No Business Like Snow Business
by Brooke Smith | 26 February 2024 10:05 am
Every year, there are reports of rooftop avalanches causing extensive property damage, personal injury, and even death. What is a rooftop avalanche? How does it happen? How can it be prevented?
A rooftop avalanche is the sudden release of snow off a rooftop. When the roof is super-cooled, snow and ice form a temperature-sensitive bond to its surface. As that roof is warmed from sunlight passing through the translucent snow (or from building heat loss), the bond is broken, and a film of melt water lubricates the roof. This sudden release can dump tons of snow beneath the eaves in a matter of seconds, presenting a major life-safety issue and financial liability.
The best way to mitigate avalanche danger is by installing a certifiably tested, engineered snow retention system specific to the snow loads expected on the metal roof. Snow retention mechanically provides a frictional interface between the roof and the snow, so snow evacuates the roof by evaporation and thaw, rather than by a sudden and dangerous rooftop avalanche.
Techniques for mounting snow retention
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. There are three techniques for mounting snow retention to a metal roof. Two use mechanically attached snow guards.
The distinction between the two involves clamping, which grips the standing seam in some fashion without puncturing the panel material (non-penetrating), versus fastening screws through the roof material (penetrating) into the structure.
The third technique uses a chemically attached “stick-on” adhesion method, which involves gluing individual snow guards to a roof’s surface. Since the strength of the adhesives diminishes with age from UV rays, moisture, and temperature change, reliable engineering and design are not possible. Industry associations, including the Metal Construction Association (MCA), strongly caution against the use of non-mechanically fastened snow retention systems. (See Qualifying Snow Retention Systems for Metal Roofing from the MCA.)
Types of snow guards
Snow guards comprise a system of components assembled onto a roof surface to immobilize (restrain from sliding) a blanket of snow. There are two different approaches to restraining snow on a metal roof. One uses continuous horizontal components, commonly known as snow bars or snow fences, which are assembled laterally across the roof. The other consists of individual discontinuous parts known as snow stops or cleats, generally spot-located in some pattern of rows or staggered.
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 types of snow guards rely upon the compressive strength of the snow blanket at its interface with the snow guard devices. This strength is greatest at the base of the blanket (i.e. the portion toward the eave and immediately adjacent to the roof surface). The snow blanket is considered a monolithic slab with significant cohesive strength. Both types rely on the cohesive and shear strength of the snow blanket to “bridge” between continuous rows or laterally from one discontinuous unit to the next adjacent one.
Both types of snow guards have demonstrated satisfactory performance when tested, engineered, and installed properly and adequately.
Understanding the math and science
Appropriate population and/or spacing of snow guards are critical for any roof project. If they are inadequate, the system will fail at below-design snow loads—if overly adequate, money is wasted. Appropriate population and/or spacing is determined by math—not by guesswork.
The correct population is found by calculating the service loads to be resisted and matching them to the tested, allowable load resistance of the specific snow retention system proposed for the project.
Determine the vector force
The force applied to a snow guard system is a simple calculation. However, it varies from job to job with site specifics, including the design roof snow load; the roof slope; and the length from eave to ridge (rafter).
These three variables determine the force that a system must resist for any “slippery roof surface” and are included in the design and specifications. The buyer should demand proof of the engineered system design from the snow guard vendor.
Common mistakes
The most common mistake is using a snow guard product that has not been tested adequately, correctly, or at all. This is “rolling the dice,” putting the contractor or designer in a dangerous position of liability, should the system fail.
A second mistake is to “guess” at a snow guard system’s design, holding strength, or service loads to which it will be exposed. Such guesswork amounts to “luck of the draw,” often resulting in system failure and/or roof damage on the one hand and overspending on the other.
New industry standard
Until recently, there was no industry standard or mandate for producing and testing snow retention devices; this market space has been completely unregulated. Many applications have not been appropriately engineered for specific design loads, posing a threat to public safety.
A new industry consensus standard for testing and certifying snow retention devices—Evaluation Criteria (EC) 029-2018, Standing Seam Metal Roof-Mounted Rail-Type Snow Retention Systems—has been established by the International Association of Plumbing and Mechanical Officials (IAPMO).
An EC document from IAPMO is a consensus of professionals concerning the specifics of how a product or system is to be evaluated and how the results are interpreted and applied. The EC is analogous to the Acceptance Criteria (AC) document from the International Code Council-Evaluation Service (ICC-ES). In the absence of building code that covers snow retention, compliance with EC 029-2018 is “code equivalent” and may be used by specification to qualify proper design, testing, and production.
Evaluation Reports (ER) from IAPMO certifying compliance to an EC are widely accepted by jurisdictional code compliance agencies, whereas other means of evaluation often are not. Of note, this new standard is harmonious with the MCA report.
To date, only one snow retention manufacturer has achieved this new standard. The hope is that this new standard will set the bar for the importance of tested and certified snow retention systems, ultimately protecting public safety.
Rob Haddock, CEO and founder of S-5!, is a former contractor, award-winning roof forensics expert, author, lecturer, and building envelope scientist who has worked in various aspects of metal roofing for five decades. S-5! recently achieved the International Association of Plumbing and Mechanical Officials (IAPMO) industry standard for testing and certifying snow retention devices. For more on the science of snow retention, visit S-5.com or metalconstruction.org.
Source URL: https://www.metalconstructionnews.com/articles/features/no-business-like-snow-business/