1 Your measurements

Type feet and inches the way you'd write them, like 12' 6" or 30 3/8. Decimals work too.

ft
Measure the footprint, overhangs included. Snow falls straight down, so roof pitch does not add area.
ft
in
Use the snow on the roof, not on the ground. Check it from a window or from the ground; don't climb onto a snow-covered roof to measure.
Not sure? Use wet, heavy snow: it gives the higher load.
in
lb per sq ft
From your construction drawings, or ask your building department.

2 Snow load

Load on the roof
21 lb per sq ft
wet, heavy snow
Total weight
25,200 lb
12.6 US tons over 1,200 sq ft
Snow12" at 21 lb per sq ft per foot = 21 lb per sq ft
If the snow is light and dry3 lb per sq ft
If the snow is wet and heavy21 lb per sq ft

Snow drifts deeper at valleys, dormers and where a higher roof meets a lower one, so the load is rarely even. Check the depth in several spots, from the ground or a window, and use the deepest.

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