Flat-roof dormer: the Dutch default, done well
Short answer
A flat-roof dormer has a horizontal (or slightly sloped) top instead of a pitched one. It gives the most interior headroom per square metre of roof, is the cheapest type to build, and lasts decades if the roof covering and the edges are detailed properly.
What it is
A vertical front wall with windows, two short side walls, and a flat top finished in EPDM, bitumen membrane or zinc. The flat top is never truly flat: a 1–2° fall is built in so rainwater runs to the back or side, not towards the joint with the existing roof.
Pros and cons
- Pro: the most usable headroom for a given dormer footprint.
- Pro: the cheapest type — simpler frame, fewer materials.
- Pro: easier to fit a wide dormer, since there is no pitched roof to manage.
- Con: visually heavier than a pitched dormer; not always the right look for older houses.
- Con: the flat covering is a wear item — plan for replacement every 25–40 years.
The roof covering
EPDM rubber is the most common today: one sheet, very few joints, 30+ year lifespan. Bitumen is cheaper and well understood but has visible joints. Zinc is the premium option — beautiful, very long-lived, but the most expensive and the most sensitive to detailing.
Details that matter
- Built-in fall of at least 1° so water actually runs off.
- Drip edge at the front, no water running down the cladding.
- Upstands at the back and sides, 80–150 mm high.
- Roof covering turned up and over the upstands, not just sealed.
- Inspection access — every dormer roof should be reachable for cleaning.