A truss is a triangular or A-shaped load-bearing frame in a roof construction that supports the weight of the roof and transfers it to the walls or columns. Multiple trusses side by side, connected by purlins, together form the roof structure.
How does a truss work?
The triangular shape is the key: a triangle is the only geometric shape that cannot deform without breaking one of its sides. This makes the truss extremely stable.
A truss consists of at least:
- Two rafters (sloped beams) — form the roof slopes
- A collar beam or tie beam — connects the rafters horizontally and prevents them from pushing outward
For larger spans, additional elements are added: struts, compression members and tension members that distribute forces efficiently.
Types of trusses
Simple truss (A-frame)
Two rafters with a collar beam. Suitable for spans up to approximately 6-8 metres. The most common form for sheds and garden buildings.
Lattice truss
A truss with multiple compression and tension members in a triangular pattern. Can handle large spans (10-20 metres) with relatively lightweight materials. Often in steel.
Scissor truss
A decorative truss where the rafters and collar beam remain visible. Popular for open roof constructions (e.g. a barn or rural dwelling).
Prefab truss (W-truss)
Factory-made truss of thin wooden members, connected with steel nail plates in a W-pattern. Lightweight, affordable and quick to install. Standard in new-build homes.
Truss spacing
The spacing between trusses (centre-to-centre) determines the load per truss:
| Truss spacing | Application |
|---|---|
| 60 cm c/c | Lightweight roof covering, small structures |
| 80-100 cm c/c | Standard for sheds and garages |
| 100-120 cm c/c | Heavier trusses, larger structures |
The closer the trusses are together, the lighter each individual truss can be.
Trusses in self-build projects
When building a shed, garden building or garage, trusses are the core of the roof construction:
Making your own
For simple constructions (spans up to
5-6 metres):
1. Cut rafters to size — Calculate the length based on span and roof pitch
2. Birdsmouth joint or steel angle brackets — Join the rafters at the ridge
3. Attach collar beam — At 1/3 from the ridge, with bolt or screw connection
4. Place on the wall plate — Secure the truss with angle brackets or birdsmouth joint
Having them made
For larger constructions or when you need many identical trusses, you can have them cut and assembled by a carpentry firm. This is more precise and saves time.
Common mistakes
- Forgetting the collar beam — Without a collar beam the rafters push the walls outward
- Rafters too lightweight — The rafters must carry the roof weight plus wind and snow loads
- Trusses not plumb — Crooked trusses result in an uneven roof. Use temporary bracing to hold them upright during installation.
Related terms
- Roof structure
- Purlins
- Rafter
- Ridge purlin
- Wall plate
- Collar beam