What is a Cross Beam?
A cross beam is a horizontal structural beam placed perpendicular (crosswise) to the main span direction of a structure, intended to distribute forces, stiffen the construction, or bridge an opening. Cross beams are found in floors, roofs, bridges, and truss structures.
Cross beam vs. joist/beam
| Element | Direction | Function |
|---|---|---|
| Beam / joist | Along the span | Carries the load across the full span |
| Cross beam | Perpendicular to the beam | Connects and distributes forces between beams |
| Frame / bent | Entire frame assembly | Combination of beams, cross beams, and uprights |
Applications
Floor construction
In timber floors, cross beams are fitted between the main joists to:
- Limit deflection
- Stiffen the floor
- Reduce vibration
Roof construction
In purlin and rafter roofs, cross beams are used as:
- Ridge connections between rafters
- Collar ties — horizontal members that connect opposing rafters and prevent roof spread
Truss structures
In steel or timber trusses, cross beams are the diagonal and horizontal connecting members that stabilise the entire structure.
Sizing for DIY builders
A cross beam in a timber floor or deck:
- Span up to 2 m: 45 × 120 mm C24 softwood
- Span up to 3 m: 45 × 145 mm C24
- Span up to 4 m: 45 × 195 mm C24
- Centre-to-centre spacing: 400 or 600 mm
> Always verify with a beam calculator or consult a structural engineer for larger spans.
Related terms
- Beam
- Joist
- Roof frame
- Truss
- Ridge beam
- Centre-to-centre spacing
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