An inverted roof has the waterproofing beneath the insulation. The insulation is exposed to water in this position, so choose a product approved for that use. Suitable grades of XPS are common. Before considering a switch to PIR, check the documented scope of the system. Rigidity and a closed-cell structure alone do not establish suitability.

Options include bituminous roofing membranes, synthetic membranes and other waterproofing systems specified for the particular roof. Check the requirements for the deck, seams and terminations. Consider weather exposure and how people will use and maintain the surface. A waterproofing product intended for a bathroom needs separate evidence of suitability before it can be considered for roof use.

Check material compatibility at every interface. Some combinations need a separating layer, and an adhesive must suit both surfaces. Foil-faced boards and boards with other facings can have different permitted uses. Only torch a bituminous membrane directly onto a board if the instructions for that specific construction expressly allow it.

To compare thermal performance, use each product’s declared thermal conductivity and the resistance required of the layer. PIR with low conductivity can need less depth than typical EPS or mineral wool to reach the same resistance. This can be useful at parapets and thresholds. Calculate the difference from the actual product values, since the saving in thickness varies.

Also assess loads, moisture exposure, acoustic performance and fire protection. Mineral wool boards intended for flat roofs have different properties from soft insulation fitted between rafters. Individual grades of EPS and XPS also have different uses. Set the requirements for the complete roof, then choose a product whose performance in that system meets them.

Water vapour can enter the roof by diffusion or with air leaking through joints and gaps. Assess the moisture behaviour of the whole roof as well as the water resistance of the insulation core. Specify the layers on the room side and make their connections to walls and penetrations airtight. The conditions depend on how the rooms below are used.

Check that the deck and materials are dry enough before closing the roof. Trapped construction moisture can cause problems even if the insulation provides the intended thermal performance. During refurbishment, inspect the existing layers and find the cause of dampness or leaks before choosing a repair. Covering a wet deck between layers with low vapour permeability can make subsequent drying difficult.

Terrace paving and growing medium add weight and increase the need to protect the waterproofing. A green roof needs appropriate resistance to root penetration, drainage and layers suitable for planting. Keep the roof outlets accessible for inspection. These details are as much a part of the design as the thickness of the insulation.

Check the pressure beneath equipment feet and supports. A compressive stress quoted at a specified deformation is insufficient to determine the permissible load over decades. Account for creep and the way the support spreads the load. Apply the same reasoning to photovoltaic installations, maintenance walkways and heavy items placed on the roof later.

Check the roof attachment for wind uplift. The substrate and the design determine the type and spacing of fasteners, the bonding method or the ballast required. Pay particular attention to edges and corners. Design the fixing pattern for the roof and its different zones. Coordinate the details at service penetrations and parapets with the rest of the build-up.

At handover, inspect waterproofing joints, trace the drainage route and check access to outlets. Keep the product information and inspection requirements. The insulation limits heat loss and the waterproofing protects the building from rain. Their service life depends on how well they work together in the completed roof.