Building components · Frame Members
King Post
Explains the king post as a position inside a truss form, and corrects the usual assumption that a central vertical member must be carrying something downward.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a king post is
A king post is the vertical member on the centre line of a truss, connecting the apex where the sloping members meet to the horizontal tie that runs between their feet. It belongs to one truss and stops at that truss: it does not continue below the tie, and it does not reach the ground.
Its direction of working is the part most often misread. In the classic form the tie beneath is the member that would sag under its own weight and whatever it carries, and the king post hangs it up from the apex — so the post is commonly in tension, pulling upward, rather than in compression propping the apex down. That single fact governs how it is jointed at both ends, because a joint that resists a push is not the same joint as one that resists a pull.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a column. A column collects load from what it supports and carries it downward to a foundation or to structure below, arriving as a concentrated force at a point; a king post neither reaches a foundation nor carries the building, and in the usual arrangement it works upward in tension rather than downward in compression.
- It is not a queen post. A king post is single and sits on the centre line; queen posts are a pair set either side of the centre, and the space the pair leaves between them is precisely the space a king post would occupy.
- It is not a collar tie. A collar is horizontal, runs between a pair of opposing rafters above the tie beam and resists them spreading apart; a king post is vertical, runs from the apex down to the tie, and in the usual arrangement carries the tie's own weight upward rather than restraining anything sideways.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- centre post
- king rod
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The joints at each end, since a member in tension depends entirely on connections that can resist a pull rather than merely hold a member in place.
- Which way the truss form actually puts it to work, because a member that looks the same can be in tension in one arrangement and in compression in another.
- The truss being treated as a designed whole, since the forces in every other member follow from the arrangement this one is part of.
- Whether the truss was cut on site or procured as a fabricated unit, which changes what the joints are and what may be done to them.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A member working in tension transfers its force entirely through its end connections, so the joints carry the same significance as the member itself.
King posts sit in the middle of a roof space and are commonly the first thing a conversion wants out of the way, which is where the structural scheme and the intended use come into conflict.
Where the tie below sags, the question is whether the hanging action was ever present, has been lost at a joint, or was never what the truss relied on.
The joint at the tie is a place where moisture can collect in an exposed or poorly ventilated roof, and the condition of a tension joint is not readable from the surface of the timber around it.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That the king post props the ridge up. In the classic king post truss it does the opposite — it hangs the tie beam from the apex and works in tension.
- That a king post truss and a prefabricated roof truss are the same thing. Prefabricated roof trusses are a published material entry procured as a complete unit; the king post is a position within a truss form that may or may not be present in any given roof.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this king post working in tension or in compression in the arrangement it belongs to?
- What are the connections at the apex and at the tie, and are they sound?
- If the roof space is to be opened up, what carries the tie once the post is altered?
- Was this truss cut on site or fabricated, and what does that mean for altering any of its members?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes a position within a truss. It states no tension capacity for the member and no capacity for the joints at either end, and neither can be inferred from it.
- Altering any member of a truss changes the forces in all the others and is structural alteration requiring qualified design.
Structural Frame Members
The columns, beams, joists, rafters, plates and bracing that make up a load-carrying frame, identified by the position each occupies.
Browse all frame members entries →