Why does a stick built roof system have a bird's mouth cut into the rafter and a truss doesn't?

Answer 1

The birds mouth is to give the rafter more surface contact(for better/easier/stronger nailing) with top plate Truss's are constructed so that is not as big a concern.Also a good builder will use metal clips to hold both down(they are called storm clips).Truss's also are stiffer(less bending) and weight more.