Use this procedure to miter 2 intersecting slabs or roof slabs.
Both slabs or roof slabs are trimmed along the intersection of their perimeter lines. Vertices are added or removed as required. The new edges of the slabs or roof slabs are set to angles required for a true miter cut. If the slab thicknesses of the two edges are different, the angles are different.
If you want to miter slabs or roof slabs that do not intersect, you can miter them by edges.
Mitering roof slabs by intersection
- Select one of the two slabs or roof slabs you want to miter.
if the object selected is… then… a slab click 
. 
a roof slab click . 
- Enter i (Intersection) to miter at the intersection of the two slabs or roof slabs.
- Select the first roof slab at the side of that slab to keep.
The opposite side is trimmed.
- Select the second roof slab at the side of that slab to keep.
The opposite side is trimmed.