The following is an excerpt from the article “Preventing Roof Collapse on Homes.” For more information please visit disastersafety.org.
ESTIMATE HOW MUCH THE SNOW ON YOUR ROOF WEIGHS USING THESE GUIDELINES FROM IBHS:
Fresh snow: 10-12 in. of new snow is equal to one in. of water, or about 5 lbs per square foot of roof space, so you could have up to 4 ft. of new snow before the roof will become stressed.
Packed snow: 3-5 in. of old snow is equal to one inch of water, or about 5 lbs per square foot of roof space, so anything more than 2 ft. of old snow could be too much for your roof to handle.
Total accumulated weight: two ft. of old snow and two ft. of new snow could weigh as much as 60 lbs per square foot of roof space, which is beyond the typical snow load capacity of most roofs.
Ice: one in. of ice equals one ft. of fresh snow.