What is a Roof Truss?
A roof truss is a load-bearing structure of timber beams that together form a repeatable unit in a roof structure or industrial building. The truss consists of two angled rafters connected by a horizontal tie beam. Multiple trusses placed side by side form the skeleton of a roof or hall.
Parts of a Roof Truss
A classic roof truss consists of:
- Posts — the vertical corner uprights
- Rafters — the angled beams that form the roof slope
- Tie beam — the horizontal beam that holds the rafters together and prevents spreading
- Ridge point — where the two rafters meet at the top
- Braces — diagonal stiffeners for additional stability
Types of Roof Trusses
| Type | Description | Application |
|---|---|---|
| Common truss | Rafters run all the way to the ridge | Classic barns, farmhouses |
| Rafter roof | Without tie beam, rafters directly on wall plate | Light roof constructions |
| Hammer beam truss | With a short horizontal hammer beam at mid-height | Large spans, historical |
| Lattice truss | Steel or timber lattice | Industrial halls, large spans |
Why Use Roof Trusses?
- Span — a truss can span large spaces without intermediate supports
- Repeatable — trusses are placed in series with a fixed centre spacing (the “bay”)
- Self-supporting — the triangle system (rafters + tie beam) is structurally stable
Spacing Between Trusses
The spacing between trusses is called the bay width or truss spacing:
- Light constructions (shed, storage): 1.2 – 2.4 m
- Heavy constructions (barn, hall): 3 – 6 m
Building a Truss Yourself
When building a garden shed or barn, prefabricated timber trusses can be bought ready-made. This saves a great deal of sawing and they are tailored to the roof pitch.
Prefab truss costs (indicative):
- Small truss (garden shed, 3 m wide): €80 – €200 each
- Large truss (garage/barn, 6 m wide): €300 – €600 each
Related Terms
- Rafter
- Purlins
- Ridge purlin
- Roof structure
- Timber framework
- Timber frame construction
More guides and plans available at fredsdiyplans.com