Building components · Frame Members
Truss Chord
Explains the chord as one edge of a triangulated assembly, and why its force comes from the whole truss rather than from what sits on it locally.
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 chord is
A truss has two edges and a set of members between them. The chords are the edges: continuous members running the length of the truss, one along the top and one along the bottom, connected at intervals by the members in between. Everything the truss spans over resolves into force along those two edges.
The resolution is what makes a truss efficient. A solid member resists bending through its own depth; a truss separates that action into a chord being pushed along its length and a chord being pulled along its length, with the distance between them doing the work that solid depth would otherwise do. That is why a chord's force depends on the whole assembly and on where along the truss it sits, rather than on what rests on it at that point.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a beam. A beam resists bending within the depth of one member, and its worst condition is commonly at midspan; a truss resolves the same bending into compression along one chord and tension along the other, so the chord is one half of a couple rather than a member acting alone.
- It is not a truss web member. The chords run continuously along the edges of the truss; web members sit between them, each connecting one joint to another, and are commonly discontinuous by design.
- It is not a prefabricated roof truss. That is a published material entry — the whole truss procured and delivered as a unit; the chord is a position within such a unit, and the same position exists in trusses fabricated in steel or cut on site.
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.
- truss boom
- rafter chord
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Which chord it is, since the chord being pushed and the chord being pulled behave in entirely different ways and fail in entirely different ways.
- Restraint along its length where it is in compression, because a long member being pushed along its length depends on being held in line.
- The joints where web members connect to it, which is where its force changes and where the assembly either works or does not.
- Continuity, including any splice along its length, since a chord that is not continuous is only as continuous as the joint that makes it so.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Force in a chord changes along the truss rather than being uniform, which is why the same member can be lightly worked at one point and heavily worked at another.
A truss deflects as an assembly, and a brittle finish fixed to the bottom chord is responding to the whole truss rather than to its own local support.
Bottom chords are the most convenient thing in a roof space to hang, fix to or load, and none of those uses is part of what the truss was arranged to do unless the design says so.
Trusses are commonly fabricated to one standard of accuracy and the structure they land on built to another, and the chord ends are where that difference has to be resolved.
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.
Clarifications
Commonly misunderstood
- That the top chord of a trussed rafter is simply a rafter and can be cut or notched like one. Its force comes from the whole truss, and cutting it changes every other member.
- That the bottom chord is a ceiling joist. It commonly carries a ceiling, but its defining duty is resisting the truss's tension, and loading it as a storage floor is a different question that belongs to the design.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this chord in tension or in compression along the part being considered?
- What restrains the compression chord along its length, and does any proposed work remove that restraint?
- Is the chord continuous, and where are its splices?
- What may be fixed to, hung from or stored on the bottom chord in this particular roof?
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 splice position along a chord and no spacing of the restraint a compression chord depends on, and neither can be inferred from it.
- Cutting, notching or loading a truss member is structural alteration requiring qualified design, including where the member looks like an ordinary rafter or joist.
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 →