Dormer roof leaking: where water gets in

Published 6 October 2026 by SA Roofing

Three leak points

The flat roof on top, the cheeks and their flashings, and around the window

Most common

Failed flashing where the cheeks meet the main roof

Typical life

Around 20 to 30 years for a two-layer torch-on roof

Access

Often needs scaffolding, so do all the work in one visit

How a dormer keeps water out

A dormer is a small structure built out of the slope of a main roof, usually to add headroom and a window in an attic conversion. Most dormers in Dublin have a flat roof on top, covered in felt or torch-on, with two side walls called cheeks where the dormer meets the slates or tiles of the main roof. Each of those parts sheds water in its own way, and a leak can start at any of the joints between them. Dormer roofs are covered on our flat roof repairs page, and the cheeks and flashings fall under our roof repairs.

Attic conversions with dormers are common across Dublin, from older semis in Dundrum to estates in Lucan and Dublin 15. Because dormers are high and hard to see from the ground, a problem can run for a long time before it shows inside.

Leaks through the flat roof on top

The flat roof on a dormer has the same faults as any flat roof, with less room for error. The roof is small, so a split, a blister or a failed seam is never far from the edge. Water that ponds in a dip has nowhere else to go, and our guide to water pooling on a flat roof explains why that shortens the life of the felt. The drip edge at the front, where the roof runs into a gutter or over the face of the dormer, is a frequent weak point, as is the back edge where the dormer roof tucks under the slates of the main roof.

Older dormers often have a single layer of felt that has gone brittle. A modern two-layer torch-on felt roof lasts considerably longer, though torching near timber cladding needs care and cold-applied felt is sometimes used at the edges instead.

Many dormer roofs drain to a small gutter at the front or straight onto the main roof below. If that gutter blocks with moss and grit washed off the felt, water backs up over the drip edge and runs down the face of the dormer, where it can get in around the window head. Clearing it is a small job, but it is often missed because the gutter is out of sight from the ground.

Leaks at the cheeks and flashings

The cheeks are where most dormer leaks start. Where each side wall meets the main roof, the slates or tiles are cut to fit against it and a flashing, usually stepped lead or lead soakers tucked under each slate, turns the water away from the joint. When the lead splits, lifts or was never dressed properly, rain running down the main roof gets in behind the cheek and appears inside on the side wall of the attic room or at the corner of the ceiling.

Cheeks clad in tiles, slate or timber can also let water in through the cladding itself, especially on the side facing the prevailing wind. A stain that appears after wind-driven rain from one direction often points to the cheek on that side.

Leaks around the window

Water can also get in around the dormer window. The seal around the window frame, the flashing or sill detail below it and the joint between the frame and the cheek all age. A leak here usually shows directly around the window inside, often at the bottom corners. It helps to know which it is, because a failed frame or glazing unit is a job for a window installer, while the flashing and the roof around it are roofing work. A roofer can check the roofing details around the window and tell you whether the frame itself is the likely problem.

How long a dormer roof lasts

A dormer roof lasts about as long as any flat roof of the same type, so older built-up felt typically gives 10 to 20 years and a well-laid two-layer torch-on roof around 20 to 30. Lead flashings on the cheeks can last much longer if they were fitted properly. Our article on how long a roof lasts in Ireland has typical figures for each material.

Repair or re-cover

A single split, a lifted drip edge or one failed section of cheek flashing can be repaired on its own. When the felt is brittle across the whole dormer roof, the deck feels soft, or the roof has been patched more than once, re-covering it is usually better value. Because dormers often need scaffolding, it makes sense to deal with the flat roof and the cheek flashings during the same visit rather than paying for access twice. Our guide to flat roof repair costs covers what drives the price, and a leak where an extension roof joins the house, covered in our extension roof leaks guide, often has the same causes. A re-cover is also a good moment to check the insulation and ventilation in the dormer roof, because condensation inside a poorly ventilated dormer can rot the deck from below.

Getting it traced

Note where the stain appears inside, on the side wall, the ceiling or around the window, and whether it follows wind from one direction or any heavy rain. Photos of the dormer from the garden or the street help us judge access before we visit. We check the flat roof, the cheeks and the flashings, explain which one has failed and give you a free quote for the repair.

Related

• Flat roofs
• Roof repairs
• Roofers in Dundrum
• Extension roof leaking where it meets the house
• Water pooling on a flat roof: what is normal and how to fix it
• Torch-on felt roofs: how long they last and what to expect