with tiles or sheets, a felt underlay and open rafters is the easier case. Foam can be applied to the underside, provided the covering is sound and dry. Open-cell foam is the usual choice, and our page on open-cell and closed-cell spray foam explains why. If you park a car or use the space for hobbies, remember that fumes from vehicles and humidity from drying clothes or washing will be in the air. Also remember that combustible foam in a workshop with a heater or electrics needs the same clearances discussed in our page on whether spray foam is a fire risk.

Roof access is another practical point. Outbuilding roofs are often low, and it is tempting to climb up and look yourself, but older garage roofs can be brittle, particularly cement-fibre sheets, which can crack underfoot and drop you through. A photograph from a ladder at the eaves, or from a window, is a safer start, and we survey roofs like these using proper access.

Sheets and cold buildings

Metal sheeted garages and outbuildings are another case. Foam on the underside of a profiled metal sheet is common because it deals with condensation drips, and it can work where the sheet is sound and there is no timber to rot. The weakness is rust. Foam against steel can hide corrosion, and if the sheet is already failing, the foam makes it hard to see. Cement-fibre sheets of older garages may contain asbestos, and the rules for these are set by HSE, which says careful removal of undamaged sheets is non-licensed work, while breaking them up can make it notifiable.

Use matters as much as construction. A garage that is a car park and a dry store needs little; one with a washing machine, a drying rack, a hot tub next door or a room used as a gym or office has more moisture and more need for a sound design. If you are converting a garage to living space, the roof becomes a thermal element subject to the regulations, and foam should be part of a considered build-up with ventilation, not a quick spray to tick a box.

What to do

Before you foam anything, have the roof checked. We survey garage and outbuilding roofs, photograph the covering, the deck and the fixings, and tell you whether insulation can go in as planned. If foam is already there, the same survey tells you whether it is safe to leave. If you want a safer alternative, our page on spray foam alternatives compares them.

The wider service is described on our spray foam page.