Creating 3D Geometry - AutoCAD 2011 and AutoCAD LT 2011: No Experience Required - page 899

FiGuRE 16.85 The boxes to be used to subtract the roof
FiGuRE 16.86 Extend the outside edges of the boxes.
5. Move the boxes so that they protrude through the roof using the end-
point of one edge as the first point of displacement and the midpoint
of the same edge as the second.
When you're finished, the model should look similar to the one
shown in Figure 16.87.