Why does my garage roof drip inside?

Timing tells you the most, and a dry spell is the best test, because a drip with no rain in the last few days is very unlikely to be a leak. A leak shows after rain, often at the same few places, and it stops once the roof has dried. Condensation shows on cold, still, damp mornings, even when no rain has fallen for days, and it appears across a wide patch of ceiling or sheets rather than at a single point.

Look at the staining too. A leak leaves a tide mark around one spot, often beneath a lap, a fixing or a rooflight. Condensation leaves small beads across the underside, black spotting on timber and a general damp smell. Metal bolts and brackets weep first, because they are the coldest points in the building.

If you cannot decide, take a few photographs over a week and note the weather beside each one. Our page on a leaking garage roof covers the other side, where water really is coming through the covering.

The air in a garage carries moisture from wet cars, damp tools, stored furniture and the concrete floor itself. When that air touches a surface colder than it is, some of the moisture drops out as water. A thin corrugated sheet or an uninsulated boarded deck is that kind of surface, since the underside follows the outdoor temperature within minutes.

That is why old fibre cement and steel sheet garages drip even when the roof keeps rain out. Nothing is wrong with the covering. It is a cold surface in a damp room, and swapping one cold sheet for another will not change that.

The useful question to put to any roofer is what will sit under the new covering to keep the underside warm, or to let the air move. A good answer names a product and a position. A vague one means the drips are likely to return.

Moisture needs somewhere to go. BS 5250, the code on managing moisture in buildings, sits behind most roof ventilation advice, and BSI currently lists its 2021 edition as under review. Marley's advice for cold roofs lined with a high-resistance underlay is a 25 mm gap along the eaves when the pitch is 15 degrees or less, and 10 mm when it is steeper.

Those figures describe houses with lofts, so they are not a rule for every garage, but the idea carries across. A cold roof needs a clear path for air to enter at the eaves. A flat roof needs either that path or a warm-roof build, where the insulation goes on top of the deck and keeps the structure at room temperature.

On a small garage the answer is often simpler: an air brick or vent in an end wall that lets damp air out, plus a covering built to a proper fall so water never sits on it. Our repair or replace page explains how we decide between those routes.

Insulation lifts the temperature of the surface that was dripping, and that stops most condensation. Our page on insulating a garage roof goes through the options in detail.

Badly placed insulation can trap moisture instead. RICS's consumer guide on spray foam cautions that poor installation may bring dampness, and uncontrolled condensation among it. The Energy Saving Trust advises against foaming a roof before a leak has been put right. Lender attitudes are mixed: as of November 2024 some said they would refuse, while others judged each property on its merits. That matters when the garage shares a wall with the house.

The order of work follows from that. First find out whether it is a leak, then fix any leak, and only then decide on insulation and ventilation together, so the two jobs support each other.

Some habits cut the drips at no cost. Keep wet coats, wet dogs and washing out of the garage. If you can, leave the door open for a while after parking a wet car. Lift stored cardboard and fabric off the floor, because they hold moisture and give it back.

A small fan or a dehumidifier running through a cold morning is another quick test of the theory. Check the floor as well. A damp slab is a steady source of moisture in the air. A plastic sheet laid under stored boxes will often show it as droplets on the underside within a day or two.

If none of that helps, a roofer's survey separates the cause from the cure. Our survey covers the ceiling side of the roof as well as the weather side, ending in a plain verdict and a written price as one total. For the whole subject of replacing a garage covering, see our garage roof replacement page.