Commercial buildings use roof systems that differ significantly from roofs used on residential structures. The underlying roof structure of a commercial building consists of steel trusses, which are designed to carry heavier loads over longer spans. Many commercial buildings have flat or nearly flat roofs. Flat roofs require a special built-up roof that is well-drained and impervious to moisture penetration. In this module, you will learn about the different types of supporting structures used on commercial buildings and the methods used in construction metal and built-up roofs.