Tim Moore's blog on Stereotomy.
The art of representing objects in section, elevation and plan in order to cut them out. - Louis Mazerolle
Tim has some excellent graphic image drawings of devers de pas. The DP line or the angle of projection line and he has done a great job of explaining how to develop the devers de pas.
Geometry is a language without words.However, without being able to read French it's hard to tell if Louis Mazerolle explains how to develop the foot print of the skewed rafter at point G in the drawing on page 52. If all of the rafters are the same width, then you need to develop the real roof surface at the skewed rafter side of the Trapezoidal Ground Plan. The width of the skewed rafter on the ground plane is developed from the width of the rafter on the real roof surface and then the lines parallel to the DP line can be drawn in on the ground plan to represent the foot print of the skewed rafter.