Bottom chord (truss)

The bottom chord is the lower continuous member of a truss, in tension under gravity load and usually carrying the ceiling. It is the member most easily damaged by single-point lifting on long trusses.

A truss is a triangulated frame of a top chord, a bottom chord, and internal webs, all connected with nailplates. Under normal (gravity) load the top chord is in compression and the bottom chord is in tension, the two work as a pair so the truss spans further than a single beam could. The bottom chord also commonly carries the ceiling lining and, in some designs, a limited storage or services load if it was designed for it.

The reason AS 4440 makes a point of handling is that a truss is strong in its own plane but weak the other way: a long truss lifted by a single point in the middle (a crane choke at the apex, say) puts the bottom chord into bending and compression it was never designed for, and it can buckle or crack a nailplate joint. Trusses are designed to be lifted at the panel points the supplier nominates, often with a spreader bar, and braced as they go up.

For a builder the practical points are to lift and handle trusses as the supplier and AS 4440 specify (panel-point pick-up, spreader bar on long spans, keep them vertical), not to cut, notch, or hang unplanned loads off the bottom chord (it is a designed tension member, not a handy beam), and not to store heavy materials on bottom chords unless the truss was designed for it. A damaged chord or a sprung nailplate found after install means an engineer’s repair detail, not a quiet fix.

Also known as: Tie, ceiling tie (in some framing).

Category: Roof framing / Trusses.

Related

• AS 4440 truss installation
• Nailplate

See also

• Roof framing with trusses

References

• AS 4440 Installation of nailplated timber roof trusses, Standards Australia (verified 2026-06-01)

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Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.