The short answer
A garage roof that drips on a dry day is usually suffering from condensation, not a leak. Warm, damp air inside the garage meets the cold underside of the sheets, cools, and turns to water that gathers and falls. Bare metal sheets are the worst offenders. The remedy is to stop that air reaching the cold surface, by ventilating, insulating or lining the roof, rather than sealing the outside.
What condensation is doing up there
Air always carries some water vapour. Warm air holds more of it than cold air, and when a mass of warm, damp air touches a cold surface it can no longer hold everything it was carrying. The surplus condenses into droplets. A car that has just come in from a wet road, a wet coat, a tumble dryer vented badly or a damp floor all load the air in a garage with vapour.
The underside of a single skin of metal sheeting is about the coldest surface in the building. It follows the outside temperature closely because nothing separates it from the weather. Vapour reaches it, condenses, runs along the corrugations and falls from the lowest point. Gather enough droplets over a cold night and the drip looks very like a leak by morning.
The same thing happens under a thin felt roof over bare boards, or on the inside of a plastic skylight. It is a building physics problem more than a roofing fault, which is why re-sheeting the roof sometimes makes no difference to it at all.
Telling condensation from a leak
The two are easy to mix up and need different cures, but a few observations will separate them.
- Timing. A leak shows during or after rain. Condensation shows on cold, damp mornings and may be worst when it is dry and still outside.
- Position. A leak drips from one spot, often under a joint, a fixing or a crack. Condensation appears across large areas, lines of corrugation or the whole underside.
- Fixings and laps. Staining around a screw head or sheet lap points to water getting in from above. A uniformly damp underside points to vapour.
- The pattern through the week. If the dripping follows days when the car has been washed or damp laundry hung up, think vapour.
- The sheets themselves. A cracked sheet or a lifted ridge lets rain in. Intact sheets that still weep are more likely sweating.
Both can occur together, and a garage with both is common. If you are unsure, a roofer can look from above and below on a damp day and tell you which problem you have. The page on a leaking garage roof covers the rain-entry side.
Which roof coverings sweat most
Single-skin profiled metal is the classic case. Steel box profile and corrugated sheet conduct cold very efficiently and have no thermal mass to slow the change. Typical UK sources put the life of metal roofs at 40 to 70 years, and the sheets can outlast the use of the building, but condensation under them is a recurring complaint.
Fibre-cement sheets, including older asbestos cement ones, are less prone to dripping because the material is thicker and a little more absorbent, though they still run cold. If your garage has these sheets, the drips are less likely to be condensation, and a crack or a failed fixing becomes a bigger suspect. Because asbestos is a possibility on any older sheeting, do not drill, scrub or brush it to investigate. Our page on painting or sealing an asbestos roof explains why.
A flat roof of felt or single-ply laid over timber can suffer a different version. Vapour passes into the deck from the garage below, meets cold timber, and condenses inside the structure, where it rots joists and stains ceilings. That is harder to spot because nothing drips at first.
What cures it
The aim is always the same: keep the warm, damp air away from the cold surface, or stop the surface being cold. There are four routes, and they can be combined.
- Cut the moisture at source. Dry the car in the open, do not hang washing in the garage, vent any dryer outside and fix damp floors and walls.
- Ventilate. Air vents high and low let damp air escape. Without airflow the vapour just accumulates.
- Line the underside. Many metal sheets are sold with an anti-condensation felt fixed to the underside, which absorbs droplets and releases the moisture as the air dries out. It can also be retro-fitted as a coating or lining.
- Insulate. A properly insulated roof keeps the inner surface warm. Insulation without a vapour barrier below it can move the problem rather than solve it.
Replacing the covering with an insulated panel or a warm-deck flat roof deals with the cold surface at the source. Our page on whether to insulate your garage roof covers the options and the trade-offs.
What not to do
The commonest mistake is treating condensation as a leak. Paying someone to seal laps, smear mastic along every fixing or paint the sheets from outside costs money and leaves the drips untouched, because the water was never coming from outside.
Spray foam applied to the underside needs real care. The RICS consumer guide cautions that foam can lead to condensation that nobody controls, and some lenders have been wary of it. A garage is a less sensitive building than a house, but wet timber behind foam still rots unseen.
Sealing the garage tight is the other trap. Closed doors, no vents and a full load of damp air is the recipe for heavy condensation. A little deliberate airflow, even a pair of small vents, is better than a perfect seal.
Shared and listed outbuildings
If your garage is part of a block, the cure may be agreed jointly, as the roof is shared. The page on who pays for a shared garage block roof explains how that works. A listed outbuilding or a garage standing within the curtilage of a listed house is another special case, since altering the roof covering or lining can need consent. Our guide to a listed outbuilding roof sets out the questions to ask first.
Whatever kind of building it is, we begin by checking what is actually happening: the sheets, the fixings, the airflow and the way the garage is used. Where the roof really does need replacing, our garage roof replacement service builds the right lining and ventilation in from the start, so the drips do not return.
