Section 1605

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SECTION 1605 - ROOF DESIGN

1605.1 General. Roofs shall sustain, within the stress limitations of this code, all dead loads plus unit live loads as set forth in Table 16-C. The live loads shall be assumed to act vertically upon the area projected upon a horizontal plane.

1605.2 Distribution of Loads. Where uniform roof loads are involved in the design of structural members arranged so as to create continuity, consideration may be limited to full dead loads on all spans in combination with full live loads on adjacent spans and on alternate spans.

EXCEPTION: Alternate span loading need not be considered where the uniform roof live load is 20 pounds per square foot (0.96 kN/) or more or the provisions of Section 1605.4 are met.

For those conditions where light-gage metal preformed structural sheets serve as the support and finish of roofs, roof structural members arranged so as to create continuity shall be considered adequate if designed for full dead loads on all spans in combination with the most critical one of the following superimposed loads:

1. Snow load in accordance with Section 1605.4.

2. The uniform live load set forth in Table 16-C on all spans.

3. A concentrated gravity load of 2,000 pounds (8.9 kN) placed on any span supporting a tributary area greater than 200 square feet (18.58 ) so as to create maximum stress in the member whenever this loading creates greater stresses than those caused by the uniform live load. The concentrated load shall be placed on the member over a length of 21/2 feet (762 mm) along the span. The 2,000-pound (8.9 kN) load need not be applied to more than one span simultaneously.

4. Water accumulation as prescribed in Section 1605.6.

1605.3 Unbalanced Loading. Unbalanced loads shall be used where such loading will result in larger members or connections. Trusses and arches shall be designed to resist the stresses caused by unit live loads on one half of the span if such loading results in reverse stresses, or stresses greater in any portion than the stresses produced by the required unit live load upon the entire span. For roofs whose structures are composed of a stressed shell, framed or solid, wherein stresses caused by any point loading are distributed throughout the area of the shell, the requirements for unbalanced unit live load design may be reduced 50 percent.

1605.4 Snow Loads. Snow loads full or unbalanced shall be considered in place of loads set forth in Table 16-C, where such loading will result in larger members or connections.

Potential unbalanced accumulation of snow at valleys, parapets, roof structures and offsets in roofs of uneven configuration shall be considered. Where snow loads occur, the snow loads shall be determined by the building official.

Snow loads in excess of 20 pounds per square foot (0.96 kN/) may be reduced for each degree of pitch over 20 degrees by as determined by the following formula:

(5-1)

For SI:

WHERE:

= snow load reduction in pounds per square foot (kN/) per degree of pitch over 20 degrees.

S = total snow load in pounds per square foot (kN/).

For alternate design procedure see Appendix Chapter 16, Division I.

1605.5 Special Roof Loadings. Roofs to be used for special purposes shall be designed for appropriate loads as approved by the building official.

Greenhouse roof bars, purlins and rafters shall be designed to carry a 100-pound-minimum (444.8 N) concentrated load in addition to the live load.

1605.6 Water Accumulation. All roofs shall be designed with sufficient slope or camber to assure adequate drainage after the long-time deflection from dead load or shall be designed to support maximum loads, including possible ponding of water from any source, including snow, due to deflection. See Section 1608 for deflection criteria.