Homestead
Timber Frames

King Post Truss

June 1, 2024
pavilion with king post truss

What is a king post truss

A King Post Truss is a timber framing system used to construct roofs. The key elements are the horizontal tie beam spanning the room below, the vertical post that supports to top called the "king post" extending up from this tie beam to the roof apex, two diagonal struts leaning against the king post, and supplementing rafters that may subdivide the spans. These members form a triangular, roof-supporting truss. 

King Post Design

Through-Tenon 

The critical connection is the through-tenon joint between the king post and tie beam. Instead of the king post simply resting on the beam, its bottom end has a rectangular tenon that projects horizontally into a mortise (hole) cut through the tie beam's thickness. The tenon passes completely through this mortise hole so its end protrudes out the other side.

Hardwood wedges are then driven into the gaps between the tenon and mortise hole. This wedging action locks the joint tightly together. The through-tenon prevents the king post from being pushed or pulled through the tie beam below.

When roof loads push down on the principal rafters, the compressive forces get channeled into the vertical king post. These downward forces flow through the solid tenon and directly into the horizontal tie beam, rather than getting stopped at the beam's surface. The through-tenon allows a direct transfer of forces between members.

The triangular geometry of the truss, combined with the through-tenon's force transfer, gives this framing system its strength using only wood joinery without metal fasteners. Loads can span long distances through the truss members.

When properly constructed with tight-fitting, precisely cut joints, the through-tenon King Post Truss provides a sturdy roof structure. The forces follow a clear load path - down the rafters, into the king post, through the tenon, and into the tie beam. This efficient structural design has been used successfully in structures like barns, pavilions, and Timber frame homes.

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