Metal panels are not typically thought of as options for low-slope roofs. Some metal panel systems, however, can be used on very low-slopes. Although some manufacturers tout their systems as being suitable for slopes as low as 1/4:12 (2 percent), NRCA recommends a minimum slope of 1/2 inch per foot as the minimum design slope for hydrostatic roof assemblies and 3 inches per foot as the minimum design slope for hydrokinetic systems. The greater the slope, the more reliable the leakage protection.
This section addresses metal panels suitable for use on slopes of 3:12 (25 percent) and less. These panels can also be used on slopes in excess of 3:12. See Steep-Slope Roofs for metal panels that are only suitable for slopes greater than 3:12.
When installed on low-slopes (particularly slopes approaching 1/2:12 (4 percent) or less) a metal panel system needs to provide water resistance all across the roof surface. Thus, low-slope metal panel systems should be designed and installed with the intent of making them membrane-like. To achieve this, the panel joints must be soldered or sealed together with sealant tape or sealant, or both. Also, fasteners that penetrate the panel at end-joint splices or flashings must be sealed with gasketed washers. In addition to making all of the metal joints watertight, they must remain watertight while undergoing extensive movement from thermal cycling. Over time, thermal movement of the metal can tear through fastener gaskets and enlarge holes at fasteners.
One should be cautious about using continuous sheet metal in a flat roof situation. Sheet metal is prone to wider, more extreme temperature swings because of its dense nature as a material, especially in the sunlight on a roof. This will cause significant expansion/contraction movements in the sheet metal surface. The movements themselves are difficult to manage, but combined with necessary roof penetrations for vents, drains, curbs, and wall corners, which bind the inevitable movement, tears or seam breaks in the sheet metal are highly likely. Consider employing sheet metal in flat roofs only where there are no penetrations and the movements can be accommodated. It is more difficult to achieve a reliable and long-lasting watertight system on a low-slope roof with metal than it is with the other low-slope membrane materials.
Galvalume-coated sheet steel or aluminum panels are typically specified for low-slope standing seam panels. On historic projects copper, terne-coated copper, or terne-coated stainless steel may be used.
For corrosion protection on steel panels, current practice is to specify 55% aluminum-zinc alloy (commonly known by the trade name Galvalume). Until the late 1990s, unpainted aluminum-zinc alloy panels had a factory-applied lubricant to facilitate roll forming. The lubricant eventually weathers away, but installation smudges and fingerprints result in uneven appearance for a while. A thin clear acrylic coat can be specified to provide a more even appearance and show the effects of weathering more gradually, as the acrylic weathers away. Acrylic-coated Galvalume is sold under trade names such as Galvalume Plus and Acrylume.
Factory-coated low-slope panels are recommended. There are several finish options. The most common factory-applied coil coating is polyvinylidene fluoride (PVDF), commonly known by the trade names of Kynar and Hylar. PVDF is typically specified since if offers a large range of colors and is extremely resistant to color change over time. Painting can also be specified when a high emissivity is desired.
Internal gutters and parapets at the eaves of low-slope metal roofs should be avoided, as it is less problematic to have the water flow over the end of the panels and fall directly to grade or drop into an external gutter that is below the plane of the panels.
Some panels have snap-together seams, while others are mechanically seamed with an electrically powered mechanical seaming tool. On slopes of 1:12 (8 percent) or less, it is recommended that mechanically seamed panels be specified.
There are two basic types of standing-seam panel profiles, the trapezoidal rib and the vertical rib. Because of its appearance, the trapezoidal rib panel is typically used on industrial buildings and warehouses. The trapezoidal panel is difficult to make watertight at hips and valleys.
In addition to the standing seam panels, through-fastened panels (also referred to as R-panels) with exposed fasteners are available for low-slope systems with slopes in excess of 2:12 (17 percent). They should be considered hydrokinetic systems. This is a relatively inexpensive system. It has largely been replaced by standing-seam systems, which eliminate leakage problems that are often associated with exposed fastener systems. Other exposed fastener systems include corrugated panels and 5-v crimp panels.
To avoid leakage problems at panel end-joint splices, it is preferable for the panels to be continuous from eave to ridge. If panels are quite long, job-site roll forming may be necessary. However, full-length panels are sometimes impractical and can expand and contract 1″ or more, making detailing very difficult.
The Metal Roof Systems Design Manual by the Metal Building Manufacturers Association provides further guidance pertaining to metal roof systems.