Introduction and requirements
For all types of roof (including cut roofs, prefabricated trusses and flat roofs), the requirements of the roof remain the same. The roof must prevent moisture penetration and excessive deflection, and it must transfer the loads from the roof correctly to the load-bearing walls of the house. The principles of roof construction are relatively straight-forward.
Typical roof types
Diagram D1 - Types of roof
Spacing, performance factors, and guidelines
Prefabricated timber trussed rafters are used in the construction of roofs in the vast majority of modern houses. These rafters are usually spaced at 400 mm and 600 mm centres. Factors on which trussed rafters’ successful performance rely include: appropriate design and fabrication, sufficient bracing, correct distribution of point loads, and adequate fixings to hold the roof down.
BS EN 1995-1: 2005 Eurocode 5: Design of timber structures – Part 1, which is often called Eurocode 5 for short, outlines the guidelines for design that should be followed when designing timber structures and structural elements, including trussed rafters.
With the introduction of Eurocode 5, all conflicting national standards were withdrawn. One such conflicting standard is BS 193: Timber Trussed Rafters for Roofs. This standard has been withdrawn and a revised standard, which is still under review.
The minimum bracing table for trussed rafters (as referred to in this section) is reproduced from the revised standard outlined above. Details of the roof and design drawings should be available to the suppliers to facilitate design and manufacture of the trusses.
BS EN 141250: 2010 Timber Structures – Product requirements for prefabricated structural members assembled with punched metal plate fasteners outlines the tags required on timber trussed rafters. These tags should contain the following:
A reference which proves the truss manufacturer is approved.
The truss manufacturer’s customer name.
A reference to BS EN 14250.
The strength class of the timber.