In protected membrane assemblies, the membrane sheets are placed below the roof insulation. Although any type of attachment system can be used, the most common is a loose-laid system. Concrete surfaces and other roof surfaces will require a separation layer to be installed over the deck to prevent damage to the membrane from ridges or fins in the deck surface. The membrane is installed as described previously. A separation sheet is placed over the completed membrane prior to the laying of the insulation. This is to prevent contact between the polystyrene insulation and the membrane. Once the separation sheet has been installed, the insulation is then laid loose over it.
When the insulation has been installed, a filter fabric is installed over the entire area. Overlap the fabric a minimum of 300 mm (12 in) and turn up above the top of the ballast at all perimeters, curbs and penetrations. Apply the gravel ballast, spread evenly, at a minimum rate of 50 kg/m² (10 lb/ft²) or as required by the membrane manufacturer and/or the project design. Additional ballast may be required due to specific wind uplift requirements. Never install more gravel than has been specified or you may overload the roof.
Protected membrane roofs must drain properly. They should not be installed on roofs with slopes less than 1:50 (2%).