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Roofs shall be designed for the controlling of environmental snow loads. Live load reductions do not apply to environmental loads.
(a) Flat-Roof Snow Loads. The snow load pf on an unobstructed flat roof shall be calculated as 25 pounds-force per square foot. “Flat” as used herein refers not just to dead-level roofs but to any roof with a slope of less than one in./ft. five degrees).
(b) Sloped-Roof Snow Loads, ps. All snow loads acting on a sloping surface shall be considered to act on the horizontal projection of that surface. The slope-roof snow load ps shall be obtained by multiplying the flat- roof snow load pf by the roof slope factor Cs:
ps = Cspf Equation 68-5.3
Values of Cs for warm roofs and cold roofs are as follows:
Slope | Cs |
less than 30° | 1.00 |
less than 40° | 0.75 |
less than 50° | 0.50 |
less than 60° | 0.25 |
70° and greater | 0.00 |
(c) Roof Slope Factor For Curved Roofs. Portions of curved roofs having a slope exceeding 70 degrees shall be considered free from snow load. The point at which the slope exceeds 70 degrees shall be considered the “eave” for such roofs. For curved roofs, the roof slope factor Cs shall be determined by basing the slope on the vertical angle from the “eave” to the crown.
(d) Roof Slope Factor For Multiple Folded Plate, Sawtooth, And Barrel Vault Roofs Including Bowstring Trusses. No reduction in snow load shall be applied because of slope (that is, Cs = 1.0 regardless of slope, and therefore ps = pf).
(e) Unloaded Portions. The effect of removing half the balanced snow load from any portion of the loaded area shall be considered. For unsymmetrical loading refer to ANSI A58-1-1982.
(f) Unbalanced Roof Snow Loads. Winds from all directions shall be considered when establishing unbalanced loads. See ANSI A58.1-1982 for unbalanced snow load configurations.
(g) Drifts On Roofs. Roofs shall be designed to sustain localized loads from snow drifts that can be expected to accumulate on them in the wind shadow of higher portions of the same structure. The surcharge load due to snow drifting shall be determined as provided for in Section 7.7 of ANSI A58.1-1982.
(h) Roof Projections. A continuous projection longer than 15 feet may produce a significant drift on a roof. The loads caused by such drifts shall be considered to be distributed triangularly on all sides of the obstruction that are longer than 15 feet. The magnitude of drift surcharge loads and the width of the drift shall be determined by using the method developed for lower roofs in Section 13-52-280(g).
(Prior code § 68-3.6; Added Coun. J. 11-29-89, p. 8387)