Snow on two-level flat roofs — measured vs. 1990 NBC loads

Abstract

Between 1967 and 1982, depths and specific gravities of snow were recorded on 44 single- and multi-level flat-roofed buildings between Halifax and Edmonton. The average density of snow in the drifts where the roofs change elevation was about 3.0 kN/m3, the value used consequently in the 1990 National Building Code of Canada (NBC). This is some 25% higher than the value used in the 1985 NBC. Data on drift geometry and maximum loads in the drifts are presented and compared with provisions in the 1990 NBC. As well, the paper presents measured values of average and maximum roof-to-ground load ratios for upper level roofs and for lower roofs away from the drifts. These compare favourably with those recommended in the 1985 and 1990 NBC. The statistical variabilities of snow loads and densities are given, since these are required to establish load factors used for limit states design in the NBC. Further research needs are identified. Key words: snow loads, snow drifts, uniform snow, flat roofs, snow density, snow load variability, snow load survey.