The short answer
If a metal garage roof drips on a dry day, especially in cold or damp weather, the cause is almost certainly condensation. Metal sheets chill quickly, and when warm moist air inside the garage touches them, vapour turns to water on the underside. The cure is to cut the moisture, add ventilation, and stop the sheet from getting cold on the inside, usually with an anti-condensation backing or insulation.
Why metal sheets drip
Air always holds some water vapour, and warm air holds more than cold air. When that warm air touches a surface colder than the temperature at which it can hold its vapour, the surplus turns to droplets. Metal is a very good conductor, so a steel or aluminium sheet follows the outside temperature closely. On a cold night or a damp morning it is much colder than the air in the garage, and the underside becomes wet.
The drips can look like a leak, but there is no hole. Timing is the clue: condensation appears in cold, still, humid weather and on clear nights, often as the sun comes up, while a leak appears during rain. It can also appear over a roof that has just been replaced, because the new sheets are tight and no longer breathe through gaps.
Where the moisture comes from
A garage is often damp for reasons that have nothing to do with the roof. A car that comes in wet brings in water, and its engine then warms the air. A floor with no damp-proof membrane gives off moisture. Washing, tumble dryers, paint, gas heaters and stored firewood all raise humidity. In a small closed space those sources add up quickly.
Reducing the moisture helps. Let the car dry outside, fit a vapour barrier or sealed finish to the floor, and avoid drying laundry inside. Look at the other sources before you spend money on the roof. The page on why a garage roof leaks will help you tell the two problems apart.
Ventilation
Moist air has to be able to leave. A sealed garage with a metal roof holds its humidity, and the roof gets wet. Vents at eaves and ridge, or high and low in the walls, set up a natural flow that carries vapour out. A gap along the eaves that is too small, or blocked by debris or insulation, is a common fault. Ventilation will not cure condensation in every case, but it reduces the amount that forms.
Anti-condensation sheets and insulation
The most direct fix is to keep the underside of the sheet from reaching the dew point. Sheets are sold with a fleece or felt backing that absorbs moisture and releases it slowly as the air dries, so that it does not drip. They work well for the usual run of cold nights, but they can become saturated in a prolonged damp spell. Insulation does the opposite job, keeping the inner face warmer. Rigid boards fitted under the sheets, or insulated composite panels, reduce temperature swings.
Fitting insulation under existing sheets needs thought, because it hides the underside of the roof, where leaks would show, and it can trap moisture if the vapour path is wrong. A void with proper ventilation above the insulation is the safer design. Our guide on whether to insulate a garage roof goes through the options.
Why a new roof can make it worse
Older roofs often leaked air through their gaps and laps, which let moisture drift out. A replacement with tight laps, sealed ridges and closed eaves stops that natural drying, and a garage that used to be fine can start to drip. If you have just fitted new sheets and the drips began soon after, the answer is usually to open up a ventilation route, not to blame the sheets.
Dealing with the drips in the meantime
While you sort out the cause, protect what is underneath. Move tools, stored paper and cardboard away from the dripping areas, and put a cover over the car. Wipe wet sheets when you can, and open doors on dry days to let the air change. Rust is the long-term cost. Steel sheets that stay wet underneath rust from the inside, and a rust spot at a fixing or a lap soon becomes a hole. Treat early spots with a rust converter and paint, and watch the screws and washers, which fail first.
When the roof itself needs replacing
Sometimes the drip is the sign of a worn roof with rusted sheets, failed fixings or gaps at the laps, where a real leak and condensation overlap. Where a roof is replaced, the choice of covering can solve the problem at source. Our comparison of garage roof materials weighs metal against alternatives that suffer less from condensation.
If you cannot tell which problem you have, we survey for free, take pictures of the underside and the fixings, and say whether it is condensation, a leak or both. The fixed written quote names what we propose, and ten years of written workmanship cover back any new roof. More is on the garage roof replacement page.
Every roof is different. A free survey with photos settles it for yours.
Get a free survey