In Maine, the state building code requires a roof to be designed to resist a snow load of between 60 psf (on the coast) and 100 psf (in the northwest). These loads can be reduced as the slope of a roof increases above 6 in 12 and should be increased where drifting is likely. On a 3 in 12 slope roof these loads would translate to fresh wet snow depths of between 5 and 9 feet. You must design the roof for the expected load or buy a snow rake and use it as needed. These loading requirements have a lot of safety factors built in which explains why more roofs don't collapse every winter.