Snow Loads and Strength of Small Roofs in Canada

Two types of conventional roof frames were tested by short-term loading and long-term loading. The conventional roof frames were tested both on fixed-end supports and on roller supports. The W-trusses tested had nailed plywood gusset plates. The W-trusses were loaded both short-term and long-term. Conventional frames were constructed of CLA#1 eastern or white spruce and 3-1/2 inch and 4 inch common nails. Nailing conformed to the requirements of the 1953 National Building Code of Canada and the 1958 Housing Standard. On non-specified joints nailing considered to be good building practice was used. Lumber used for the trusses was CLA#1 construction grade eastern or white spruce, plywood was 1/2-inch sheathing grade Douglas-fir, and nails were 3 inch common unclinched. Failure loads on the better designed conventional roof frames varied from 18 to 125 psf depending on heel joint details, size of rafter, and type of end support. The poorer designed conventional roof frames failure loads varied from 56 to 125 psf. In both types of construction the frames were considerably stronger when 2 by 6 or 2 by 8 rafters were used instead of 2 by 4 rafters. Results of the tests on the W-trusses gave performance data on the effects of roof slope, truss span, truss member size, nailing, and duration of loading. A number of factors which affect the strength, stiffness, and long-term performance of the trusses were also investigated. Truss strength depends to some extent upon truss stiffness.

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