Low-slope metal roofs have simple geometries, and are easier and less expensive to install that medium-to steep-slope roofs. Low-slope metal roofs require less material, which reduces the overall load on the building’s structure. Low-slope roofs are not totally flat, their roof slope generally ranges from 1/4:12 to 3:12. Minimum metal roof slope depends on the roof profile, building design, climate and site location. Typically, large-footprint, single-story warehouses and pre-engineered metal buildings will utilize low-slope metal roof systems.
Jim Bush, vice president of sales at ATAS International, Allentown, Pa., believes large buildings with long-slope runs often are best applications for low-slope roofs as the overall height of the building can be minimized with low-slope systems. Frank Resso, PE, president, Resso Engineering, Newnan, Ga., agrees, saying that (generally) low-slope metal roofs tend to be used on buildings that are larger in overall area with long runs from eave to ridge, to reduce the total height of the roof and to minimize the structural requirements of the supporting framing. “Buildings with low-slope metal roofs tend to be more concerned with system performance than the aesthetic benefit that a more visual roofline could promote,” Resso says. “From a budget and aesthetic standpoint, flat built-up roofing (BUR) -to-metal retrofits are prime candidates for low-slope metal roofing.”
Charlie Smith, national recover manager at McElroy Metal, Bossier City, La., claims the best low-slope installations are on square-shaped buildings with no hips or valleys, with dimensions that allow for single-run panel lengths with no step downs. “Site forming with the correct panel and clip can allow single runs to 300 feet,” he says. “Conventional structures with a steel deck with rigid installation under the metal roof provide the best results. Open-frame metal buildings with batt insulation are the most common, but create most waterproofing and installation challenges during the initial installation. One-half inch slope is much better than 1/4-inch slope. With a 1/4-inch slope there is too much chance for ponding water, due to irregularities in the structure, especially near the eave. A metal building structure is designed to have a metal roof on it because the metal roof channelizes the water flow and maintains equal loading across the structure. The key here is that the water remains channelized which maintains equal loading of the purlins.”
Potential for Ponding Water
To prevent damage, low-slope roofs must be correctly designed and installed to divert moisture in the form of rain, snow, condensation and ice away from the building. Ponding water will find its way into a building at seams, fastener points and penetrations.
“Low-slope details must incorporate waterproof elements within their design to accommodate the potential for ponding water,” Bush says. “An extensive amount of sealant is used within all details to ensure weathertightness. Steepslope roofs rely on water-shedding principles and typically rely on more diversion channels within flashing assemblies.”
Resso concurs that low-slope metal roofing differs from steep-slope roofing in that resistance to water infiltration through panel seams is an even more crucial aspect of the system design. “Many times, the low slope is dictated by the building geometry: wide-width roof areas that simply must be lower slope to limit the overall height of the roof/building as a practical matter,” he says. “Thus, low-slope metal roofing characteristically involves very long-length metal panels directing a large volume of rainfall, and draining that water at a slower velocity due to the comparatively flatter roof surface. All of this leads to a much higher risk of the metal panel joinery being inundated—even submerged—in water.”
Snap Together Versus Seamed
Although some snapped-seam or through-fastened, overlapped roof panels are used on lowslope metal roofs in non-critical applications, the preferred method is to use a mechanically seamed system that often incorporates sealants within the seam design. “The mechanically seamed systems rely on the multiple folds of metal with the sealant to create a weatherproof joint design,” Bush says. “The seams are also elevated above the roof surface commonly 2 inches or greater to minimize the potential for ponding water to try and penetrate the seams. Snap-together seam design principles rely on slope to shed water and are not considered weatherproof seam designs.” Snap together panels do not have the same level of compression seal on the sealant in the seam after installation as a mechanically seamed panel. Alex R. Detmer, manager of field technical services at Kingspan Insulated Panels, Deland, Fla., claims that sealing with a seamer tightens up the seam completely.
Resso stresses that this seam sealant should be factory applied. “Sealant installed on the job site, either manually or by machine, can be inconsistent and can be impaired by dirt and debris if installed in an uncontrolled outdoor environment,” he adds. Also, “The snap panels that are designed to go over open framing, get compressed while being snapped together, but relax once the panel engagement is complete, loosening the compression seal,” Smith says. “When it is cold outside, many times the sealant is very difficult to compress, so it makes it difficult to engage the seam properly. There has been at least one significant lawsuit over the use of snaptogether panels over open framing on low slope.” Resso cites the ASTM E2140 testing standard, which was developed in large part with the intent of establishing a performance benchmark for panels systems installed at a low slope, including systems that may potentially be subjected to panel seams being submerged in water during heavy storm events.
Infiltration Prevention
Apart from increasing the roof slope in design, what can be done to prevent ponding and water infiltration on low-slope metal roofs? Detmer suggests making sure you eliminate any end laps where a panel laps another. “A single-drop panel from ridge to eave will eliminate many issues with leakage,” he adds. “Also, applying the proper sealant in the proper area with the proper size bead will definitely help.”
Ben King, contracts manager at Kingspan Insulated Panels, also stresses the importance of the sealant and how you apply it for the air and water seal. “This is critical to establish since the insulated nature of the product separates two environments that could condense water if they don’t remain separated.”
Attention to sealant placement within flashing designs is indeed a critical installation concern on low-slope metal roofs. Because low-slope metal roofs have a higher chance of wind holding water on the roof and holding water up against termination flashings, all of the flashing details including the eave, rake, ridge, headwall, sidewall and penetration details need to be watertight. “To accomplish this, those conditions will require the use of exposed fasteners holding two or three layers of metal together with butyl tape sealant compressed in between,” Smith adds.
Smith believes that because the chance of water going over the top of the seams has a much higher likelihood on low-slope roofing than on steeper slopes—especially as the length of the run from eave to ridge increases—a structural, hydrostatic roof system with hydrostatic installation details may be required. “Structural systems are designed to go over open framing and hydrostatic means they are watertight,” he says. “Also, most asymmetrical, structural panels will require additional, field-applied tube sealant around the clip in order to be watertight. This is even though there is factory-applied seam sealant in the seam and inside the clip in most cases. There is still a break in the sealant around the clip that has to be addressed with field-applied sealant.”
