by Harry J. Lubitz, CSI, CDT
The use of snow retention devices originated several hundred years ago in areas of Scandinavia and the Alps. In these cold regions, builders and homeowners placed stones and logs on rooftops to increase their friction with the snow. The purpose of a snow guard is to facilitate the evacuation of rooftop snow in a predictable and controlled fashion—evaporation (sublimation) and thaw—rather than by a sudden and dangerous rooftop avalanche.
Keeping snow in place was important to the structure owner as this construction predated the use of modern insulation. A good layer of snow on the roof in these Alpine climates helped reduce heat loss and keep inhabitants warm. Retaining snow on the roof provided the additional benefit of protecting anyone or anything (such as farm implements and animals) around the structure from accidental release of accumulated snow pack, and subsequent potential injury from its release.
Stones placed on the roof acted as individual ‘cleats’ to engage the snow bank and keep it in place (Figure 1). Logs were akin to continuous fences that engaged the bank of snow. Both methods proved effective in retaining snow and preventing a rooftop avalanche.
In these regions, people have lived with a constant threat of avalanching snow for centuries. Snow depths can reach up to 10 m (32 ft) annually in these parts of the world (Figure 2). Consequently, they learned how to deal with and reduce the hazards of sliding snow.
There is a science to understanding rooftop avalanches. When snow blankets a roof surface, a frictional, temperature-sensitive, adhesive bond is created between the snow particles and the roof material. A weak cohesive bond is also created at the ridge of the roof connecting the snow packs on each side of the roof. The vertical weight of the snow translates to vector forces—‘drag’ or gravity loads.
At first, the adhesive and cohesive bonds together are sufficient to resist the drag load. However, when the weather clears and the sun’s ultraviolet (UV) rays pass through the translucent bank of snow to the roof surface below, the radiant heat is absorbed by the roof’s surface. The heat energy raises the surface temperature, altering the frictional co-efficient as the bond turns into meltwater. This can happen when ambient temperatures are well below freezing due to the insulating characteristic of the bank of snow. With the frictional (or adhesive) bond now jeopardized, the drag loads exceed the strength of the cohesive bond at the ridge and the snow bank quickly releases, causing a rooftop avalanche.
This science applies to all roof surfaces, the difference being the texture and porousness of the roof materials. For example, metal roofs are more prone to dangerous avalanche hazards because of their slick surfaces, particularly since their polyvinylidene fluoride (PVDF) coatings are chemically related to non-stick Teflon—whereas a granular roof surface would pose a lower immediate risk.
Using snow guards
A snow guard should be used to:
- protect building occupants and pedestrians;
- limit liability;
- shield vehicles and equipment;
- safeguard landscaping;
- protect building and roof elements; and
- reduce maintenance cost.
Several provincial and municipal jurisdictions, along with local school boards and national retailers, have added this design requirement to their building designs. Good practice for snowfall regions suggests roof orientation be diverted away from pedestrian traffic. However, this is often impossible and pedestrian traffic may have to pass below unprotected eaves. Clearly, the safety of individuals walking by a building outweighs the minimal cost of adding snow guards to a project design.
There have been several high-profile court cases where people were injured or killed by snow falling from roofs. For example, in 2011 at Super Bowl XLV in Dallas, Texas, at least six people suffered a range of injuries when warming weather increased the rooftop temperature and caused layers of ice and snow to avalanche from the stadium roof.