What do I need to consider for a low-pitch roof?
When opting for a low pitch roof design, several important considerations need to be made. At a low pitch, some elements of the roof system are working at the limit of their capabilities, meaning that there is a higher chance of failure under extreme weather conditions.
Manufacturers put their products through rigorous testing to ensure they are fit for purpose and to provide a minimum pitch result, which is the minimum angle that a tile can be installed at as part of a watertight roof. For example, Wienerberger’s Sandtoft 20/20 clay plain tile is designed to cope with low pitch installations, with a 15° pitch achievable with a 100mm headlap.
Although individual roof components have been thoroughly tested and perform perfectly well at the given pitch, constructing the roof with the same accuracy as that used under test conditions takes skill. It only takes a small gap or two in the tiling where it passes over a flashing and the risk of water ingress from rainfall or wind driven rain is increased.
In terms of effective water drainage, a roof with a gentle slope discharges rainwater at a slower rate than a steeper roof, simply due to gravity. When a roof structure also features long rafters, this can mean that a large amount of water can linger on the surface and find its way into any gaps, damaging underlay and battens over a prolonged period. Therefore, roof tile manufacturers often recommend a limited maximum rafter length. The pitch of a roof should be increased 1° for every half metre distance that the rafter length is over the recommended maximum length.
During rainfall, water runs from one single or double lapped tile to the course below, with sufficient side lap and pitch to prevent sideways or upwards creeping of water. It is important to pay close attention to the recommended headlap stated on manufacturer’s instructions, as often the lower the pitch, the larger the headlap should be.
As low-pitched roofs are often constructed on building extensions, they may have to deal with water shedding from a larger roof above, sometimes in a point load through a down pipe shoe, or in concentrated loads from valleys or dormer side abutments. Therefore, careful design is important, with such point loads avoided if possible.
Rather than relying on standard roofing construction, which is fine for steeper roofs, with low pitches it is advisable to detail and construct the roof to protect vulnerable areas such as eaves, valleys, abutments, roof window surrounds and other such junctions. These areas need to be considered even more carefully with low pitched structures to make sure that they are contributing to a watertight roofing system.
To find out more about low pitch roofs, download our technical guide here.