The Shadaswell Guide to Building A Dormer Window. Step 1

Install a pair of treble width rafters either side, then verticals, top and sides to make the side triangles and square opening :


The front timbers align vertically with the centre line of the stone course of the wall. The mullion will therefore sit inside this frame. I am not sure about the top horizontal though. Maybe it is temporary to give the structure rigidity, because I would have thought it would have impeded building up the stone off the top of the mullion. Maybe they are not planning to do this ? I must find out tomorrow.

The builder did try to explain to me why a triple rafter was needed each side, but I didn't follow the argument. Hopefully it will all become clear.....