King Post Truss

A king post truss is a triangular frame made of two sloping principal rafters, a horizontal tie beam across the bottom, and a single vertical post (the king post) running from the centre of the tie beam up to the apex. It is one of the simplest truss forms and has been used for centuries in timber roofs and bridges.

How it works

The rafters push down and outward; the tie beam resists the outward spread in tension, so the frame does not push the walls apart. The king post connects the apex to the middle of the tie beam. In many designs the post actually hangs from the apex and helps hold the tie beam up, so it works in tension rather than as a prop. Diagonal struts are often added from the foot of the king post up to the rafters to shorten their unsupported length.

In historic British usage the king post is described as the vertical timber supporting the ridge. Historians distinguish it from a crown post, which rises from the tie beam only as far as a collar and carries a collar purlin rather than the ridge.

Use today

The king post form remains common in short-span modern trusses, including small prefabricated timber trusses and garages, and as an exposed decorative feature in timber-framed buildings, porches and gable ends. For longer spans, builders move to queen post trusses or more complex web arrangements.

Related terms

• Queen post truss: two vertical posts instead of one, with a straining beam between them.
• Crown post: a central post from tie beam to collar, typical of medieval south-east England.