Use this procedure to miter two roof slabs that do not intersect.
You can select particular edges to miter. You can miter edges of overlapping roof slabs without trimming the entire slab. To accomplish this, however, you may need to insert additional vertices. Only one edge of each roof slab is mitered.
If you need to miter multiple edges of a roof slab, stretch them to overlap the other roof slab and then miter them at their intersection.
Mitering roof slabs by edge
- Select one of the two roof slabs you want to miter.
if the object selected is… then… a slab click 
. 
a roof slab click . 
- Enter e (Edges) to miter selected edges of the roof slabs.
- Select one edge on the first roof slab.
- Select one edge on the second roof slab.