By David Kowch
With the growing trend of more environmentally conscious construction, metal roofs are becoming the choice of architects and builders. The lifespan of metal roofs is anywhere from 40 to 70 years. They are also environmentally friendly, as they can be recycled at end of their life cycle.
Every metal roof needs a properly designed and engineered snow guard system. A snow guard increases friction between roof and snow, retaining a snowpack on a roof, so it evacuates in a predictable and controlled fashion (evaporation and thaw) rather than by a sudden and dangerous rooftop avalanche.
Why use a snow guard?
Falling snow forms a temperature-sensitive bond to the surface of a metal roof. As that roof warms—either from the sun or building heat loss—the bond with the snow is broken and a thin film of melt water serves to lubricate the roof. This can have dramatic results as a several-ton blanket of snow suddenly slides off the roof and avalanches upon anything in its path—gutters, vehicles, landscapes, and even people. Once piled up below, that same snowbank can damage the exterior walls either directly or indirectly by funnelling melt water into, rather than away from, the wall. Snow guards allow snow to leave the roof slowly, either in small amounts of snow or as melted water, thereby avoiding avalanches.
The use of properly designed and engineered snow guards can:
- limit liability;
- protect building occupants and equipment such as air-conditioner units installed below upper roofs;
- safeguard structural and roof elements such as vent stacks, signage, lighting, and architectural elements; and
- reduce maintenance costs because snow can damage gutters and landscapes.
Sliding snow along the roof is impacted by various factors with each one affecting the snowpack in different ways.
When snow blankets the roof, the ambient air is typically at or below the freezing point. This creates a frictional bond, allowing the snowpack to remain on the roof. If the ambient air rises above freezing, the snowpack begins to melt from the top, and the heavier, wetter snow moves downward toward the roof panel. As this cycle continues, the heavier, wetter snow forms melt water along the roof panel, thus weakening the frictional bond between the snowpack and roof. The frictional bond would eventually be reduced to the point where the snowpack slides or avalanches down the roof.
Solar thaw works in the opposite manner of ambient thaw; the snowpack is melted from the bottom along the roof. Solar thaw occurs when sunlight passes through the snowpack and heats up the roof surface. This, in turns, creates melt water along the roof and snowpack. It is interesting to note thawing is a common factor for sliding snow especially when the roof colour is darker, which has a higher absorption value than a lighter hue.
Heat loss thaw
Similar to solar thaw, “…heat loss thaw occurs between the snowpack on the surface of the roof. Heat loss thaw allows for heat from within the building escaping through roof construction warming the surface of the roof to temperatures above outside ambient air,” per the Metal Construction Association’s (MCA’s) Metal Roof Designs for Cold Climates.
The ability of snow guards to retain sliding snow on the roof is dependent on many factors. Snow loads are one of the most important, but commonly overlooked, considerations.