Both washed aggregate and dense concrete paving slabs are suitable as ballast for use with ROOFMATE™ SL A insulation.
This gives a good appearance at an economical cost and should be 20-40mm nominal diameter, clean, washed and reasonably free from fines.
When boards are overlaid with a suitable separating layer - such as ROOFMATE MK - lapped 300mm, then a 50mm depth of 20/40 aggregate may be sufficient to counter flotation of the insulation. Additional ballast may, however, be needed in those areas subject to wind uplift, such as exposed perimeters and corners.
Aggregate should be replaced by paving slabs:
- to form walkways where regular foot traffic is expected
- where the kerb at the roof edge is too shallow to retain the aggregate
- at perimeters, where calculations indicate aggregate will provide insufficient resistance to wind uplift or will be affected by wind scour - see BRE Digest 311.
Pressed concrete pavers 600mm x 600mm and 50mm thick should be raised off the insulation on spacers to allow drainage and to avoid rocking.