1607.11.2 Reduction in Roof Live Loads
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 building official. Such structures shall be designed for a minimum roof live load of 12 psf (0.58 kN/m2).
Lr = Lo R1R2 (Equation 16-25)
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 16-26)
R1 = 1.2 — 0.001At for 200 square feet < At < 600 square feet (Equation 16-27)
For SI: 1.2 — 0.011At for 18.58 square meters < At < 55.74 square meters
R1 = 0.6 for At ≥ 600 square feet (55.74 m2) (Equation 16-28)
At = Tributary area (span length multiplied by effective width) in square feet (m2) supported by any structural member, and
R2 = 1 for F ≤ 4 (Equation 16-29)
R2 = 1.2 — 0.05 F for 4 < F < 12 (Equation 16-30)
R2 = 0.6 for F ≥ 12 (Equation 16-31)
F = For a sloped roof, the number of inches of rise per foot (for SI: F = 0.12 × slope, with slope expressed as a percentage), or for an arch or dome, the rise-to-span ratio multiplied by 32.