Year-after-year rooftop avalanches cause hundreds of millions in property damage, personal injury and even death. Snowpack can suddenly release, dumping tons of snow below the eaves in seconds. Inadequate snow guard systems (or none) create a life-safety issue and potential liability.
The best way to mitigate this avalanche danger and reduce liability caused by sliding snow and ice is by installing a scientifically tested and engineered snow retention system specific to the snow loads expected on your particular project.
Understanding the Math & Science
The forces of snow on your rooftop can be mathematically calculated…and should be for any snow guard system. The service loads applied to a snow guard system are a relatively simple calculation. They vary with site specifics which should be known to the design team: 1) the design roof snow load, 2) roof slope, and 3) rafter length (eave-to-ridge). These three factors determine the vector force (gravity load) that a system must resist for any roof and should be included in plans and specifications requiring an engineered system.
Snow Retention System Design Considerations
Snow Retention Population and Placement
Snowbanks typically accumulate and densify in a cross-sectional wedge pattern. All snow guards rely on the snow’s own compressive strength to restrain its movement. Gravitational forces compress the snowbank most at its interface with the roof surface, especially toward its eave end—so this is where compressive strength is greatest. The interface of snow retention devices at this location has proven to be strongly preferred worldwide and considered most effective.
System Design
Two snow guard system designs are quite common. One utilizes continuous components, assembled laterally across the roof in “fence style” and installed at or near the eaves. Depending on specific job conditions and load-to-failure characteristics of the devices, they may also be repeated in parallel rows up the slope of the roof with greater concentration near the eave area.
The other design consists of small, discontinuous individual units or “cleats,” spot-located at or near the eave and repeated in a pattern progressing up the slope of the roof, again with a greater concentration near the eaves. This style also relies upon the shear strength within a snow bank to “bridge” between the individual units.
Both styles of snow guards (continuous and discontinuous) have demonstrated satisfactory performance when tested, engineered and installed properly and adequately.
Snow Retention Techniques
There are three techniques for mounting snow retention systems to a metal roof.
1) Clamping a Snow Guard to a Standing Seam Type Roof
This method uses a seam-clamping attachment, and is a secure option for a standing seam metal roof profile when proper certified testing and engineering is employed. Clamps attach directly to the roof seam using setscrews that do not penetrate the roof (mechanically-attached).
2) Attaching to the Building Structure Itself
The second method is a system that attaches snow guards through the roof and into the building structure (also a mechanical-attachment). Attaching to the building structure provides a secure and reliable method if properly tested, designed, installed and waterproofed. They should not be used on standing seam profiles.
3) Using a Stick-On Guard
The third type is a stick-on (adhered) part (chemical-attachment). Some variations employ a factory-applied adhesive and others a field-applied one. At a glance, stick-on snow guard systems appear to be a very convenient and a lower cost option, but they are only lower in cost when you consider individual part costs vs. total project costs, including labor and quantities required to do the job.
Moreover, the replacement costs over the life of the roof are extensive when (not “if”) the adhesive diminishes in holding strength.Replacement time is often relatively soon after installation, depending on orientation to the sun and other environmental factors.
How to Choose the Best Snow Retention System for Your Project
Numerous snow guard vendors have appeared in the market, each claiming to be the best, ultimate, strongest, etc. There are no industry standards or mandates for design, manufacture, use or testing of snow guards—no “snow guard police.” This market space is completely unregulated!
Vet and Specify
Manufacturer transparency is requisite to adequately vet a snow guard system. A vendor who lauds the capabilities of his system but fails to provide proof of those claims may be blowing smoke. The buyer should scrutinize manufacturer qualifications/certifications to ensure an engineered application and long-term service. This transparency should extend from raw material through manufacture and product hand-off. Scrutiny of vendor’s evidence provides credibility and assurance to the buyer. A qualified manufacturer should gladly provide proof of claims.
Proof of Testing
Anchorage of clamps to the roof specimen should be tested on the specific material, gauge and roof profile. A minimum of three test repetitions should be conducted (fewer are not an acceptable statistical average). Testing of all system components should be conducted by a third-party ISO 17025 accredited lab and specific to the exact roof and brand of its manufacture.
It is also not acceptable to apply a test result from one set of test specimens to another similar style roof product. Request test reports from the vendor, proving values of the appropriate testing used in engineering calculations. Ideally, panel-specific test results should be available to you and published on the vendor’s website.
Proof of Engineering
Project-specific engineering provided by the vendor should incorporate the tested strength of the system with appropriate factors of safety applied. Insist calculations are provided prior to product selection. Ideally, the vendor should offer a web-based calculator with real-time output showing calculations and allowable loads specific to your project, including product names of snow guard systems and specific brand of roof manufacturer.
At a minimum, require these calculations with quotation. Or, have the vendor provide them stamped by a registered Professional Engineer.
Proof of Certified Manufacturing
How can you know the product tested is really the product purchased? Systems may look the same, but alloys, tensile strength, yield and other mechanical properties should be verified through certified manufacturing with third-party audits in an ISO 9001-15 compliant facility, not because one looks the same as the other. Ask to see the current ISO Certificate. It should be displayed on the vendor’s website.
Warranties
Does the manufacturer offer meaningful product defect and performance warranties? Obtain copies prior to purchase—and read the fine print! They should be displayed on the vendor’s website. Will the vendor be in business for the long-term to honor them if needed? Has the vendor substantiated its track record? How long the company has been in business is irrelevant. The question is, “How long and on how many projects has the system you are buying been in use?”
Why Use Snow Retention?
A smart and low-cost investment in a snow retention system specific to your metal roof profile not only protects the roof and its elements, but protects the rest of the building and its occupants. It reduces short-term and long-term costs and most of all, reduces potential liability for building owners, designers and contractors. For information, videos and webinars on this topic and more, please visit www.s-5.com or www.metalconstruction.org.
