- Rafters cut through without a replacement structure
- Trimmer beams that are too light or poorly connected
- Sagging of the roof beside the dormer
- A dormer wider than the existing roof structure can carry
Building detail · Dormer
Dormer: trimming of rafters and purlins
A dormer needs an opening in the roof. Rafters interrupted by it still have to pass on their load. That is done with trimmer beams and strengthened rafters beside the opening.
- Trimmer beams above and below the opening
- Strengthened or doubled rafters along the opening
- Check of purlins and bearings
Principle sketch
Build-up of the detail
Plan view of the roof structure around the opening for a dormer. The numbers in the sketch refer to these parts:
- Continuous rafters
- Doubled rafters beside the opening
- Trimmer beams above and below the opening
- Interrupted rafters, picked up by the trimmer beam
- Purlins
- Opening for the dormer
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 width of the opening and the number of interrupted rafters
- The sizes and connections of trimmer beams and rafters
- The load of the dormer itself, with snow and wind
- The condition and bearings of the purlins
- Whether the roof structure stays stable after the change
A large dormer or a full-width dormer may need a structural calculation. Structural calculations
Detail FAQ
Frequently asked questions about this detail
What is a trimmer beam?
A cross beam that picks up the interrupted rafters and transfers their load to the continuous rafters beside the opening.
Does a dormer always need a calculation?
Not always, but for larger dormers or a weak roof structure a structural check is wise and sometimes required.
May a purlin be cut for a dormer?
That calls for a separate structural solution, because purlins carry the rafters. Such an intervention must always be calculated.
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