What is a trussed roof?
A trussed roof is a roof structure made up of a series of light trusses at a fixed, close spacing. Instead of a few heavy trusses with purlins between them, each truss carries its own strip of the roof. In Dutch it is called a spantenkap; the term is also used for a couple roof, in which each pair of rafters acts as a truss. In current practice it usually means prefabricated timber trusses, often joined with steel nail plates.
Build-up
- Light trusses. The trusses are usually spaced 600 or 1200 mm apart. Each truss is a triangle of two sloping top chords, a bottom chord (tie) and diagonals.
- Roof sheathing or membrane. On top of the trusses comes sheathing of board material or a vapour-permeable membrane, with counter battens and tiling battens for the tiles.
- Bottom chord. The bottom chord can serve as a ceiling hanger or as support for a light, non-accessible loft floor.
- Insulation. The insulation can be placed on the ceiling or between and on top of the top chords.
Application
Trussed roofs are widely used on new-build houses, garages and outbuildings with a non-habitable loft. They are light, quick to erect and need few heavy beams.
- Do not just cut. The diagonals and chords work together as one unit. Removing a diagonal for a loft ladder or storage space can make the truss fail. Always ask the supplier or a structural engineer first.
- The loft is often not designed for storage. Light trusses are often not designed for heavy floor loads. Check this before putting boxes or a boiler up there.
- Stability. Without wind bracing and diagonal braces, the trusses fall over like dominoes. Install them according to the erection drawing.
- Converting to a loft room. If you want to make the loft habitable, a new load-bearing structure is usually needed. Expect a design by a structural engineer.
Common mistake: treating a trussed roof like a traditional purlin roof in which you can freely cut and build. In a trussed roof, every member counts.
Related terms
- Truss
- Roof truss
- A-frame truss
- Roof structure
- Purlin roof
- Collar beam
- Horizontal thrust
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.