Dormer face

Objective

Find the position of the rear dormer face.

Principle

The rear face has to retain a minimum of 200mm of existing roof tiling at the eaves, measured along the pitch.

Depending on the angle of the roof and the and the overhang of the fascia board. This means that the inside face of the dormer may be set back further within the building than the existing brickwork on the second floor. This is fine for most cases, but may impact the stairs..

The thickness of the dormer depends on whether the face has to carry doors or windows -

• If it carries windows, then the wall thickness is smaller

• If it carries doors then it has to be thicker. The critical point for the position in the dormer is set by the extent of your new joists.

How to do it

Find the point out which the rafters intersect for the top of the new joist. Position the external face of the dormer at this point.

Check that the minimum 200mm tiling is retained.

If it is not then move the dormer face backwards until the 200mm tiling margin is achieved.

The interior of the structural part of the dormer face sets the span of the flat roof joists.

Notes

• If the wall thickness changes then the span will change also and that can mean a thicker or thinner flatroof joist.

• If the if the client is going for a London roof style loft then it may be a good idea to bring this further back in order to not exceed the maximum volume of a new loft extension.