A dormer is generally constructed from timber. The main parts that form a dormer are the roof, side walls (cheeks) and front wall which faces the garden. The cheeks can be supported in one of two ways:
- The rafters can be doubled and bolted together with the cheeks then constructed off the rafters.
- If the dormer width means the cheeks are at the edges of the roof then the cheeks can be taken down to the floor and supported off the floor joists (which are doubled) or on a beam, or in some cases by the party or external walls.
The front wall of the dormer can be supported off the external wall, or if it is to be set back from the external line of the house, it can be supported off the new floor joists, which should be designed to cater for the extra load of this wall (see also external walls).
The dormer may well need to be constructed so as to give resistance to a fire spreading to or from a neighbouring property – the nature and extent of the construction to give this fire resistance will be dependent on the size of the dormer cheek and its proximity to the boundary.
Removal of rafters
To enable a window, rooflight or dormer to be installed when creating new room(s), it is normally necessary to cut an opening in the existing rafters.
The remaining sections of the cut rafter(s) can be supported by the new dormer or, in the case of a new window/rooflight, will need to be supported by installing new timbers (known as trimmers) across the head (top) or sill of the new opening.
Depending on the size of the new opening, these may need to be two timbers fixed together (double trimmer) so that they can adequately transfer the load to the existing rafters on either side of the new opening.
It is generally good practice to strengthen the rafters on both sides of the opening as they are now taking more load. This can be achieved by bolting another rafter of the same size and length to the existing one.