Specifying snow retention systems

by sadia_badhon | December 23, 2020 9:42 am

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by Rob Haddock

Rooftop avalanches cause property damage to the tune of hundreds of millions of dollars, personal injury, and death each year. A snowpack can suddenly release and dump tons of snow below the eaves in a matter of seconds, endangering building elements, adjacent roofs, landscapes, vehicles, property, and, even worse, pedestrians. This is clearly a life/safety concern.

Inadequate snow guard systems—or the lack of them—can potentially create liability issues for building owners, designers, and contractors.

Liability concerns

While engineered snow retention systems complement the roof, this market space is unregulated. Snow guards are not governed by codes or standards. Many applications are not specifically engineered for design loads. The only policing within this field is by the architect, specifier, and, too often, the contractor.

Plan callouts stating, “furnish snow guards, typical,” or specification language noting, “…as recommended by manufacturer,” is just rolling the dice, making the contractor the ultimate decision-maker for adequacy and product selection. If the system is devoid of appropriate design and testing it can fail even when installed per the manufacturer’s instructions, thereby exposing the designer and/or contractor to liability. System failure can threaten anything on the ground below and also damage the roof. Protection from this liability starts long before the project is bid.

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Rooftop avalanche causes

During snowfall, sunlight is occluded, and snowpack is retained on the roof by two bonds between the snow blanket and the roof. The first is a strong, but temperature-sensitive, adhesive bond between the blanket of snow and the roof surface. The second is a cohesive bond at the ridge where snowpack on one plane of the roof interlaces with snowpack on the other (Figure 1).

A vertical load (weight of snow) is added to the roof surface, which translates to a vector load parallel to the roof surface, often referred to as “drag load” or “gravity load”—the force of snow trying to slide off the roof. A rooftop avalanche occurs when vector forces exceed the strength of the two bonds on a slippery roof.

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Infrared rays from the sun pass through the translucent snowpack, warming the roof below and freeing the temperature-sensitive bond. The meltwater lubricates the interface at the roof surface. (Since the snowbank insulates the roof, this happens even when ambient temperatures are well below freezing.)

The vector force of the snow blanket is now resisted only by the cohesive bond at the ridge, which is too weak, thus causing the blanket of snow to split leading to a sudden release of snow from the roof. This can happen on one side of the ridge or both simultaneously, depending on orientation of each to the sun (Figure 2).

Site-specific variables

A snow guard is a device or system that mechanically provides a resistance interface between the roof and the snow, so snow evacuates the roof in a predictable and controlled fashion—through evaporation (sublimation) and thaw rather than by a sudden and dangerous rooftop avalanche. The resistance of the snow guard system must exceed the vector force in service, otherwise it will fail.

The vector force applied to a snow guard system is a relatively simple calculation but varies with site specifics, such as:

• design roof snow load;
• roof slope; and
• roof (rafter) length from eave-to-ridge (Figure 3).

These three variables determine the force a system must resist for any slippery roof surface and should be included in plans and specifications that require an engineered system (for more information, read the Metal Construction Association’s [MCA’s] technical bulletin on “Metal Roof Design for Cold Climates”).

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Vector force

Vector force is found by reducing the vertical load by the sine of the roof angle. The product is then multiplied by the tributary area to the snow guard system. The loads for the entire length from eave to ridge are tributary to the snow guard system restraining it. For standing-seam metal roof (SSMR) profiles, tributary force is distributed to each point of system attachment—the standing seams. So, the tributary vector force to each seam is found by multiplying the vector force by the roof length by the panel width (Figure 4).

Snow retention system design

Snowbanks typically accumulate and densify in a cross-sectional wedge pattern with greatest depth at the eave. All snow guards rely on the snow’s own compressive strength to restrain its movement. Gravitational forces compress the snowbank the most at its interface with the roof surface, especially toward its lower (eave) end, so the compressive strength is greatest there. (Figure 5).

The interface of snow retention devices at this location is considered the most effective. Even when multiple rows are required by calculation, the global practice is to locate them within the downslope half of the roof surface. Further and exact placement details are more art and aesthetics than science. The MCA makes some general recommendations (for more information, read the MCA’s technical bulletin on “Metal Roof Design for Cold Climates”).

Since compressive strength is so great at the snow-roof interface, snow guard devices of only a few inches in height have demonstrated success even when the snow is many feet deep.

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Snow guard design concepts

Two design concepts are common. One utilizes continuous horizontal components, assembled laterally across the roof in the style of a ‘fence.’ Such assemblies are usually 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, but with greater concentration near the eave area. The snowbank creates a bridge between rows to distribute vector loads.

The other design consists of small, discontinuous individual units used as ‘cleats,’ generally spot-located at or near the eave, or repeated in a pattern progressing up the slope of the roof, again with a greater concentration near the eaves. This style also relies on the shear and compressive strength within a snowbank to ‘bridge’ between the individual units in both axes.

Both styles of snow guards (fence and cleat) have demonstrated satisfactory performance when tested, engineered, and installed properly and adequately (Figure 6).

Additional considerations

Aside from adequate testing for holding capacity, other design considerations include verifying metals’ compatibility, matching serviceability of the device/system with that of the metal panels, and color matching if required. The objective is to select a system that will perform satisfactorily for the life of the roof without harming it.

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Devices utilizing air-dried paints to match the roof color are easy initially, but may produce a poor outcome after weathering a few years. This is due to inferior characteristics of air-dried paints when compared to factory-applied finishes of metal panels. Powder coating may provide greater longevity in terms of color stability, but generally is not as durable as factory-applied polyvinylidene fluoride (PVDF) panel finishes.

Snow retention techniques

There are three techniques for mounting snow retention systems to a metal roof. Two utilize mechanically attached snow guards. The distinction between the two involves clamping, which grips the standing seam in some fashion without actually puncturing the panel material (non-penetrating) versus fastening screws through the roof material (penetrating) into the structure. The third method uses a chemically attached, ‘peel-and-stick’ adhesive tape or pumpable glue. This is often used for individual, discontinuous (cleat style) snow guards.

Clamp mounting to standing seam

This (mechanically attached) method uses a seam-clamping fixity and is a measurably secure option for a SSMR profile, only if evidence of adequate testing is presented to prove it. The clamps attach directly to the roof seam using setscrews that do not penetrate the roof.

Clamp-to-seam attachments can easily be mounted after the roof has been installed and will not fatigue due to thermal heating or cooling of the roof (Figure 7). If the correct quality checks are followed in product selection, they will prevent potentially voiding the warranties of a long-life, premium roof. Both styles (continuous and discontinuous) are available in the marketplace.

What to expect with clamp-mounted attachments

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The sliding force of the snow is transferred to the clamp and then into the roof panels, specifically the seams. The holding strength of such a system should be quantified through rigorous testing and proven to resist the service loads on any given job. Test results vary widely on different SSMR profiles and materials. Therefore, test results should be specific to the brand of roof manufacture, profile, material, and gauge thickness used on the project. It is recommended to consult the vendor’s website for test results and lab reports.

Attaching to the building structure

The second method for mounting snow retention 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 (Figure 8). It is only suitable for a roof that is attached the same way (i.e. with screws penetrating the roof’s surface). It should not be used on a SSMR, as it violates the latter’s design intent.

What to expect with structure attachments

Snow guards that attach directly to the structure require penetration for anchorage of snow guard brackets, hence weatherproofing for the long-term service life of the roof is of paramount importance—including sealant protection from ultraviolet (UV) exposure and over-compression of the seal. Such waterproofing requires a high degree of expertise in design, sealant chemistry, selection/sourcing, etc. If sealants/gaskets are not factory applied by the vendor, it falls in the hands of the contractor to select and apply appropriate waterproofing.

The strength of the structural attachment mounting method should be proven and matched to the in-service loads to which the snow guard will be exposed. Again, testing should be specific to the deck and/or structural substrate being actually used on the project. These proven holding strengths then determine snow guard population to the specifics of the project scientifically, not by guesswork.

Using an adhered guard

The third type of snow retention is a stick-on part (chemical attachment). Some variations employ a factory-applied adhesive while others use a field-applied one. Adhered products are available in both plastic and metallic alternatives, but in discontinuous styles only. At first glance, stick-on snow guard systems appear to be very convenient and a lower cost option, but that is not the case when labor and material quantities are taken into account. Further, the replacement costs over the life of the roof can be extensive.

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Replacement is not ‘if’ but ‘when’ the adhesive diminishes in holding strength. All adhesives weaken when exposed to heat, cold, UV, and moisture. Replacement time is often within a few years after installation, depending on orientation to the sun and other environmental factors (Figure 9).

Due to diminishing holding strength over time, testing is not feasible (e.g. at what age should testing be done?). When they fail, the adhesives can also rip away paint coatings, leading to corrosion (Figure 10). Industry groups, such as MCA, Metal Building Manufacturers Association (MBMA), and the U.S. Army Corps of Engineers (USCOE) strongly advise against their use.

What to expect with adhered attachments

At first glance, installation is simple. However, a close look at the detailed instructions reveals the process is a complex one—careful surface preparation is required as well as many days of curing time (specific temperatures for some). Expect to re-adhere stick-on parts or replace them every three to six years or so.

How to choose the best snow retention system

Since there are no industry standards or mandates for design, manufacture, use, or testing of snow guards, it is a ‘buyer-beware’ scenario, as to the appropriateness and proof of testing and engineering performed by the vendor.

How does one discern prudent product selection from sales rhetoric? In addition to understanding the art and science behind snow retention systems, one should vet the vendor and their product offerings.

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Vet and specify

Vendor transparency is requisite to adequately vet
a snow guard system. The designer should scrutinize vendor/manufacturer qualifications/certifications in order to ensure a safe, engineered application and long-term service (read MCA’s technical bulletin on “Qualifying Snow Retention Systems”).

Proof of testing

To resist the forces applied to any system, the failure point of the snow retention system must be known, and the appropriate factors of safety employed to determine allowable structural capacity. Then, the population and frequency of the system is determined, so it cannot fail. Since the system may comprise multiple components, a ‘load chain’ could result. The calculated vector forces are transmitted through this chain into the building structure. Each link (component) in the chain must be proven by testing and/or engineering analysis. The weakest link determines the strength of the chain. The requisite engineering cannot be completed without reliable, extensive testing.

Anchorage of clamps or brackets to the roof specimen should be repetitiously tested on the specific substrate of the project. A minimum of three test repetitions should be conducted. In the case of SSMR clamped systems, the testing should be specific to roof materials, gauge, manufacturer, and seam profile of the project, as holding capacities differ widely.

Testing of these system components should be conducted by a third-party lab with the International Organization for Standardization (ISO) 17025, Testing and Calibration Laboratories, accreditation. It is also not scientifically acceptable to apply a test result from one set of specimens to another similar-styled roof product or assembly.

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Proof of engineering

Project-specific engineering should be provided by the vendor. It must incorporate the tested ultimate strength of the system with the application of an appropriate factor of safety. It is important to review the calculations prior to product selection. Ideally, the vendor should offer a web-based calculator with real-time output showing calculations and allowable loads specific to a project.

At a minimum, require these calculations with submittals. It is also recommended to specify they are stamped by a registered professional engineer.

Proof of certified manufacturing

How can you know if the product tested is truly the one you 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 a facility compliant with ISO 9001-15, Quality management systems — Requirements.

Warranties

Does the manufacturer offer a meaningful performance (not just material) warranty? Obtain a copy prior to specification, and it is advisable to read the fine print. Will they be in business for the long-term to honor the warranty, if it is 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 in question been proven effective?”

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Further

It is important to enforce the specification. A lack of code mandates enables a free pass to unqualified systems and potential designer liability. Require the vendor to provide a written statement of compliance with MCA’s guidelines (details at MCA’s technical bulleting on “Qualifying Snow Retention Systems for Metal Roofing”).

Conclusion

Metal roofs are known for their durability, sustainability, and versatility. However, they are slippery and can cause rooftop avalanches in the discharge areas below the eaves, thus resulting in property damage and personal injury or even death.

A scientifically tested and engineered snow retention system mechanically resists sliding snow, so the buildup of snow and ice on a roof evacuates in a predictable and controlled fashion rather than by a sudden release of snow.

To determine the best snow guard system for a metal (or other slippery) roof, one should understand the art and science behind snow retention systems, and most importantly, vet the manufacturer and their product offerings.

A smart and rather low-cost investment in a snow retention system specific to the project’s metal roof profile not only protects the roof and roof elements, but also the rest of the building and its occupants, pedestrians, vehicles, equipment, and landscaping below. It reduces short- and long-term maintenance costs, and, mostly, reduces potential liability for building owners and designers.

[12]Rob Haddock, the inventor of S-5! attachment technology, is a former contractor, an award-winning roof-forensics expert, author, lecturer, and building envelope scientist. Haddock was awarded the CSI 2018 Ben John Small award, and has worked in various aspects of metal roofing for nearly five decades.

Source URL: https://www.constructionspecifier.com/specifying-snow-retention-systems/

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