Why Dormers Leak and How They Are Fixed

Why dormers are prone to leaks

On an open roof, water runs straight down the slope and off the edge. A dormer interrupts that path with vertical walls, a second small roof and new joints on every side.

Each of those joints relies on flashing to keep water out. Wherever a piece is missing, short, lapped the wrong way or sealed only with caulk, water finds its way in, often during heavy or wind-driven rain.

The usual dormer leak points

Building trade guides and home inspectors point to the same handful of spots. Check these first when a dormer leaks.

The top corners

The top corners are the most common trouble spot, especially on shed dormers. That is where the flashing along the sidewall meets the flashing across the top of the dormer.

A gap at that junction lets water run behind the flashing. Taller flashing pieces and a patch of self-sealing membrane at each corner are a common fix.

Step flashing along the sidewalls

Step flashing is a row of small bent metal pieces woven into each course of shingles where the roof meets the dormer wall. Each piece laps over the one below, so water is handed down the wall and onto the roof.

Leaks start when pieces are missing, too short, or nailed through the wall leg. Trade guidance is to fasten only the roof leg, so the joint can move without tearing, and to run membrane and wrap up the sidewall behind the flashing.

Where a valley meets the dormer

Where a valley runs into the side of a dormer, a large amount of water is pushed toward one spot. The junction at the bottom of that valley is a known weak point.

Our article on roof valley types explains how valleys carry water. At a dormer, that flow has to be turned cleanly onto the roof below, or it backs up into the wall.

The dormer roof itself

A dormer has its own small roof, with its own ridge, flashing and edges. If that roof is shallow, it needs roofing suited to its slope, and worn or damaged shingles there leak like anywhere else.

Dormer windows and siding

Many dormer leaks are not roof leaks at all. Water can get in around the dormer window or behind the siding on the dormer walls, then show up on the ceiling below.

Siding that runs too tight to the roof surface, or a missing kickout flashing where the dormer wall meets the roof edge, can send water into the wall. Our guide to mold behind siding explains how that water gets in, and our siding repair service fixes the wall side.

Gable dormers and shed dormers

A gable dormer has a peaked roof, and a shed dormer has a single sloped roof. Building research guidance notes that shed dormers generally create fewer flashing problems than gable dormers.

A gable dormer acts a bit like a wide chimney cricket, splitting water to both sides. That puts more water against both sidewalls and into the valleys where the dormer meets the main roof.

How to narrow down a dormer leak

Start with where the water shows up inside and when it happens. A stain below the dormer window points to the window or the wall around it, while a stain at a ceiling corner often points to the top corners or the valley.

Note whether the leak appears only in heavy or wind-driven rain. Our article on why some roofs leak only in heavy rain explains how wind pushes water into flashing gaps on the side facing the storm.

If you can reach the attic safely, look for wet framing beside the dormer with a flashlight. Our guide to water stains on the ceiling explains how water travels before it shows.

How a dormer leak is fixed

A lasting repair opens the area, replaces what failed, and rebuilds the flashing in the right order. The exact work depends on where the water gets in.

1. Open the area. Shingles and, where needed, siding are removed along the leaking side or corner.
2. Check the wood. Wet or soft decking and sheathing are replaced before anything is covered up.
3. Add membrane. Self-sealing membrane goes on the roof and up the dormer wall at the joint.
4. Reset the flashing. New step flashing is woven into each course, with counter flashing or the wall wrap and siding lapped over it, and patches at the top corners.
5. Close up the wall. Siding goes back with proper clearance above the roof, and a kickout is added where the dormer wall meets the roof edge.

Because the roofing and the siding both have to be right, it helps to have one crew handle both sides of the joint. Our roof flashing repair work covers dormers, sidewalls, chimneys and skylights.

Why caulk is not the fix

Caulk and roof cement can slow a leak for a while, but they break down with sun, cold and movement. A flashing joint stays watertight because the pieces overlap, not because the gaps are glued.

When to call for a dormer repair

Call when you see a stain near a dormer, damp wood in the attic beside it, or peeling paint and soft trim on the dormer walls. The sooner a dormer leak is fixed, the less sheathing and framing needs replacing.

Buckshot Exteriors is family-owned, has served homeowners since 1979, and has offices in Madison, Wisconsin and Centerville, Iowa. Our on-site inspection is free, and we handle both the residential roofing and the siding around your dormers.

Call (608) 909-9109 to schedule your free inspection, or send us a message and we will set a time. If you would rather check first, the stain-location guide above helps you narrow it down.