The Ontario Building Code | Specified Snow Load Specified Snow Load

(1) The specified load, S, due to snow and associated rain accumulation on a roof or any other building surface subject to snow accumulation shall be calculated from the formula,

S = Is [Ss (CbCwCsCa) + Sr]


Is = importance factor for snow load as provided in Table,

Ss = 1-in-50-year ground snow load, in kPa, determined in accordance with Subsection 1.1.2.,

Cb = basic roof snow load factor in Sentence (2),

Cw = wind exposure factor in Sentences (3) and (4),

Cs = slope factor in Sentences (5), (6) and (7),

Ca = shape factor in Sentence (8), and

Sr = 1-in-50-year associated rain load, in kPa, determined in accordance with Subsection 1.1.2., but not greater than Ss(CbCwCsCa).

Importance Factor for Snow Load, IS

Forming Part of Sentence


Column 1

Column 2

Column 3

Importance Category

Importance Factor, Is



















(2) The basic roof snow load factor, Cb, shall be 0.8, except that for large roofs it shall be,

(a) 1.0 – (30/lc)2, for roofs with Cw = 1.0 and lcgreater than or equal to 70 m, or

(b) 1.3 – (140/lc)2, for roofs with Cw = 0.75 or 0.5 and lcgreater than or equal to 200 m,


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

w = smaller plan dimension of the roof, in metres,

l = larger plan dimension of the roof, in metres.

(3) Except as provided for in Sentence (4), the wind exposure factor, Cw, shall be 1.0.

(4) For buildings in the Low and Normal Importance Categories as set out in Table, the wind exposure factor given in Sentence (3) may be reduced to 0.75, or to 0.5 in exposed areas north of the treeline, where,

(a) the building is exposed on all sides to wind over open terrain as defined in Clause , and is expected to remain so during its life,

(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 CbCwSs/γ metres, where γ is the unit weight of snow on roofs, and

(c) the loading does not involve the accumulation of snow due to drifting from adjacent surfaces.

(5) Except as provided for in Sentences (6) and (7), the slope factor, Cs, shall be,

(a) 1.0 where the roof slope, α, is equal to or less than 30°,

(b) (70° - α)/40° where α is greater than 30° but not greater than 70°, and

(c) 0 where α exceeds 70°.

(6) 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 α is greater than 15°, but not greater than 60°, and

(c) 0 when α exceeds 60°.

(7) The slope factor, Cs, shall be 1.0 when used in conjunction with shape factors for increased snow loads as given in Clauses (8)(b) and (e).

(8) The shape factor, Ca, shall be 1.0, except that where appropriate for the shape of the roof, it shall be assigned other values that account for,

(a) non-uniform snow loads on gable, arched or curved roofs and domes,

(b) increased snow loads in valleys,

(c) increased non-uniform snow loads due to snow drifting onto a roof that 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,

(d) increased non-uniform snow loads on areas adjacent to roof projections, such as penthouses, large chimneys and equipment, and

(e) increased snow or ice loads due to snow sliding or meltwater draining from adjacent roofs.