Pitched roofs may consist of rafters and purlins alone with the purlins supported on posts, masonry walls or primary trusses. They may also consist of an arrangement in which the lower ends of each pair of rafters are tied or trussed together.
The most common form of roof construction, particularly for housing today are trussed rafters which have a tie between the lower ends of each pair of rafters and some form of cross bracing between. They are normally designed and made by a specialist manufacturer using computer controlled design and fabrication. The members are usually 38 or 35mm wide and are all in line with each other because the joints are made with punched metal plates. Proprietary trussed rafters are used extensively in domestic construction because they are fast and easy to erect.
Prefabricated trussed rafters can also be designed to accommodate rooms in the roof – called attic trusses.
Prefabricated roof panels are increasingly being used as an alternative to roof trusses, particularly in association with timber frame buildings. They can provide unobstructed roof spaces for occupation. The panels are similar to timber frame wall panels and enable a weatherproof structure to be achieved rapidly on site.
Trussed rafters are sometimes purpose-designed by an architect or engineer for an individual project and constructed by a carpenter, on-site or in a workshop. The members in these are usually lapped at joints and fixed with nails, bolts or timber-connectors.
More traditional forms of roof include the use of primary trusses and site constructed, or 'cut' roofs using an arrangement of rafters and purlins.
Primary trusses are designed to support purlins which in turn support rafters. They are of heavier construction than trussed-rafters and are spaced at intervals which are normally a multiple of the rafter spacing eg 1800 or 2400mm.
Purlin-supported rafters need no tie members. The rafters are notched or ‘birdsmouthed’ over the purlins or otherwise fixed to avoid displacement and to keep all loads vertical. The purlins are fixed to posts, walls or primary trusses and all or any of these supports are designed and built to remain in a state of equilibrium. The main reason to use purlin-supported rafter structures is usually either to allow the roof-space to be used or for it to be seen.
TRADA Publications available from the Bookshop
Timber Frame Construction. Chapter 8 Roofs. This chapter of the 256 page TRADA book covers roof construction of all types with descriptions and drawings.
TRADA Wood Information Sheets
Room in the roof construction for new houses. WIS 0 – 12.
Principles of pitched roof construction. WIS 1 – 10.
Trussed rafters. WIS 1 – 29.
Timber Engineering Guidance
Bracing for non-domestic timber roofs. GD 8.
