The short answer
A metal garage roof usually drips inside because of condensation: moist air in the garage cools against the cold underside of the sheet and turns to water. It is most obvious on cold, damp days and after the car has been parked wet. A true leak drips after rain, from a fixing, lap or crack. Fixes include an anti-condensation backing, ventilation, and removing sources of moisture.
Condensation or a leak: how to tell them apart
The two problems look alike from underneath, but they behave differently. Condensation forms when the air inside is warmer or wetter than the surface it touches. Steel and aluminium sheet follow the outside temperature quickly, so the underside is cold on a winter morning while the air in the garage holds moisture from wet cars, damp tools, drying washing or a floor that has never had a damp-proof membrane.
A leak, by contrast, follows rainfall. It tends to appear in one place, below a fixing screw, a lap between sheets, the ridge or a damaged edge. Look at the pattern as well as the timing, from inside with a torch. Drips spread evenly across the whole underside, with beading on bolts and purlins, point to condensation. A single dark track running down from one point points to a leak.
A useful test is to check on a dry day after a cold night. If the roof is wet underneath and nothing has rained, it is condensation. Our general guide to leaking garage roofs covers the causes of genuine leaks.
Why bare sheet metal is so prone to it
Sheet steel is a thin, conductive skin with nothing behind it. Warm air in the garage reaches it with no insulation or air gap in between, so it cools quickly and gives up its moisture. A timber deck with felt or a plastic roof is warmer on the inside and less prone to the problem, because the inner face stays closer to room temperature.
Some box-profile sheets are made with a fleece or felt backing that absorbs moisture and releases it slowly, which is why a bare sheet is the usual suspect. Check the maker's data for what a given backing is designed to do, because products differ. We look at what is on your roof first, and our page on box-profile metal sheet roofs describes the products.
Older garages add another factor: no ventilation. Gaps at the eaves and ridge used to let air through by accident. Tightly sealed garage doors and sealed roof edges can leave damp air with nowhere to go.
Fixes that work, in the order we would try them
Cut the moisture at source
Park the car dry where you can, keep wet equipment out, and avoid drying laundry in a closed garage, where the moisture has nowhere else to go. If the floor is bare concrete and damp, sealing it helps.
Add ventilation
Airflow at the eaves and at the high point of the roof lets damp air leave before it condenses. BS 5250 deals with managing moisture in buildings, including roof void ventilation, and each manufacturer publishes its own figures for its products. Vents also need to be insect-proof and weather-proof, and they must not undo the roof's watertightness.
Add an anti-condensation layer
A fleece or foil-faced backing can be fitted under existing sheets if there is access, but often the more reliable route is to replace the sheet with a product that has one built in.
Insulate with care
Insulation under metal can help, yet it can also trap damp against the steel if it is done badly. RICS warns that spray foam applied without condensation risk checks can lead to dampness, so we would not recommend foam under a metal roof without proper advice.
When the answer is a new roof
If the sheets are rusted, bowed, or fixed with failing screws, adding a liner will not save them. Rust on the underside of steel can come from condensation that has been dripping for years. At that point it makes sense to replace the roof with a coated, backed sheet, and to correct the ventilation at the same time.
If the existing covering is old cement-based sheet, it may contain asbestos and the rules change. Careful removal of intact sheet is non-licensed work under HSE guidance, but sheeting cannot be broken up or drilled casually. Take a look at how asbestos sheeting is disposed of before you decide.
Whichever way you go, the fixings matter: corrosion-resistant screws with sealing washers, set at the spacing the sheet maker specifies, stop the second source of drips, which is water tracking in at each hole.
Some owners use the moment to change the shape of the garage roof. Our page on making a flat garage roof pitched looks at the structural and planning side of that.
Cost and what we will put in writing
National costs vary with garage size and roof type. MyBuilder's 2026 garage roof guide gives single flat roofs £700 to £1,000, double flat roofs £900 to £1,400 and pitched roofs £2,500 to £4,500. A sheet replacement on a pitched or monopitch garage falls within the pitched figures, and a lining or ventilation job costs less than a full replacement.
On site we start from the inside, checking for beading, rust and bare fixings, then look at the roof from outside. The written quote covers the sheets, backing, vents and labour, with the old covering taken away. To start, phone 01453 933613, use WhatsApp, or upload photographs of the underside of the roof with the enquiry form. We guarantee our workmanship for 10 years, in writing. You can also read about the wider range of jobs on our garage roof replacement page.