Most flat roofing systems require some kind of rigid insulation under the new roofing system. The biggest reason is that all flat roofing systems require a substrate between the structural decking and the new roof system. The second and more obvious reason is for higher R-values, which building owners are asking for, and building code throughout New England is requiring.
The most common types of flat roofing rigid insulation are below. For more information, click on the name of the insulation.