The Wind Exposure Factor

The Slope Factor (i.e. Slippery Roof Factor)   1995 NBCC Conditions under which you can assume the roof is “Slippery”

a.      the roof surface is “slippery” (i.e. steel, glass) and,

b.      snow and ice can slide completely off the roof

 Conditions under which you cannot assume the roof is “Slippery”

a.      the roof surface is not “slippery” (i.e. not steel nor glass)

b.      in cases where snow and ice cannot slide completely off the roof

c.      in a valley section of a roof

d.      in cases where snow slides or melt water drains onto a lower roof 

The sketches below give some examples of which buildings qualify as “Slippery” and which do not. The sketches assume that each of the roofs is clad with steel and is steep enough to use the slope factor Cs.

The entire roof is slippery.

Both the upper and lower roofs are slippery despite the snow accumulation on the lower roof .

In both of these examples, the upper slope on the left side of the building is slippery. As far as the upper slopes on the right sides are concerned, it could be argued that they are not slippery because the snow and ice may not be able to slide completely off the roof, depending on the distance to the lower roof. The lower slope on the right side of the building is not slippery because snow and melted ice will fall from upper roof to lower roof except in figure 3(a) where lower roof can be considered slippery provided the slope on the lower and upper roofs are the same.

FIGURE-(3a)                                                                                               FIGURE-(3b)

In this example, the upper slope on the left side of the building is slippery. The upper slope on the right side is not slippery because the snow and ice cannot slide completely off the roof. The lower slope on the right side of the building is not slippery because snow and melt water drain onto them from a higher roof.

Both upper slopes are slippery because snow and ice can slide completely off the roof. Neither of the two lower slopes is slippery because snow and melt water drain onto them from above.

In this example, the roof may be considered slippery everywhere except in the valleys. The trusses directly below valley framing should therefore not be designed using the slippery factor.