Condensation dripping from a garage roof

1Why a single skin of sheeting sweats

Air always holds some moisture, and warm air holds more than cold. Where it brushes a surface cooler than itself, the surplus condenses into droplets. A single skin of corrugated sheeting, metal, plastic or fibre cement, is thin and has no insulation, so on a cold night its underside is cold. In the morning, as the garage warms and the air picks up moisture, the droplets form and run to the lowest point of each corrugation, then drip.

It can look like a leak. The pattern is different, though. Condensation tends to appear across a large area of the underside at once, all along the troughs of the sheets, rather than under one lap or fixing. It often happens on dry days and after cold nights, and it stops when the weather changes. A true leak follows rain.

Flat roofs behave similarly when the deck is cold and the ceiling is uninsulated, although the water usually collects on the timbers and the boarding rather than dripping from a sheet. The first step is deciding which you have.

2Where the moisture comes from

A garage is not a sealed box, but it holds more moisture than people expect. A car that arrives wet from the road, a tumble dryer vented into the space, damp washing hung on a line, a leaking gutter, wet concrete and stored cardboard all add water to the air. A floor with no damp-proof layer feeds it constantly.

Cutting the moisture at the source helps more than any roof treatment. Dry the car outside if you can, vent a dryer outside, keep stored goods off the floor and mend any dripping gutters. Our page on garage roof guttering covers the water side of that.

Ventilation helps too. A fixed vent high and one low allow damp air to move out rather than sit under the roof, and a small gap at the eaves, left open, can do much of the work. Stopping up every gap to beat the draught makes condensation worse, because the humid air is trapped under the roof.

3Anti-condensation linings, felt backing and insulation

Where sheets are the problem, there are several ways to stop the drips, each with limits. A felt or fleece backing, glued to the underside of metal or plastic sheets, absorbs some moisture and releases it slowly as the air dries, which stops the drops forming. Anti-condensation coatings do something similar. Both are cheaper than replacing a roof, and suit sheets in sound condition.

Insulation addresses the cause: a warmer inner surface that does not reach the dew point so easily. Boards or insulated panels fitted beneath the roof structure, or a replacement insulated roof system, can solve the problem for a garage you want to use as a workshop or store. Our page on garage roof insulation covers the options.

With any lining, ventilation still matters, because a layer that traps moisture against a timber deck can cause rot. On flat roofs, the way the layers are built and ventilated decides whether a fix holds, so we examine the build-up at the survey and photograph it.

4Asbestos cement sheets: do not scrub, drill or line them yourself

If the garage was built before 2000, HSE advises assuming asbestos is present, and large corrugated sheets of that age may be asbestos cement. That affects what you can do about dripping. Fixing felt backing means working on the underside with adhesive or fixings. Drilling, scraping or brushing the surface can release fibres.

HSE labels roofs of this material fragile, so climbing on top to deal with the problem is not an option either. Read our page on whether an asbestos garage roof is safe to walk on before anyone goes near it. The overview of garage roof replacement lists the alternatives.

If you have a suspect roof, the best response is to leave the surface alone, manage the moisture, and have it assessed before any lining goes in. Sometimes the most sensible long-term fix is a different roof, and the guide to removing asbestos sheets yourself explains why that is not a casual weekend job.

A damp-looking roof on a suspect garage is a survey, not a DIY, question.

5Telling condensation from a leak, and what it costs to fix

A few checks separate the two. Look on a dry, cold morning: if drips fall with no rain in the previous day, it is condensation. Look for staining around fixings and laps after rain, which points to a leak. Feel the underside of the sheets: cold and wet across the whole surface suggests condensation, while damp in streaks beneath a particular lap suggests water coming in.

Both can exist together. A small leak in a roof that also sweats is common, and sealing the outside does nothing for the second problem. So the cause comes first: we trace it, and show you photographs of your own roof.

Cost depends on whether you want a lining, a replacement roof or an insulated system. National guides give typical UK ranges for a full garage roof replacement, which our page on garage roof cost sets out with the source and date. The quote, in writing and as a single total, follows the survey. Photographs of the underside and the outside help us judge the next step.