Once the walls have dried (several hours or overnight), place your diorama inside if you have one, then set the roof on the house, one half at a time. There may be a gap at the top, but you can fill that in with frosting or you may take a paring knife and carve a mitered corner to fit (only for obsessive people).
Prop up your roof while it dries with mugs or books of the proper height.
