I guess I could send the question to math.se, but I think we can figure that one out ourselves; it's just a bit of trigonometry after all, right?
- The large side is 20 ldu (standard brick width).
- The small side is 16 ldu (2 plates) - 4 ldu (half a plate), that is 12 ldu (half the height of a brick).
So the angle is tan-1(12/20), which is ...
You have a Fabuland Roof Support Slope, 6 x 2.
If it's like the similar ones I've seen in Duplo, it probably came pre-assembled into one of these:
Fabuland Roof Support with Gray Roof Slope without Chimney Hole
Fabuland Roof Support with Red Roof Slope with Chimney Hole
Fabuland Roof Support with Red Roof Slope without Chimney Hole
These curves have been approximated by the contributors to the LDraw project. You can download free software to view and use the part files, from which you can approximate other points on the curve. For example, here is an LDraw part file shown in "wire-frame" mode:
LDraw parts are drawn in increments of the "LDraw Unit" (LDU) in which a standard 1x1 brick ...