Section 4.8.1/Roof Live Load Reduction
ASCE 7-22 Section 4.8 permits reduction of roof live loads based on tributary area and roof slope for ordinary roofs, reducing the design load from the base 20 psf.
The minimum uniformly distributed roof live load is 20 psf for ordinary flat, pitched, and curved roofs. Section 4.8.1 permits this load to be reduced based on two factors: tributary area and roof slope. The reduced roof live load Lr = Lo x R1 x R2, where Lo is the base roof live load (20 psf). R1 accounts for tributary area: R1 = 1.0 for At less than or equal to 200 square feet, R1 = 1.2 - 0.001 x At for At between 200 and 600 square feet, and R1 = 0.6 for At greater than or equal to 600 square feet. R2 accounts for roof slope: R2 = 1.0 for flat roofs (F less than or equal to 4), R2 = 1.2 - 0.05F for slopes between 4 and 12 (where F is the number of inches of rise per foot), and R2 = 0.6 for slopes of 12 or greater. The minimum reduced roof live load is 12 psf (R1 x R2 cannot reduce below 0.6 x 0.6 = 0.36, and the minimum is set at 12 psf). The roof live load must not be confused with the ground snow load or roof snow load, which are calculated separately and may govern over the roof live load in snow regions.
Why this section exists
Roof live loads represent temporary loads from maintenance workers, equipment, and materials during reroofing or repair. Unlike floor live loads, roof live loads act on large areas infrequently. The tributary area reduction reflects the statistical improbability that a large roof area will be simultaneously loaded by maintenance activity. The slope reduction reflects the reduced likelihood that workers and materials accumulate on steep roofs. The 12 psf minimum ensures a baseline capacity for any roof regardless of size or slope. In many locations, snow loads exceed the roof live load and govern the design, but the roof live load still applies in snow-free regions and must be checked independently in load combinations.
What plan reviewers look for
Plan reviewers check the structural general notes for the roof live load and verify the reduction is correctly applied. They confirm the tributary area used for R1 matches the framing plan. They verify the slope used for R2 matches the architectural roof plan. They check that the reduced roof live load is not less than 12 psf. They verify the roof live load is checked in load combinations separately from snow load (the two loads do not act simultaneously in ASCE 7 combinations). They verify that special roof uses (landscaped roofs, occupiable decks, photovoltaic arrays) use the appropriate floor live load, not the reduced roof live load.