The triangle formed by a pair of rafters and a joist is the most basic form of truss, and has been in use in one form or another for centuries. The two rafters can be envisioned as leaning against each other, just as you might lean a ladder against a wall. The vertical forces on the rafters are all transferred to the outer corners of the base of the triangle, which typically bears on the building walls below. The load transfer creates a horizontal force, which I refer to as rafter thrust. This thrust is restrained by the horizontal joist, which provides a restraining force to keep the base of the triangle (and the walls
