Why Flat Roofs Are Hardest on Snow
Flat roofs carry the full snow load with no shedding, plus rain-on-snow and ponding risk. What to watch for and how to design.
Flat and low-slope roofs are the toughest case for snow because they get no help from gravity (nothing slides off), and they invite a couple of extra load cases.
No slope shedding
With essentially no slope, the slope factor Cs is 1.0, so the roof carries the full flat-roof load Pf. There is no reduction for shedding the way a steep roof gets.
Rain-on-snow and the minimum
In mild-winter areas (Pg ≤ 20 psf) a low-slope roof picks up a 5 psf rain-on-snow surcharge, because rain soaks into snow that can't drain. Low-slope roofs also must meet the §7.3.4 minimum load, which can govern in light-snow regions.
Drainage and ponding
Beyond snow, a flat roof must drain meltwater. Blocked drains plus melting snow lead to ponding, which adds weight and can spiral. Keep drains and scuppers clear through winter, and design positive drainage.
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