Each year, rooftop avalanches tragically claim lives and cause extensive property damage in snow-prone regions. To mitigate the risk of these hazards, designers should specify a certifiably tested and scientifically engineered snow retention system that ensures both the safety and longevity of the metal roof.
The specification must not only account for the design snow load but also consider the roof’s unique profile. This requires consideration of many factors. Seven key considerations are understanding the importance of an engineered system, types of snow guards, aesthetics and architectural integration, durability and service life, metallurgical compatibility, system safety and reliability, and quality assurance specification to execution.
Importance of an engineered system
The most critical consideration when specifying snow retention for metal roofs is ensuring the system is engineered to resist the forces it will encounter in service. Failure to properly execute this step can put the system at risk of failure, ultimately endangering life and safety. Due to site-specific variables, it must be engineered individually for each project.
horizontal snow bars or snow fences assembled
laterally across roofs.
Three key variables are the design roof snow load, roof slope, and roof run (dimension from eave-to-ridge). A fourth variable is the system’s tested holding capacity, or ultimate (or failure) load, when affixed to the specific roof type, material, and profile. Factors of safety are then applied to this failure load to determine the allowable load, assuring that the system will not fail in service. All four of these variables must be considered for each project site and for different roof surfaces within the same site.
Correctly calculating the required population of snow guards is critical for any given project. An inadequate population will fail under less-than-design snow loads, while an overly robust population will result in unnecessary costs. All variables should be defined by the plans or a structural engineer and are needed for proper design.
The correct population is determined by calculating the tributary service loads to the system and then matching those loads to the tested allowable load resistance of the specific snow retention system proposed for the project.
A force applied to a snow guard system is a relatively simple calculation, but results vary with the variables mentioned.
For reliable system design, request evidence of three quality assurances from the snow guard vendor: certified testing specific to the roof type, certified manufacturing processes to assure that the products tested are the same as those shipped, and proof of engineered calculations.
Types of snow guards
A snow retention system consists of components assembled onto a roof surface to immobilize a blanket of snow. There are two primary approaches to restraining snow on a metal roof:
Continuous systems: Horizontal components, commonly known as snow bars or snow fences, which are assembled laterally across the roof.
Discontinuous systems: Individual parts known as snow stops, blocks, or cleats, generally spot-located in some pattern of rows or staggered.
Both systems are typically installed at or near the eaves and may be repeated at intervals toward the ridge but with greater concentration near the eave area. The appropriate frequency of installation (population) is determined by the specific job conditions and load-to-failure characteristics of the chosen devices.
Both types of snow guards rely on the compressive strength of the snow blanket at the interface with the snow guard devices. This strength is greatest at the base of the blanket (i.e. the area toward the eave and immediately adjacent to the roof surface). The snow blanket is a rather large, monolithic slab with significant cohesive strength within itself. Both types of snow guards rely on the cohesive and shear strength of the snow blanket to bridge between rows or laterally from one discontinuous unit to the next.
Mechanically (not adhesively) attached snow guards of both types have demonstrated satisfactory performance when tested, engineered, and installed properly and adequately.
cleats, generally spot located in rows
or staggered patterns.
Aesthetics and thoughtful architectural integration
While safety should always be the primary concern when specifying snow retention systems, architects must also factor in the aesthetic appeal and its permanence. The objective is to select a system that complements the building’s overall architectural style while integrating into the roofscape, maintaining durability and performance for the entire expected roof life.
By choosing a system that matches the roof’s materials and color, architects can preserve the building’s aesthetic integrity. In other cases, the designer may opt for a visual statement by selecting a snow retention system with a bold contrasting finish.
Durability and service life
Materials and finishes should have a service life equal to the roof itself. Systems and components using organic materials (plastics) will embrittle, fade, or yellow over time and, thus, are not generally suitable for use on a 50-plus-year metal roof. Adhesives degrade over time due to age and exposure, reducing their holding strength and leading to failure well before the roof’s expected service life. This not only compromises performance but also leaves behind unsightly residues and can cause damage to the roof’s finish.
Metallurgical compatibility
Metals compatibility is another important consideration. Metal roofing materials are typically made from pre-painted steel or aluminum, copper, or zinc and have distinct characteristics in terms of durability, weight, and thermal expansion.
Snow retention devices should be made from materials compatible with the metal roof to prevent galvanic corrosion and ensure longevity. High-tensile aluminum alloys in snow retention systems, with stainless steel fastening hardware, are the most common. Brass or stainless steel is also used
to ensure metallurgical compatibility with certain roof materials.
Architects should confirm that the snow retention system is made from certified materials consistent with those used in testing to verify holding capacity. Certified manufacturing processes, including third-party audits conducted at an ISO 9001:2015 compliant facility, ensure that products sold and shipped are the same as those tested and have documented traceability. Proof of compliance should be provided via an up-to-date ISO certificate.
System safety and reliability
Architects must prioritize the safety of the snow retention system. If the system fails, snowpack can dump tons of snow below the eaves in seconds, endangering building elements, lower roofs, landscape, vehicles, property—and people. Inadequate snow guard systems, or none, create a life-safety issue and expose design professionals to potential liability.
The consensus standard for testing and certifying snow retention devices is the International Association of Plumbing and Mechanical Officials (IAPMO) Evaluation Criteria 029-2018 for Standing Seam Metal Roof-Mounted Rail-Type Snow Retention Systems. The evaluation report issued by IAPMO is evidence of compliance with this standard. This is a critical document to request when vetting snow guard systems.
Quality assurance specification
to execution
Snow bars or snow fences can be installed either by a clamping technique (non-penetrating) or by fastening screws through the roof material into the structure (penetrating). Snow stops, blocks, or cleats are attached using the same methods. Peel-and-stick adhesive tape or pumpable glue are not advised by the Metal Construction Association (MCA) as these adhesives degrade on a roof application.
The design team should work closely with vendors who understand the complexities of selecting snow guards, rails, or fences without damaging the roof or compromising its waterproofing integrity. Attachments should be consistent with the roof technology—non-penetrative for standing seam roofs and penetrative for face-fastened roofs, provided that the sealing methods meet or exceed industry best practices for the roof type.
Project management should require the vendor to provide a detailed list of project-specific variables outlined above, accompanied by load test reports with an appropriate factor of safety applied. The vendor should provide engineering calculations with or before product submittals and, ideally, offer a web-based calculator for real-time output to ensure the calculations align with the specific project and roof profile. Even better, require by specification that a registered Professional Engineer stamps calculations.
The MCA Technical Bulletin for “Qualifying Snow Retention Systems for Metal Roofing” is harmonious with EC 029-2018 (above), and compliance with both can also be utilized in the specification.
Finally, another consideration is the warranty. Does the manufacturer offer a meaningful performance warranty that covers both material and system performance for the long term? Obtain a copy before specification—and read the fine print, ensuring the manufacturer will stand by their product if issues arise.
Rob Haddock, director at Metal Roof Advisory Group Ltd., and CEO and founder of S-5!, is the inventor of S-5! attachment technologies. Haddock 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.