Can I use long rafters with a low pitch roof

01 December 2016

Can I use long rafters with a low pitched roof?

The key consideration is to ensure that a roof designed with a low pitch and long rafters will provide adequate watertight performance. A low pitch and long rafters can be used, provided they do not result in a low rainwater discharge rate, and that the site conditions do not make the roof vulnerable to rainwater ingress.

It is important to note that rainwater is discharged at a slower rate on low pitched roofs. When combined with long rafters, this can mean higher volumes of rainwater accumulating on the roof slope. This leads to localised rainwater deluge and flooding of the lower level of the roof. It will then be more susceptible to wind-driven rain, particularly at the lowest point towards the eaves, where the volume of rainwater will be at its greatest.

The factors that affect adequate performance include site exposure, building height, orientation, building geometry, number of roof penetrations (i.e. tile vents, soil vent pipes, mechanical extracts and roof windows) and detailing to internal roof junctions, such as valleys.

You should always consult the product manufacturers, particularly of tiles, slates, penetrations and membranes for guidance as early as possible, preferably in the design stages. This can help save you time and cost by preventing any potential issues when it comes to the installation.

Low pitched roofs with longer rafter lengths are used on many different types of projects, from small domestic extensions to larger commercial and residential developments. With factors such as planning restrictions prohibiting the overall height of a building, large building designs often incorporate low pitched roofs with long rafters.

With this trend continuing, it is important to specify products that are designed for purpose, and our range of low pitch roof tiles can be used to ensure adequate performance.

You can find out more about correct pitched roof specification in our guide.

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