Public Service Announcement - Snowloads

#2018-02
February 7, 2018
 

SMITHERS – The recent volume of snowfall has prompted residents to be concerned with accumulated snow loads on home roofs.

Homes built prior to 1985 or homes that are developing icicles or ice damming at the eaves may need some clearing.  Attic ventilation—ridge vents, whirly vents, flat vents—that are blocked, may be restricting air flow that cools and removes moisture from the attic.  Heat escaping from the house and trapped in attic spaces can melt snow at the shingles and cause a flow of water to the cold overhang at the soffits.  So clearing vents is a place to start!

The BC Building Code establishes the Ground Snow Load which is used to calculate the Roof Snow Load that engineers and builders factor in construction.  The Ground Snow Load has changed over the past three decades:  1985—2.2 Kpa, 1998—2.7 Kpa, 2003—2.9 Kpa, 2006 to present—3.2 Kpa.

The Town of Smithers’ current ground snow load is 3.2 kPa plus 0.2 kPa rain component, for a total 3.4 kPa, or 69 lbs/ft2. For building design, this value is generally reduced by 40% for residential buildings and by 20% for commercial buildings, known as the specified roof snow loads. This equates to 41psf for residential and 55psf for commercial.

Pure water density is around 1000 kg (or one tonne) per cubic metre or 62.4 lbs  per cubic foot. Newly fallen snow more typically weighs 70 to 150 kg per cubic metre (4.4 to 9.4 lbs/cubic foot). Typically winter snowpacks have a density of 200-300 kg per cubic metre (12.5-18.7 lbs per cubic foot).

If we use 300kg/m3 (18.7pcf), then the current design depth of snow on roofs is 2.2ft on residential and 2.9ft on commercial. Owners should consider removing the snow if the packed accumulated snow exceeds these depths.  If heavy rains fall on these snow depths, the roof loading and risks of structure failure increase significantly.

Owners of older homes and buildings that were designed to a lighter snow load should consider removing snow at lower depths than those listed  above.

1. Do not shovel alone – have a spotter
2. Use fall-restraint or a rope safety line
3. Do not over-exert yourself
4. Remove snow evenly from the roof to avoid unbalanced loading
5. Do not shovel all the way to the roof surface – leave 50mm (2 inches)
6. COMMERCIAL BUILDING ROOFS—Do not clear onto public sidewalks or roadways unless barricades are set up, and follow up to clear the right of way for public safety.

Please click here to view the following PDF documents:

Roye Lovgren, Building Official
Town of Smithers, Phone: 250 847-1600