- A lead flashing that is too short or does not lie properly over the tiles
- An interrupted underlay that lets leaking water into the construction
- A thermal bridge between the roof insulation and the dormer wall
- Condensation due to a missing vapour control layer in the dormer
Building detail · Dormer
Dormer: connection to the roof and weatherproofing
Where a dormer sits on the roof, tiles, underlay, insulation and the dormer wall all meet. A good connection keeps the roof watertight and prevents cold spots in the new room.
- Lead flashings over the tiles for a watertight transition
- Roof and dormer insulation that connect
- A front wall and window frame that suit the roof
Principle sketch
Build-up of the detail
Vertical section through the front of a dormer on a pitched roof. The numbers in the sketch refer to these parts:
- Roof tiles
- Tile battens, counter battens and waterproof underlay
- Roof boarding with insulation
- Dormer front wall (insulated timber frame)
- Lead flashing over the tiles
- Window frame with insulating glass
- Interior finish
- Flat dormer roof with roofing
Common mistakes
Where it often goes wrong
Leaks, condensation and cracks usually start at the junctions, not in the middle of a wall or roof.
In the drawing
What we look out for
We work out the detail for your home, based on a site survey and the chosen materials.
- The lap of lead flashings over the tiles and along the cheeks
- A continuous waterproof underlay
- Insulation and vapour control designed together
- The trimming structure under the dormer
- Drainage of the dormer roof
How the rafters under the dormer are supported is shown in the trimming of rafters and purlins.
For drawings and a permit for a dormer in Amsterdam: dormer drawings at Aboss Bouwadvies (in Dutch).
Detail FAQ
Frequently asked questions about this detail
What does a lead flashing do on a dormer?
A lead flashing covers the transition between the dormer wall and the roof tiles, so rainwater runs off over the tiles instead of into the roof.
Can a dormer be added without renewing the whole roof?
Often yes. Around the dormer, tiles, battens and underlay are adapted and connected. The condition of the existing roof decides how much has to be replaced.
How do I prevent condensation in a dormer?
By designing insulation, a vapour control layer on the inside and adequate ventilation together.
Can I use this principle sketch for my own home?
The principle sketch shows the build-up, but it is not a working drawing. Dimensions, materials and fixings depend on your home. We work out the detail for your situation.
Detail library