The short answer
Condensation forms when moist air touches a cold metal sheet and cools below its dew point. Anti-drip backing, a lining or insulation under the sheets, and enough ventilation reduce it. Sealing the garage tighter without ventilating can make it worse, and a real leak looks different because it follows rain, not temperature.
How a dry garage ends up with a wet ceiling
Air always holds some water vapour, and warm air holds more than cold. When that air reaches a surface cold enough to chill it, the vapour condenses into droplets. A thin steel or aluminium sheet roof is an excellent conductor and has almost no insulation, so on a cold night its underside becomes the coldest surface in the garage. Water collects there, runs along the corrugations and drips from the low points or the fixings.
The moisture comes from ordinary use. A car that has driven through rain brings a surprising amount of water indoors. So does a drying coat, a wet dog, a tumble dryer or a freshly washed vehicle. Garages with little ventilation hold that vapour until it finds the cold roof.
The pattern is the giveaway. Condensation tends to be worst on cold mornings, after a spell of use, and in winter, and it appears across the whole underside rather than at one point. A leak, by contrast, follows rainfall and drips at a particular spot. If you are not sure which you have, our page on garage roof leaks helps you separate the two.
Anti-drip backing and other cures
Many modern metal sheets are sold with an anti-condensation or anti-drip backing: a layer of felt-like fleece, or a sprayed coating, applied to the underside. It does not stop condensation forming. Instead, it absorbs the droplets and releases them slowly as vapour when the air warms, so they do not fall in a stream. It is a sensible, low-cost fix, but it has limits when a lot of moisture is being produced.
The more thorough answer is thermal. An insulated sheet, a sandwich panel or a lining board with a layer of insulation behind it keeps the inside surface above the dew point, so the water never forms. That is a more costly route, yet it also makes the garage warmer, which suits a workshop or a space where tools are kept.
For an existing roof, a retrofit lining can work, provided the build-up is designed so that moisture does not get trapped between the lining and the sheet. This is where guesswork goes wrong. Spray foam applied directly under a metal sheet, for instance, is not something we would recommend without a careful assessment, and RICS has cautioned consumers that badly installed insulation of that sort can bring damp and uncontrolled condensation.
Ventilation: the part people forget
Insulating without ventilating moves the problem instead of solving it. A sealed, insulated garage holds its moisture, which then lands on the next coldest surface: the door, the tools or the walls. Some air movement is needed to carry vapour out.
Simple measures work well. High-level and low-level vents on opposite walls let air pass through. An air brick or a louvre on the gable end is cheap and effective. Where the garage has a door with a seal around it, a small gap or trickle vent stops the air from stagnating. Habits help too: let a wet car drip outside for a few minutes, wipe it down, and do not hang wet washing in there.
Roof ventilation guidance for houses, such as the BS 5250 principles summarised by Marley, is aimed at pitched roofs with loft spaces and does not translate directly to a single-skin metal garage roof. For a garage, your roofer should size the openings by looking at the building, its use and the roof build-up, not copy a house rule.
Deciding whether to repair, line or replace the roof
If the sheets are sound and the problem is purely condensation, a lining and better ventilation will usually solve it without a new roof. If the sheets are corroded, the fixings are loose or the laps are letting water in, replacement is the better course, and it is the moment to build in insulation and a backing from the start.
Not every garage has metal sheets by choice. Some are corrugated cement sheets that were replaced with steel at some point, others still have the original cement. Because old cement sheets may contain asbestos, take care before drilling anything, and read our page on asbestos disposal if you plan to have them removed.
Flat concrete roofs behave differently and have their own condensation pattern, which our page on concrete garage roofs explains. If you are considering changing the shape of the roof as well, the page on turning a flat garage roof into a pitched one covers the structural side.
What we check, and what the rules say
Our survey covers the underside of the sheets as well as the weather side. We note where the drips fall, whether the sheets have a backing and whether it is saturated, how much air can move through the building, and what the garage is used for. A workshop with a heater needs a different answer from a store for garden furniture. You receive pictures taken on your garage, then a written quote.
If a garage is being converted, extended or attached to the house, Building Regulations may apply, including the thermal limits in Part L. Check with your building control body, which may be Swindon Borough Council's team, or see our page on garage roof regulations for the questions to ask.
If a backing and two vents cure the drip, that is the advice you will get, because the lighter fix is the better one when it works. A full new roof is for the point where the existing sheets have simply given up. See our garage roof replacement page for how we work.