Traditional Layout uses the Rabattement Surface to layout an Stereotomic (3D) view of an rafters in a 2D space/plane. There are a few French and German books showing and explaining how to draw out the Stereotomic view of the rafter, but nothing in English. This article is an attempt to explain the Rabattement Surface used in Traditional Layout in English.
Rabattement Surface = Folding Plane or Folding Surface
The first concept that needs to be understood, is drawing the Profile View of the layout. The layout of the Profile View is critical to the rest of the layout. All lines in the drawings are either perpendicular lines or parallel lines. With only a few exceptions.
In the profile view you draw the elevation of the roof above the plan view. You should leave a drawing buffer between the plan view and elevation view for clarity. The seat line of the rafters in plan view and on the Rabattement Surface are established by drawing a perpendicular line to the profile rafter from A1 to A2. Then swing an arc from center point G from A2 to A3 using the radius length equal to G -A2. This will establish the seat line on the Rabattement Surface. Swing an arc from B2 to B3 using the radius equal to G-B2. This establishes the ridge cut line on the Rabattement Surface.
Next, drop perpendiculars from A1 to A and A3 to C. These lines establish the seat lines and the internal\lower plane of the rafters on the Rabattement Surface. Line AC establishes the Internal Edge of Hip Rafter on Rabattement Surface.
Here's an example of dropping perpendiculars from a jack rafter on the Rabattement Surface to establish the bevels angle cuts on the edges and sides of the jack rafter.
The Rabattement Surface is two planes. The tops of the rafters and the lower edges of the rafters.
This next drawing shows how to locate the seat of the jack rafter at the hip rafter. Continue the seat line in plan view to the center line of the hip rafter, point H. From point H draw a line perpendicular to the hip line in plan view. It will intersect the plan view at point I. From I draw a line parallel to the hip rafter line on the Rabattement Surface. It will intersect the jack rafter on the the Rabattement Surface at L1 & L2. Drop perpendiculars from L1 & L2 to the Stereotomic view of the jack rafter.
The perpendiculars from L1 & L2 will intersect the bottom two edges of the jack rafter. From J1 draw the seat line to J3 and from J2 to J4 draw the other side of the seat line.