Trussed rafters

Most modern roofs are constructed from trussed rafters. Trussed rafters have been popular for 40 years or so and have almost totally replaced the traditional roofs shown earlier. The most common pattern is the Fink or ‘W’ truss, designed for symmetrical double-pitched roofs, although there are a variety of shapes suitable for most roof designs. The trussed rafters are prefabricated and delivered to site ready for lifting onto the supporting walls, although occasionally you will find the entire roof structure assembled on the ground and lifted into place by crane.

The timbers, which are typically around 80 x 40mm (the British Standards prescribe a minimum thickness of 35mm for spans up to 11m) in section are butt jointed and held together by special metal nail plates, which are pressed into position by machine. The truss suffers minimal deflection under load due to the triangulation of the timbers and the metal nail plates fixing the members firmly together.

Trussed rafters offer several advantages in comparison to traditional roofing methods:

-   no internal support is required from loadbearing partitions
-   spans of up to 12m can easily be achieved
-   they offer very fast construction
-   highly skilled labour is not required
-   they are relatively cheap
-   they can be designed to very shallow pitches.

Perhaps their major disadvantage is that Fink trusses severely limit the use of the roof space for storage and loft conversion is impossible without their complete replacement or the need to carry out major structural works, such as the provision of new purlins, floor beams and joists to satisfy the structural requirements of the Building Regulations.

Design and fabrication
Fixing
Strapping
Other details