The interaction of the two principal wind forces on roof tiles

Abstract Wind forces on roof tiles laid over a low-permeability underlay can be separated into two principal mechanisms. The forces due to the airflow over the steps formed by the tile courses, referred to as the surface flow force, have been used as the basis for the British Standard Code of Practice BS5534 Part 1. Forces due to internal flow between the tiles and the underlay have been shown to occur where large differences exist between external pressures over different areas of the roof. Whereas the surface flow force depends critically on the detailed design of the tiles, the internal flow force is rather more influenced by the overall roof shape and details of the “batten space” between the tiles and the underlay. The roof areas most affected by the two forces are often quite different — surface flow forces are more severe on the wind-ward slope, whereas internal flow forces can be severe on the leeward slope. This paper discusses the differentiation of these effects and possible ways of calculating their combined effect.