Matrix IMA: Roof Design in Regions of Snow and Cold

Complex roof geometry greatly increases the destructive influence of snow on roofs. Vents and chimneys and other projections through the roof surface should be at roof ridges, (Figure 3) not at eaves. Valleys that concentrate snow and ice can greatly increase roof loads. Roofs that cascade snow and ice from one level to another can cause serious damage to the lower roof. Snow, which falls from higher roofs to lower roofs, can rebound and break windows and walls.

Heated gutters at roof edges can reduce both ice damming and icicle formation, if properly designed, but if improperly constructed, they can be swept away. Attention must be given to downspouts or they will freeze. They must either be interior or, if exterior, heat traced so as to convey melt water into warm areas.

Snow on roofs can be beautiful but it adds considerable complexity to their design. The architect or engineer must allow for these special factors in the snow country.

IV. LOW SLOPE (FLAT) ROOFS

The design of low slope (flat) roofs in regions of snow and cold have their own unique parameters to integrate into the same building geometry that a designer would normally contemplate for warm weather climes. A flat roof in snow regions usually lends itself to wind stripping (i.e. some snow is blown away). Unless the building is relatively small and located in a very sheltered "hole" in the terrain, wind will play a role in removing some of the snow from the roof. Flat roof snow is usually considered to be about 70% of ground snow. This snow can be higher when the site is sheltered and may equal or even in excess of the ground snow. (ASCE 7-98)

During initial building planning, the designer should visit the site during cold and snowy conditions. Besides being able to experience the fixed geographical conditions, the designer can ascertain the unique microclimate of the site. Snow depth and storm wind direction is by far the most important climatic aspect to determine. The site reconnaissance is extremely important because surrounding terrain features can radically alter both storm wind direction and speed from available regional weather data. Furthermore, the designer can get a fairly good idea of how nearby similarly roofed buildings are performing. For example, logic will dictate that given a uniform snow distribution, the short dimension of a flat roof in line with the storm wind will allow the wind to blow the snow off the roof more readily (less length of roof snow). Conversely, a microclimate wind change of ninety degrees from the regional wind direction will put the long direction of the roof in line with the storm winds. Hence, more length of roof snow is subject to stripping and more snowdrift volume (per unit width) will be generated. The designer must be constantly aware of the storm wind direction when site planning the flat roof building. He must be aware of the snow distribution, the transport of the snow by the wind and the obstructions to snow transport.