Pitched Roofing Consultant John Mercer discusses the key considerations when constructing low pitch roofs.
Of the many enquiries I receive, one of the most popular topics relates to low pitch roof construction. Experience has taught us that the ideal range of roof pitches for roof tiles and slates is anywhere between 30 to 50º. During rainfall or wind-driven rain spells, water is shed from one double lapped tile or slate course to the next one below, the tiles and slates having sufficient lap and pitch to prevent any sideways or upwards ‘creep’ of the water through capillary action from penetrating the roof covering. Single lapped tiles perform the same function by virtue of their lap over the course below and side laps which can be either simple overlaps or more sophisticated interlocks with water bars and troughs.
Although the trend in recent years is to build homes with steeper pitch roofs to provide living spaces within the roof, there are many tiles and slated roofs constructed at low pitches to reduce construction costs, for example, or reduce a building’s impact on the horizon, or very commonly, to enable the building of a home extension to fit below the upstairs windows.
Roof tile manufacturers have developed and continue to develop roof tiles and associated systems that are suitable for use at low pitches. However, under certain weather conditions, water may be driven through the slating or tiling and must be captured by the underlay system and drained away to the gutter. Certainly, at low roof pitches, the risk of water ingress increases through or around the tiling, simply because everything is working closer to its limits. For example, though the individual components have been thoroughly tested and perform fine at the stated minimum roof pitch, constructing the roof with the same accuracy as that used under test conditions can be difficult. It only takes a small gap or two in the tiling where it passes over a flashing and the risk of water ingress is increased.
BS 5534 states that the underlay layer should provide a continuous under-roof barrier to water, snow, and dust, and should be capable of draining any moisture that might be deposited onto the surface of the underlay to the roof drainage system. Although BS 5534 does not provide specific design details on how to construct a waterproof underlay system, it does recommend that consideration be given to sealing any penetrations through the underlay, such as nail holes, with suitable tapes or sealants.
The design recommendations provided in BS 5534 are an acknowledgement that there will be occasions when rainwater finds its way through the tiling and onto the underlay, so the roof should be regarded as a system, rather than simply a collection of individual components.
It is important to follow the installation recommendations given in BS 5534 and BS 8000-6 for the underlay. Underlay laid unsupported, directly over the rafters or counterbattens, must be laid with a sufficient drape to enable water to run freely into the underlay troughs and under the tile battens towards the gutters. If the underlay is laid too tight, without a drape, water on the underlay will be trapped behind the tile battens and will eventually find its way through the batten nail holes in the underlay and leak into the roof structure.
On low pitched roofs, rainwater is more likely to be driven through the tiling during heavy wind-driven rain events, though roof design should consider other factors too. For example, manufacturers usually set a maximum rafter length for their tiles when used at the minimum recommended roof pitch. It is important to observe any such restrictions by, for example, raising the roof pitch if possible, or dividing the roof into shorter sections with a gutter part way down the slope. On single storey extensions, care should be taken to avoid draining a higher roof onto the low pitch extension roof via a valley or through a gutter downpipe. Even dormer windows can pose a risk if the rainwater from the dormer roof is drained directly onto the low pitch roof.
It is equally important to detail all junctions correctly when installing the underlay, with the correct turn ups at abutments and suitably sealed around penetrations such as pipes and roof windows.
There are no special measures or variations to sub-roof designs that relate to roof pitch in BS 5534. However, there are methods that can be adopted to ensure the underlay is watertight. For example, consider using counterbattens between the underlay and tile battens, with a bituminous nail tape fitted between each counterbatten and the underlay. The counterbattens will ensure that there is a path for water to reach the gutters without the need for a drape, and the nail tape will seal the batten and counterbatten nail holes through the underlay.
• Observe the tile manufacturer’s recommendations on minimum roof pitch and maximum rafter length.
• Avoid draining water from higher roof slopes onto a low-pitched extension roof.
• Ensure the underlay is detailed correctly at junctions such as abutments and properly sealed around penetrations such as pipes and roof windows. Use adhesive tapes where necessary.
• Consider the use of counterbattens with nail tape between the counterbattens and underlay to seal the nail holes through the underlay.