Structural metal panel is the only category of metal roof systems used in low-slope applications. Structural metal panel roof systems can be used for low slope roofs because of their hydrostatic or water barrier characteristics. It is important to note structural metal panel roof systems can also be used for steep slope roof assemblies. Most structural metal panel roof systems are designed to resist the passage of water at laps and other joints, as sealant or anti-capillary designs can be used in the seams. Structural metal panel roof systems possess strength characteristics that allow them to span supporting members. 

Roof Deck

Structural metal panel roof systems are installed over a large variety of substrates. There are two general categories of substrates: one is continuous or closely spaced decking that provides solid support for the metal roof panel, and the other is composed of spaced structural supports (such as purlins) where the metal panels must span between supports. Most structural metal panels are used over spaced structural supports without being supported by a solid roof deck.

Underlayment

Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of a metal panel roof system. An underlayment performs two primary functions: it provides temporary weather protection until the metal panel roof is installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the metal roof panels. Underlayments typically are not used with structural metal panel roof systems when intermittent supports are used to carry the roof systems. However, if there is a continuous or closely spaced roof deck, we recommend an underlayment be installed.