King post
A king post is a simple vertical support used in truss designs for roofs and short bridges. It rises from the tie beam up to the apex of a triangular truss. The king post itself is in tension, and it helps hold the tie beam in place, while the two diagonal braces press against the apex.
There are similar-looking parts with different jobs. A crown post looks like a king post but works in compression and rises to a crown plate below the collar beams, not to the apex. In England, crown post used to be called king post, but that usage is now obsolete. If you move the supports to the base of the truss, you get two queen posts; sometimes a collar beam above them leads to the term queen struts, and in some roofs a hammerbeam effect appears when a section of the tie beam is removed.
The king post is the simplest truss, using the fewest pieces: two diagonals meeting at the top, a tie beam at the bottom, and the king post connecting the apex to the tie beam. In roofs, the diagonals are rafters and the tie beam may also serve as a ceiling joist. A bridge would typically use two king post trusses with the deck between them.
Notable uses and examples:
- Pont-y-Cafnau, the world’s first iron railway bridge, is of the king post type.
- King posts appear in Roman timber-framed roofs, medieval churches, and tithe barns.
- The oldest surviving king post roof truss is in Saint Catherine’s Monastery, Egypt (built around 548–565).
- The king post design also shows up in Gothic Revival, Queen Anne, and some modern construction.
Other notes:
- A king post can be replaced by a king rod or king bolt in some designs, giving a king rod truss.
- In aircraft, a king post can support top cables, but those cables are not under tension in flight.
- In machinery and ships, king posts are used for sturdy connections or for mounting cargo-handling or fueling gear.
In short, a king post is a simple, central vertical support that makes a strong, economical roof or bridge truss.
This page was last edited on 2 February 2026, at 10:42 (CET).