Re: High Slope vs. Low Slope Roof?
Why does the slope of the roof mattter? It is accounted for in the design equation. We use 1607.11.2.1 to NOT ALLOW A REDUCTION in roof loads below what has been previously established by a state university study. Example historical 50 year study says 46 lbs minimum ICC would allow 39 lbs minimum in our area. We require 46 lbs minimum no reductions period.
1607.11.2.1 Flat, pitched and curved roofs.
Ordinary flat, pitched and curved roofs are permitted to be designed for a reduced roof live load as specified in the following equation or other controlling combinations of loads in Section 1605, whichever produces the greater load. In structures 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 equation shall not be used unless approved by the building official. Greenhouses shall be designed for a minimum roof live load of 12 psf (0.58 kN/m2).
Lr = Lo R1R2 (Equation 16-27)
where: 12 £ Lr £ 20
For SI: Lr = Lo R1R2
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 16-28)
R1 = 1.2 – 0.001At for 200 square feet < At < 600 square feet (Equation 16-29)
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 16-30)
where:
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 16-31)
R2 = 1.2 – 0.05 F for 4 < F < 12 (Equation 16-32)
R2 = 0.6 for F > 12 (Equation 16-33)
where:
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.