Ordinary flat, pitched and curved roofs, and awnings and canopies other than of fabric construction supported by lightweight rigid skeleton structures, are permitted to be designed for a reduced roof live load as specified in the following equations or other controlling combinations of loads in Section 1605, whichever produces the greater load. In structures such as greenhouses, where special scaffolding is used as a work surface for workers and materials during maintenance and repair operations, a lower roof load than specified in the following equations shall not be used unless approved by the commissioner. Such structures shall be designed for a minimum roof live load of 12 psf (0.58 kN/m2).
Lr = 20R1R2 (Equation 16-24)
where: 12 ≤ Lr ≤ 20
For SI: Lr = LoR1R2
where: 0.58 ≤ Lr ≤ 0.96
Lr = Reduced live load per square foot (m2) of horizontal projection in pounds per square foot (kN/m2).
The reduction factors R1 and R2 shall be determined as follows:
- R1 = 1 for At ≤ 200 square feet (18.58 m2) (Equation 16-25)
- R1 = 1 for At ≤ 200 square feet (18.58 m2) (Equation 16-26)
For SI: 1.2 - 0.011At for 18.58 square meters < At < 55.74 square meters
- Rt = 0.6 for At ≥ 600 square feet (55.74 m2) (Equation 16-27)
|At||=||Tributary area (span length multiplied by effective width) in square feet (m2) supported by any structural member, and|
|F||=||for a sloped roof, the number of inches of rise per foot (for SI: F = 0.12 x slope, with slope expressed as a percentage or for an arch or done, rise-to-span ratio multiplied by 32)|
- R2 = 1 for F ≤ 4 (Equation 16-28)
- R2 = 1.2 - 0.05 F for 4 ≤ F ≤ 12 (Equation 16-29)
- R2 = 0.6 for F ≥ 12 (Equation 16-30)
Roofs used for promenade purposes shall be designed for a minimum live load of 60 psf (2.87 kN/m2). Roofs used for roof gardens or assembly purposes shall be designed for a minimum live load of 100 psf (4.79 kN/m2). Roofs used for other special purposes shall be designed for appropriate loads, as directed or approved by the commissioner.
Where roofs utilize a green roof system and are not intended for human occupancy, the uniform design live load in the area covered by the green roof shall be 20 psf (0.958 kN/m2). The weight of the landscaping materials shall be considered as dead load and shall be computed on the basis of saturation of the soil. Where roofs utilize a green roof system and are used for human occupancy, the minimum live load shall be as specified in Table 1607.1 or Section 1607.11.2.2, whichever is greater.
Girders and roof trusses (other than joists) over garage areas regularly utilized for the repair of vehicles and over manufacturing floors or storage floors used for commercial purposes shall be capable of supporting, in addition to the specified live and wind loads, a concentrated live load of 2,000 pounds (908 kg) applied at any lower chord panel point for trusses, and at any point of the lower flange for girders.