Dormer Flat Roofs: Why They Leak and How We Fix Them

First, the cheeks — the vertical sides of the dormer. Whether they're clad in lead, tile or timber, the junction where the cheek meets the flat top is a movement point: the dormer structure shifts slightly with temperature and wind, and rigid details crack. Second, the back gutter — the channel where the dormer meets the main roof slope. It collects everything running down the pitch above, and if it's too narrow, laid flat, or lined with perished material, water sits in it until it finds a way through.

Third, the upstand — where the flat covering turns up against the dormer face or window frame. And fourth, the flat top itself, which is honestly the least common culprit we find. The diagnostic rule is simple: if your dormer leaks in driving rain but stays dry in steady rain, the water is being pushed sideways into a junction detail — a cheek or upstand — rather than coming straight down through the top.

A re-felt — or worse, a liquid coating — applied to the flat top addresses one of the four failure points. If the leak is at the back gutter or a cheek junction, the new covering changes nothing except the appearance, and the first proper storm proves it. We get called to a lot of dormers that have been 'repaired' two or three times this way, each time with the ceiling stain returning a little bigger.

It's also worth knowing that the flat top can be blameless while looking guilty. Water entering at the back gutter tracks along the top of the deck and drips off wherever the deck dips — so the stain in the room below can be feet away from the actual entry point. Guessing from the stain's position is how the wrong repair gets specified.

A proper job starts with the covering lifted enough to see the deck and the junction details, not just a look from a ladder. The back gutter is rebuilt with correct falls and a decent width, lined in the same system as the new top or in lead. The cheeks are re-detailed — lead or GRP depending on the dormer — with the junction formed so it can move without cracking. The upstand is dressed to a proper height against the frame.

Only then does the top get its new covering — single-ply, GRP or liquid, sized to the dormer — with code 4 or 5 lead flashing into the main roof where the systems meet. Where the dormer is being fully re-roofed we check the deck and any rotten timber is replaced as part of the quoted work, with scaffold priced up front rather than appearing as a surprise on day one.

If the deck is saturated across its whole area, if the cheeks are rotten rather than just badly detailed, or if the dormer was poorly built in the first place — inadequate falls, no real back gutter at all — a strip and rebuild of the dormer roof assembly is the honest answer. It's a bigger job, but it's a one-time job, and it ends the cycle of annual patch repairs that never quite hold.

We'll always show you photographs of what we find and price the repair and the rebuild side by side where both are viable, so the decision is yours and it's based on evidence.