Lisø PhD Dissertation Manuscript - NTNU

and number of days, N, with a wind velocity above 10 m/s (N is an average for the three
coldest months of the year). See Table 1.
In addition to the balanced load sb, a drift load part sd has to be included in order to
take into account snow accumulation on leeward side of the roof due to drifting. The drift
load sd is a function of characteristic snow load on the ground s0, exposure coefficient Ce,
thermal coefficient Ct, slope reduction coefficient μb and drift load coefficient μd:
sd = s0·Ce·Ct·μb·μd (4)
The drift load coefficient μd multiplies with μb and defines the amount and
distribution of additional load on a leeward side or part of a roof. The coefficient depends
on wind exposure and geometry of the roof. In Fig. 3 the multiplication μb⋅μd for various
exposure coefficients Ce is shown for a single pitched roof with non-slippery surface.
The slide load ss take into account slide from an upper roof onto a lower roof, or a
lower part of a roof.
Fig. 3. Drifted shape coefficients μb⋅μd for simple pitched roofs with non-slippery surface.
2.2. The historical background of the exposure coefficient
In the period 1956 to 1967, the National Research Council of Canada established more than
50 observation stations across Canada, where snow depth and density measurements were
registered once a week and immediately following major snowfalls on the ground and for a
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