Snow Retention Systems

Snow retention systems are used to retain the majority of snow on the roof and, thereby, prevent personal and property damage caused by avalanching snow.

Snow retention systems typically consist of snow guards (sometimes called snow brackets, snow dogs, or snow birds) and/or snow rails (sometimes called snow fences or pipe rails). Snow guards generally consist of a metal strap, the top end of which is secured to the roof deck and the bottom end of which contains a head, or pad, that projects above the roof surface. Snow guards have been manufactured in many different styles and shapes, including birds, stars, floral patterns, and wire loops (sometimes called “pig tails”). Snow rails typically consist of base plates that take the place of a slate shingle and metal rails that span between the base plates. In some systems, the base plates are adjustable to suit the slope of the roof. In other systems, the base plate is installed first to align with a bond line in the slate roof, flashed, and then slated around. Single, double, and triple rail systems have traditionally been available and continue to this day.

Despite their generally simple appearance, snow retention systems require proper engineering specific to each individual structure. What might serve well in one climate may not be much help at all in other climates. Factors that must be considered include roof size, roof slope and orientation, projected annual snow falls, roof construction, the weight of the snow held on the roof, fastener type(s), local code requirements, and appearance.

Whether snow guards, snow rails, or some combination of guards and rails are used, it is important to use enough snow retention devices. Be aware that the most common snow retention system, one to three rows of snow guards, staggered, at the roof eave, may be inadequate in many northern snow climates.

Snow avalanches are caused as friction is lost between the snow and the slate roof. In many cases, this is due to heat loss from the building. High daytime solar radiation can also cause snow to melt, especially at high elevations. Water from the melting snow acts as a lubricant and allows the snow to slide down the roof. A cold roof design with proper ventilation or a super-insulated roof may help reduce the chance of snow slides by keeping the temperature of the roof surface below freezing.

Where to install snow retention systems is one of the first questions that must be addressed. Sometimes the entire perimeter of a building must be protected with a snow retention system. In other cases, it may be enough to protect certain areas, such as walkways and entrances, lower roofs, valleys, skylights, gutters, landscaping, parking areas, and mechanical units.

It should be noted that in the most severe snow climates even the most robust proprietary snow retention systems may be inadequate. In certain parts of Canada, for example, custom-made iron and stainless steel fences several feet high can be seen on many large buildings.

In urban areas or where walkways are located immediately below the eaves of a slate roof, snow retention systems can also be designed to prevent broken slates from falling to grade. Such systems are typically custom-built on site and might incorporate a heavy-duty wire mesh to help retain the slates on the roof. Heat cable systems are sometimes also incorporated into snow retention systems.

Because each building’s roof and structural framing system is different and each climate is different, a comprehensive review of snow retention systems is beyond the scope of this post. At minimum, the manufacturer of the snow retention system should be consulted to determine the proper components and layout to be employed. In some situations, it may be necessary to retain the services of a structural engineer, as well. It is worth noting that sliding snow can rip under-designed snow retention systems from the roof.

Snow retention systems can both positively and negatively impact the potential for ice damming on a roof. The amount of snow fall, exposure to direct sunlight, snow drifting, ambient winter temperatures, and attic temperature are some of the factors that will influence a snow retention system’s impact on ice damming and the need for ice dam protection membrane. By holding wet masses of snow and ice on the roof, snow retention systems can allow water to build up behind them and migrate laterally. A full discussion of the ice damming potential associated with snow retention systems is beyond the scope of this post.