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.
Example of a structural metal panel roof system
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 (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, NRCA recommends an underlayment be installed.
Asphalt saturated, nonperforated organic felts are among the most common underlayments used for metal roof systems; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation. The terms Type I and Type II now are used within the industry in lieu of No. 15 or No. 30, respectively.
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.
In locations where the average temperature for January is 30Âº F or less, NRCA suggests installation of an ice-dam protection membrane. An ice-dam protection membrane generally is a self-adhering polymer-modified bitumen membrane.
An ice dam protection membrane should be applied starting at a roof's eaves and extending upslope a minimum of 24 inches from the exterior wall line of a building. For slopes less than 4:12 (18 degrees), a minimum of 36 inches is recommended. See Figure 1.
Figure 1 - Example of ice damming
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.
Vapor retarders, insulation and ventilation
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.
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 covering. In the case of metal panel roofs, manufacturers tend to cover only the metal finishes or coatings. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records.
Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard.