4.1.7.1. Specified Snow Loading (1) The specified loading, S, due to snow accumulation on a roof or any other building surface subject to snow accumulation shall be calculated from the formula   where

Ss

is the ground snow load in kPa, determined in accordance with Subsection 2.5.1.,


 

Sr

is the associated rain load in kPa determined in accordance with Subsection 2.5.1., but not greater than Ss(Cb·Cw·Cs·Ca),


 

Cb

is the basic roof snow load factor of 0.8,


 

Cw

is the wind exposure factor in Sentences (2) and (3),


 

Cs

is the slope factor in Sentences (4), (5) and (6), and


 

Ca

is the accumulation factor in Sentence (7). (2) Except as provided for in Sentence (3), the wind exposure factor, Cw, shall be 1.0. (3) The wind exposure factor in Sentence (2) may be reduced to 0.75, or in exposed areas north of the treeline to 0.5, where (a) the building is in an exposed location, so that the roof is exposed to the winds on all sides, with no obstructions higher than the roof located closer to the building than a distance equal to 10 times the height of the obstruction above the roof, (b) the area of roof under consideration is exposed to the wind on all sides with no significant obstructions on the roof, such as parapet walls, within a distance of at least 10 times the difference between the height of the obstruction and Cb·Cw·Ss/γ metres, where γ is the unit weight of snow on roofs, and (c) the loading does not involve accumulation of snow due to drifting from adjacent surfaces. (4) Except as provided for in Sentences (5) and (6), the slope factor, Cs, shall be (a) 1.0 when the roof slope, α, is equal to or less than 30°, (b) (70° – α)/40° when α is greater than 30°, but not greater than 70°, and (c) 0 when α exceeds 70°. (5) The slope factor, Cs, for unobstructed slippery roofs where snow and ice can slide completely off the roof shall be (a) 1.0 when the roof slope, α, is equal to or less than 15°, (b) (60° – α)/45° when a is greater than 15°, but not greater than 60°, and (c) 0 when α exceeds 60°. (6) The slope factor, Cs, shall be 1.0 when used in conjunction with accumulation factors for increased snow load as given in Subclauses (7)(c)(ii) and (7)(c)(v). (7) The accumulation factor, Ca (a) shall be 1.0, except that (b) for large flat upper or lower roofs it shall be (i) 1.2 [1 – (30/l*)2] but not less than 1.0, for roofs with Cw = 1.0, or (ii) 1.6 [1 – (120/l*)2] but not less than 1.0, for roofs with Cw = 0.75 or 0.5, where  l*

=

the characteristic length of the upper or lower roof defined as l* = 2w – w2/l, in metres,


W

=

the smaller plan dimension of the roof, in metres,


L

=

the larger plan dimension of the roof, in metres, and (c) where appropriate for the shape of the roof, shall be assigned other values which account for (i) non-uniform snow loads on gable, arched or curved roofs and domes, (ii) increased snow loads in valleys, (iii) increased non-uniform snow loads due to snow drifting onto a roof which is at a level lower than other parts of the same building or at a level lower than another building within 5 m of it, (iv) increased non-uniform snow loads on areas adjacent to roof projections, such as penthouses, large chimneys and equipment, and (v) increased snow or ice loads due to snow sliding or drainage of meltwater from adjacent roofs.