The buildup of snow and ice on a pitched metal roof causes challenges in many areas around the country. The collection of snow on a roof can produce significant sliding forces, which can cause the snow to fall suddenly. Snow retention systems reduce the risk of sudden rooftop avalanches and mitigate the hazards present in the area below the eaves.
What you need to know about snow guards and how to install them
By Marcy Marro
Snow Guard Styles
To restrain sliding snow, snow guards use the compressive strength and density of the snowpack immediately adjacent to the roof surface, therefore resisting the vector force of snow for a specific tributary area of the roof.
Snow guards come in a variety of shapes and sizes. There are unitized snow guards, otherwise known as discontinuous snow guards, snow cleats or pad style snow guards. A unitized snow guard can be a single- or multi-component and does not use continuous cross-member. Normally used in a redundant fashion, unitized snow guards rely on the “bridging” of a snowpack to span between adjacent devices to limit snow migration.
Additional Resources
Technical Resources from the Metal Construction Association:
- Qualifying Snow Retention Systems for Metal Roofing
- Metal Roof Design for Cold Climates
- Fastener Compatibility with Profiled Metal Roof and Wall Panels
For more information, visit www.metalconstruction.org/Tech-Resources
Continuous snow guards are otherwise known as rail type snow guards, snow fence, snow rail or pipe-style snow guard. These have at least two components: brackets or seam clamps that are attached to the roof, and cross-members that are anchored to the brackets or clamps that restrain the sliding snow. Continuous snow guards can be made up of one or multiple cross-members.
Choosing the right type of snow guard is a decision largely based on aesthetic and personal preference. “Either type can work—continuous rail or discontinuous cleat-type systems—provided that either is appropriately tested, correctly installed and proven by engineered calculation to resist the in-service loads presented on a site-specific basis,” says Rob Haddock, CEO of Colorado Springs, Colo.-based S-5! Attachment Solutions.
Jerod Webber, sales manager, at Raytown, Mo.-based Dynamic Fastener, notes that engineering and testing is one of the main focuses when choosing a snow retention system. “The panel type and material, roof size, local snow loads and aesthetics are some other issues to consider.”
Material Choices
Snow guards are considered a life/safety product, and therefore should have the service life and durability equivalent to the roof itself, Haddock says. “Because metal roofs have exhibited service lives of more than 60 years in documented field studies by the Metal Construction Association (MCA), this requirement immediately rules out the use of plastics (polycarbonates) as they do not measure up to such a service life in outdoor exposure,” he adds.
In its newest technical bulletin, “Qualifying Snow Retention Systems for Metal Roofing,” MCA recommends snow guard systems for metal roofs to be manufactured from non-corrosive metals. The compatibility between different types of metals is an important consideration when choosing a snow guard system. “Use aluminum with stainless hardware on coated steel and zinc roofing; brass alloys or stainless for copper roofing; anodized aluminum or stainless for stainless roofing,” adds Haddock. “Hardware for these systems (nuts, bolts, screws) should meet the same criteria—last as long as the roof. 300 series stainless is best for the fasteners, as it is compatible with all roof materials.”
To color match the roof material, it is recommended that the type of paint or powder coating is equivalent to the roof material in terms of fade and chalk characteristics. “Lesser quality paint finishes on the snow guards will fade more rapidly than the roof, producing a very unsightly mismatch within a few years,” Haddock notes.
Correct Placement
A combination of testing, science, mathematics and quality control are required for the proper design of snow guard systems. Since they represent a load chain to transfer the sliding forces of snow into the building structure, each link in the chain must be proven to be able to withstand the forces to which it will be exposed.
Snow guards should always be placed at the lower half of the roof plane. And, depending on the system, snow guards can be placed in a single line, or in multiple rows. “Generally speaking, if installing one row and using a continuous rail-type system, your snow retention would be installed approximately 1 to 2 feet from the eave,” explains Tom Grant, branch manager at Dynamic Fastener. “The majority of the snow load is carried on the bottom half of the roof due to heat loss, wind, sun exposure and gravity.”
“Exact placement of snow guards is a mixture of art and science,” Haddock adds. “In general, snow accumulates on a roof with greater concentration near the eaves and valleys due to wind scouring. All snow guard devices rely upon the compressive strength of a bank of snow to resist its downward migration. The compressive strength of snow increases as it compacts from its own weight, increasing its density. This results with the greatest compressive strength of the snow bank at its base (adjacent to the roof surface) and at its downslope (eave) end. For this reason, the snow bank’s downslope extremity is the best place to interface with snow guards.”
Additionally, Webber notes, “Slope is a critical component in the engineering/design of snow retention because steep slopes combined with long panel lengths are two of the factors that help determine if multiple rows of snow retention are needed.”
MCA’s technical bulletin, “Metal Roof Design for Cold Climates,” can be used as a reference guide for snow retention calculations and placement.
Installation Tips
When installing snow guards, always refer to the manufacturer’s instructions. Since snow guards are expected to last the entire service life of the roof, MCA recommends always using mechanically attached snow guards, never glued. “Relying on caulk/adhesive for attachment is not the preferred method, whereas using a mechanically fastened clamp/rail combo is the stronger alternative,” says Grant.
“Clamp designs that are seam-specific, or for use on certain metal thickness, have been tested with specific set screw torque values,” explains Webber. “To properly retain the expected snow load, following the installation instructions is crucial.”
According to Haddock, the method of attachment of snow guards should be consistent with the roof type. Therefore, a face-fastened roof should use a face-fastened snow guard attachment with butyl copolymer sealants and fasteners of no less quality than those used for the roof itself. And, a standing seam roof should use clamp-to-seam attachments that are metallurgically compatible with the roof material.
For through-fastened roofs, Haddock says sealants should be at least equal in performance to those used in the roof itself. “In most cases, this means using high-quality butyl copolymer tapes and part designs that protect the sealant from UV exposure and over-compression.”
Similarly, fasteners should also last the life of the roof. “Exposed fastener heads should be AlZn alloy or 300 series stainless steel, and washers should be black EPDM,” adds Haddock.
According to the MCA technical bulletin, sealants and washers can be used to protect the sealing material from direct exposure to sunlight. When choosing washers or sealants for waterproofing, use ones with the same chemistry as the washers and sealants present in the roof system to ensure a similar expected service life. Preferred sealants are isobutylene/isoprene (butyl) polymers or materials with similar of greater service life, while ethylene propylene diene monomer (EPDM) or similar materials are the preferred washer chemistry, using carbon black UV stabilizers.
It is important that the snow guard system is designed for each specific project. As Haddock notes, the steeper the roof, the greater the forces are that act on the snow guard assembly. “For example, a 4:12 roof that is 50 feet from eave to ridge with a 40-psf design roof snow load will exert 633 pounds per linear foot of eave on the snow guard assembly,” he explains. “If the slope of the same roof is changed to 7:12, the force changes to 1,008 pounds per linear foot.”
For installing snow guards, most of the tools needed are common to the trade. One exception, Haddock says, is for systems anchored with seam clamps, a torque wrench is needed to check that the electric driving tool is delivering the torque specified by the manufacturer. “This is important because tested holding strength is dependent upon that torque,” he says.
Engineering and Testing
When deciding on the right type of snow retention system, Haddock says to be sure the design is adequate and specific to your application, with appropriate testing. He recommends asking the manufacturer for written proof of appropriate testing on the chosen roof profile or substrate by an accredited lab. A properly tested, engineered and designed snow guard system is imperative to resist the forces induced by snow packs. Failure of a snow retention system can cause property damage and potential loss of life.
