A ballasted (gravel/paved) roof will generally require a stronger supporting structure than that for the exposed membrane one, but will allow the roof to be used as a trafficked or leisure use area.
In the case of ballasted roofs, ballast will consist of rounded gravel 20–40 mm diameter at a minimum rate of 80 kg per square metre and 50 mm thickness, or pre-cast hydraulically pressed concrete pavers of 50 mm minimum thickness and approximately 120kg/m2. Gravel ballast of 20–40 mm diameter is required in order to provide resistance to the effects of wind scour, BRE Digest 311 (July 1988) refers, which can easily remove smaller
units from the roof.
The use of timber decking can also be considered as a ballasted application but due to its general lack of sufficient actual dead weight to resist the wind forces, special measures would need to be considered.