In a protected membrane roof system, the thermal insulation must be covered with round or crushed ballast, concrete pavers or vegetation. The ballast is used to stop the floating of the insulation, provide wind uplift resistance and to protect the thermal insulation from the damaging ultraviolet rays of the sun
Most stone ballast is 19 mm – 38 mm (3/4 in – 1 ½ in) in size and is applied loosely laid over the insulation.
Stone size for the ballast is very important. Stones that are too small could work between the insulation boards or could be blown away by wind action. Stones that are too large may not provide proper protection from the ultra violet rays of the sun. When paving or patio stones are used for ballast, the weight of the individual blocks should not exceed 45.4 kg (100 lb). Most stone ballast should be 19 mm – 38 mm (3/4 in – 1 ½ in) in size. Ballast weight can be determined by using these figures:
- 60 kg/m² (12 lb/ft²) for insulation up to 50 mm (2 in) thick.
- 25 kg/m² (5 lb/ft²) for every addition 25 mm (1 in) of insulation.
To minimize the amount of dirt, debris and fine stones that can get into the insulation joints, a water permeable filter fabric or scrim is loose laid over the thermal insulation before the ballast is installed. When installing fabric over insulation, overlap all edges a minimum of 300 mm (12 in). Do not use any fabric smaller than 2.4 m x 2.4 m (8 ft x 8 ft).
Stone ballast should be installed uniformly over the filter fabric. No filter fabric should be left exposed to the elements. In some high wind areas, where gusts scour the gravel in the corners of the roof, paving slabs may be required.
Patio or paving stones should be tightly butted together and cut to fit tightly at all projections and vertical walls.