Top chord (truss)
The top chord is the upper continuous member of a truss, in compression under gravity load and following the roof pitch, connected to the bottom chord by web members.
Ask Chalkline about this →The top chord is the upper continuous member of a truss (or an I-joist). It runs along the roof pitch, is in compression under gravity load, and is connected to the bottom chord by the internal web members.
A truss works because its two chords act as a pair: under normal downward load the top chord is squeezed (compression) and the bottom chord is stretched (tension), and the webs triangulate between them. The top chord follows the roof line and carries the battens or purlins and the roofing above it, so it takes both the axial compression along its length and bending between its support (web) points.
Because it is a compression member, the top chord’s enemy is buckling: a long compression member wants to bow sideways out of plane. That is why top chords have to be braced, the roof battens or purlins, plus temporary and permanent bracing, hold the top chord straight so it cannot buckle. Under-bracing a roof, or removing bracing, is a common cause of trusses leaning or a roof racking.
For a builder the practical points are to brace the top chords as the truss layout and AS 4440 require (this is not optional, an unbraced top chord is a buckling risk during and after construction), not to cut or notch the top chord or hang unplanned loads on it, and not to alter the web connections. Fix the battens or purlins at the specified spacing because they are part of the top chord’s restraint, not just a fixing for the roof. A bowed or damaged top chord found at frame inspection needs an engineer’s detail, not a packer.
Also known as: Top boom, rafter chord.
Category: Roof framing / Trusses.
Related
See also
References
- AS 4440 Installation of nailplated timber roof trusses, Standards Australia (verified 2026-06-01)
Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.