Pitched Roof Design Considerations

The approach taken has been based on testing and experience gained across all types of roof in the most challenging conditions around the world, building both the knowledge and the technology to meet whatever problems a site may pose.

The building regulations across the UK and Ireland all base their moisture control guidance for roofing on BS5250. The BS5250 standard, formerly the code of practice for the control of condensation, but now in its draft 2020 edition for “management of moisture” covers all aspects of keeping buildings dry. The 2020 draft is currently open for consultation, so bear in mind there might be some changes to the details after the consultation is complete.

There are two main types of moisture that roofs of all types must control, moisture from internal sources, vapour and condensation, and moisture from external sources, rain and snow.

External moisture is assumed to be primarily dealt with by the external weatherproofing once the building is complete, but in order to provide a secondary layer of weather protection, roofs are fitted with an underlay membrane beneath this outer layer.

These underlay membranes contribute to the performance of the roof systems in a variety of ways – they provide; a degree of resistance to wind uplift forces, protection from wind driven moisture penetrating the structure of the roof and a temporary weather protection until the outer covering can be completed. BS5250 defines two primary types of roofing underlay:

1. HR Underlay: membrane with water vapour resistance, Sd, greater than 0.05 m (0.25 MNs/g) i.e. traditional bitumen roofing felts
2. LR Underlay; membrane with water vapour resistance, Sd, not exceeding 0.05 m (0.25 MN·s/g) i.e. typically composed of polyethylene or polypropylene.