The last couple of days we've been we've been constructing a dorma window. We started by putting some ridge on a high shelf and using that as a hip end. We then laid plates on the floor accordingly, backworking to get the correct span based on the set height of the ridge on the shelf, and filled it in as usual but using a 45degree pitch and not fixing in a jack but leaving a 900mm space in the center as seen in the diagram created by Robert Spencer.
The next step was to decide on the details of the Dorma, particularly the height making sure it wouldn't be higher than the main pitch and the size of the window (in courses) keeping in mind the purpose of the window is to let in light. With all the details decided we framed it out and pitched the roof. It was a great little exercise and I learnt a few interesting things in the process including that the angle of the valley boards that meet on the ridge is the same as the purlin edge cut and the angle of the vb's on the jack is the creeper side cut.
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