There is only one category of metal roof systems used in low-slope applications — structural metal panel. 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 be used for steep slope roof assemblies, too.
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.
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 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.
Underlayment’s 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, NRCA recommends an underlayment be installed.
If an underlayment is to be installed, NRCA recommends a minimum of one layer of No. 30 asphalt-saturated felt applied horizontally in shingle fashion on roof decks having a slope of 4:12 (18 degrees) or more. For roof decks having slopes of 3:12 (14 degrees) up to 4:12 (18 degrees), a minimum of two layers of No. 30 asphalt-saturated underlayment should be applied horizontally in shingle fashion.
NRCA also recommends a slip sheet be installed over the underlayment for metal panel roof systems. A slip sheet is a layer of smooth building paper, such as rosin-sized or unsaturated building paper. Its purpose is to protect the underlayment from damage, as the panels can adhere to and tear the underlayment.
Condensation should be expected to develop on the undersides of metal roof panels. Careful consideration should be paid to vapor retarder, insulation and ventilation issues. Because every building is in some way unique, building owners and designers may need to consult moisture-control specialists.
NRCA does not make any recommendations about which product or manufacturer to use; however, NRCA does recommend that metal roof systems meet standards established by ASTM International.
ASTM E1514, "Standard Specification for Structural Standing Seam Steel Roof Panel Systems"
ASTM E1637, "Standard Specification for Structural Standing Seam Aluminum Roof Panel Systems"
When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof membrane, and vary greatly depending up the specific material and products chosen. The Second is the workmanship. At DC Contractors, we provide a 3 year warranty covering all workmanship unless otherwise specified for your particular project.and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard.