To ensure the effective design of a natural slate roof, it is imperative that key interrelated factors are taken into account, including:
- Site exposure
- The pitch of the roof
- The type of slate selected
- The slate lap
General guidance on the most important points to be considered is given below. Further information can be obtained from BS 5534:2014, Code of practice for slating and tiling.
The degree of exposure of a building to driving rain determines the minimum lap which should be specified. The anticipated degree of exposure is given in the exposure diagram below.
Localised factors such as high buildings, buildings on the slopes or tops of hills and coastal sites, can increase the exposure grading which should be applied in a specific project.
Pitch of roof
In general, the lower the pitch of the roof, the greater the lap should be. This longer lap will help to resist both capillary action and wind uplift. On steeper pitches with free-flowing drainage, smaller slates may be used. For exposed sites, wide slates with a greater lap should be used whereas in sheltered areas, roof pitches as low as 20 degrees can be achieved using a hook fixing system.
The lap is calculated by taking account of wind uplift, exposure to driving rain and the roof pitch.
Categories of exposure to driving rain and basic wind speed: