Felt or box-profile sheets on a garage?

The slope of a roof decides what can go on it before anyone discusses price. BS 6229:2025 covers flat or curved roofs up to 10 degrees, using continuously supported flexible coverings, and felt belongs in that group. Below that angle, a covering laid as one waterproof skin is the right idea.

Box-profile sheets work differently. They are metal sheets pressed into a trapezoid profile of raised ribs and flat valleys, fixed to purlins. They shed water along the ribs and rely on laps and sealant at the joints, so they need a pitch the manufacturer states. Read the product's own data sheet for its minimum pitch, because it differs between makers and between profiles.

If the garage roof is truly flat, that points to felt, EPDM or GRP. If it slopes properly, sheets come into play. A roof that sits between the two needs a measured angle, which is the first thing the survey records.

Modern felt is a bitumen membrane laid in layers over a boarded deck, with the laps sealed by heat from a torch. It copes well with small, awkward shapes, and it can be dressed neatly up a wall or round a rooflight, which is useful on a garage with a few details. Felt also lets you add insulation beneath it in a warm-roof build, which cuts condensation.

Published life varies. Less gives torch-on felt 10 to 20 years, and the range is wide because workmanship varies so much. CORC quotes more than 30, so the sources disagree. A cautious planning figure is the lower one. What you actually get depends on the layers specified, the deck, the detailing and the falls.

Cost is the attraction. Checkatrade's October 2026 replacement guide mentions around £1,000 to £2,000, averaging £1,500, for re-felting a garage roof. Our re-felt cost page goes through what pushes the figure around.

Sheets go on quickly, they are light, and they shed water from a sloping garage with no fuss. On a roof with the right pitch and a sound structure they are a sensible, durable choice, and they also re-cover an old asbestos garage once the original sheets have been taken off.

The trade-offs are practical ones. Rain drums on bare metal, which you will hear if you use the garage as a workshop. Cold metal collects condensation on its underside, which can drip onto whatever is below; our answer on why a garage roof drips inside explains how to handle that. The fixings and laps matter too, because they are where leaks begin.

A sheet's working life follows its coating and its manufacturer. The data sheet states the expected performance, and you should ask to see it before the order is placed, in the same way you would ask about a felt specification.

Felt needs a continuous deck: boards or panels laid across the joists, in sound condition and dry. A deck adds weight and cost if it has to be replaced, and the boards go down to a fall. Our page on the fall a flat garage roof needs covers that point.

Sheets want purlins spaced to suit the profile, and a roof structure strong enough to be fixed into. Old garages with light timbers sometimes need new purlins before sheets can go on, and that adds to the bill.

In both cases the survey looks at what is underneath before it settles what goes on top. We photograph the structure, tell you plainly whether it can take either covering, and advise the smallest job that does the work. Either way, the eaves, the verge and any gable-end trim are detailed so rain runs where it should and the edges look neat from the street. If a patch will hold, the answer says so.

Put the questions in order. Is the slope flat or steep? Is the deck sound? Is any of the old covering asbestos? How much noise and condensation can you live with? How long do you want the garage roof to last?

A flat roof with a sound deck points towards felt for a lower outlay, or EPDM for fewer joints and a longer published life. A pitched roof that needs no deck points towards sheets. An old asbestos roof points towards removal or, where the sheets are sound, overcladding, followed by whichever covering suits the pitch.

Our best garage roof material page runs the whole comparison. When the survey is done the price comes in writing as a single total, with the scope stated plainly. Back at the start, the garage roof replacement page gives the overview.