Beneath the metal covering, an assembly must provide support, clear incidental water and allow moisture control. Depending on the roof, it includes a membrane, sheathing, battens or other components. No single substrate works for every metal sheet. The word “substrate” can mean different things: rigid sheathing, a membrane or battens. Their roles differ – the sheathing gives support, while the underlay mainly helps protect against water that gets under the covering.

The choice of roof shape starts with the planning rules for the plot. They may limit the pitch, shape or appearance of the covering. Within those limits, decide what the space below the roof will be used for and compare the cost and layout of the alternatives. A cold loft, heated rooms in the roof and a full upper storey have different requirements. Identify chimneys, roof windows and equipment early, since each affects the detailing and drainage. The designer must also check the snow and wind loads for the site.