Snow Retention Science

Each year, numerous incidents of accumulated snow unexpectedly sliding off metal roofs cause hundreds of millions of dollars in property damage, personal injury and even death. Also known as a rooftop avalanche, it takes just a matter of seconds for snowpack to suddenly release and dump tons of snow below the eaves. Earlier this year, a rooftop avalanche ruptured a gas line and blew up an entire home.

Installing a scientifically tested and engineered snow retention system

By Rob Haddock

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Snow guards comprise a system of individual components assembled onto a roof surface to immobilize (restrain from sliding) a blanket of snow. The best way to mitigate the dangers of avalanche is by installing a scientifically tested and engineered snow retention system specific to the snow loads expected on the particular metal roof profile.

System Types

There are two different approaches to restraining snow on a metal roof. One utilizes continuous horizontal components, assembled laterally across the roof commonly known as snow bars or snow fences. The other consists of individual discontinuous parts known as snow stops or cleats, generally spot-located in some pattern of rows or staggering.

Both continuous and discontinuous types are usually installed at or near the eaves, and may be repeated at parallel intervals going up toward the ridge but with greater concentration near the eave area. Frequency (population density) is determined by specific job conditions and load-to-failure characteristics of the specific devices used.

Both rely upon the compressive strength of the snow blanket at its interface with the snowguard devices. This strength is greatest at the base of the blanket—that is the portion toward the eave and immediately adjacent to the roof surface. The snow blanket is considered a monolithic slab with significant cohesive strength within itself. Both types also rely on the cohesive and shear strength of the snow blanket to bridge between rows or laterally from one discontinuous unit to the next adjacent one.

Attachment Techniques

The finished holding strength of any snow guard product or system is highly dependent upon how it is attached to the roof and/or building structure. Two basic methods of attachment are common.

One technique uses a chemically attached stick-on adhesion method, which involves gluing individual snow guards to a roof’s surface. The other technique is some fashion of mechanical-attachment. The challenges and nuances of these techniques include:

Chemical attachment. While adhesives may seem convenient, chemical bonds degrade over time with exposure to heat, light, moisture and ozone, so the holding capacity is constantly dropping as the chemistry breaks down. The failure strength within two or three years may be a fraction of what it was at the time of testing under ideal conditions prior to exposure. It is not “if,” but “when” it fails completely. The Metal Construction Association (MCA) therefore cautions strongly, “… non-mechanically fastened snow retention systems are not recommended …”

Mechanical attachment. Because the holding strength of mechanical attachments do not change with age, their performance is measurably predictable for the long haul and preferable. There are two methods of mechanical attachment—one for standing seam (or concealed-fastened) profiles and another for exposed (or face-fastened) metal roofs. The distinction between them involves clamping, which grips the standing seam in some fashion without puncturing the panel material (nonpenetrating) versus fastening screws through the roof material (penetrating) into the structure.

In either case, mechanical attachment should have appropriate failure load testing documented by lab reports from an ASTM 1703-qualified independent lab so the complete system can be engineered to withstand the service loads to which it is exposed.

Other considerations are materials compatibility, appropriate factors of safety, prudent waterproofing for penetrative attachments and proven durability that lasts the life of the roof—to name a few.

Rob Haddock is a metal roof consultant, author, speaker, inventor, and the CEO and founder of Colorado Springs, Colo.-based S-5! He is director of the Metal Roof Advisory Group Ltd., an international consulting firm. Haddock has nearly 50 years of experience in the metal construction industry, is an in-demand speaker and is a member of the inaugural class for the Metal Construction Hall of Fame. To learn more, visit or find the MCA technical bulletin referenced at

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