Roof Snow Load Calculator

Why depth alone tells you nothing

Snow load is density times depth, and density swings by a factor of eight. Fresh powder runs about 7 pounds per cubic foot; settled snow 12; wind-packed 15; wet spring snow 20; and solid ice 57. So "a foot of snow" on your roof weighs anywhere from 7 to 20 psf depending on what kind of foot it is, and a modest-looking ice layer under it can quietly double the number. That is exactly the math the first mode of this calculator runs.

For scale: a 2,000 square foot roof under 15 psf is carrying 30,000 pounds, about eight pickup trucks. Roofs are engineered for this, which is why the number that matters is not the weight itself but the weight against what your roof was designed to hold.

What roofs are designed for

Building codes set a ground snow load for your area, and the roof structure is designed from it using factors for exposure (windswept roofs shed, sheltered roofs collect), thermal condition (heated buildings melt some load off; unheated garages do not), and slope (steeper sheds better, with design relief starting around 30 degrees). The design mode of this calculator walks that simplified chain so the code numbers stop being mysterious. Across most of the middle of the country the resulting roof design loads land between 20 and 40 psf; mountain and lake-effect regions go far higher.

Older homes, additions built without permits, and outbuildings are the caveat: they may predate the map or ignore it. An unheated detached garage with 2x4 rafters on 24 inch centers does not care what the code says your house can hold.

Drifts, valleys, and the loads averages miss

Uniform snow is the friendly case. Wind builds drifts against dormers, chimneys, and the step where a one-story roof meets a two-story wall, and those drifts can locally double or triple the load. Valleys collect slide-off from both planes. Solar panels create their own catch edges. When you measure depth for the calculator, measure the deep representative areas, and treat a major drift over a small roof section as its own problem.

What to actually do about heavy snow

• Rake from the ground. A roof rake on the eaves and first several feet of slope removes real weight and fights ice dams at the same time. Leave an inch or two on the shingles.
• Never climb a loaded roof. The fall risk is obvious; less obvious is that your concentrated weight is exactly what a stressed structure does not need.
• Watch the warning signs, not just the number. Sagging, sticking doors, popping sounds, new cracks. Signs plus load means leave and call for help.
• Mind the ice dams. If eaves ice up every winter, the roof is telling you about attic insulation and ventilation, not about snow. That is a fixable problem with its own guide.