Steel trusses versus timber; triple timber roof

Why steel trusses are popular compared to timber

1. Higher strength: larger spans (30-100 m) are possible with lighter members.
2. Timber is scarce and costly, while steel is available in standard sections.
3. Durability: not attacked by termites, fungus or rot; long life with painting.
4. Fire safety: steel does not burn, unlike timber (though it should be protected, as it loses strength at very high temperatures).
5. Prefabrication: quick erection with bolts or welding, little wastage, and members can be reused or recycled.
6. Uniform quality and predictable strength, suited to seismic design.
7. Fewer and slimmer members, which makes them economical for sheds and halls.

Triple timber roof (types)

A triple (framed) roof is used for spans above about 9 m, typically for halls and sheds. It consists of a framework of trusses placed at 3-4 m spacing:

• Principal rafters (of the trusses) carry the purlins laid across them.
• Common rafters rest on the purlins and carry the battens or boarding.
• The trusses are tied by a tie beam and braced with a king post or queen posts, struts and straining beams.

Since three sets of members (trusses, purlins and common rafters) are used, it is called a triple roof. Its types, by the truss used, are:

• King post truss for spans of 5-8 m.
• Queen post truss for spans of 8-12 m.
• Mansard truss (double slope) for roofs needing an attic room.
• Combined (king-and-queen post) truss for spans above 12 m.