In Great Britain they use mud, tiny rocks and twigs to build so it should be easy.
Or, is this question a trick? It is a good question since it identifies a primary dilemma in BIM: how much to model, how much to fudge, how much to draft???
Of course you mean a truss or rafters sitting on walls.......
If you make the roof with a simple ROOF TOOL, and place the reference line of the roof against the INSIDE of the perimeter wall, the descending roof element will collide with the wall in an approximation of the bird'smouth notch often placed in conventional rafters.
Otherwise you might consider creating trusses with the TRUSSMAKER Add-On, or the ROOFMAKER that automatically analyzes the plan and spaces placed rafters according to your spec. The roof tool can then be used to place sheathing and other planar elements layered on top of the rafter array.
The section or detail taken through the joint is then gussied up with drafted fills, annotations and linework.
So, fire a roof on there and raise and lower it until it looks right. Cut your detail and draft away with things like vapor barrier, tie clips and whatever.
No SEO's are needed. Walls can trim to roof planes, but this technique is crude if you expect to detail the connection.
Dwight Atkinson