Snow Load Calculator

Shovel snow off your roof when the snow depth approaches the structural limit of your roof design. As a general rule, remove snow when it exceeds 12-18 inches of heavy wet snow or 24+ inches of light powder snow. Warning signs include: sagging ceiling tiles or drywall, cracking or popping sounds, doors sticking or not closing properly, visible deflection of roof beams, ice dams causing water backup. If your roof was designed for 30 psf ground snow load, typical residential roof can hold about 2-3 feet of light powder snow or 1 foot of wet snow before reaching critical load.

Snow density varies significantly by type: Light/fluffy new snow: 3-6 lbs per cubic foot (0.5-1 inch of water per foot). Packed/old snow: 10-15 lbs per cubic foot (2-3 inches water). Wet/heavy snow: 15-25 lbs per cubic foot (3-5 inches water). Ice: 57 lbs per cubic foot. A cubic foot of average snow (10 lbs) on a 1,000 sq ft roof at 1 foot depth adds 10,000 lbs of load. This is why wet snow is much more dangerous than powder - it can be 3-5 times heavier for the same depth.

Roof snow load is typically less than ground snow load because of wind scouring, roof slope shedding snow, and heat loss through the roof melting snow. The formula is: Roof Snow Load = Ground Snow Load × Exposure Factor × Thermal Factor × Importance Factor × Slope Factor. Exposure factor ranges from 0.7 (windy exposed areas) to 1.3 (sheltered). Thermal factor: 1.0 for unheated, 0.85 for heated structures. Slope factor reduces load for steep roofs (over 30 degrees). Most building codes use ground snow load maps as the starting point.

Key factors: Roof slope - steeper roofs shed snow naturally; flat roofs accumulate more. Roof temperature - heat loss melts snow from below but can create ice dams. Wind exposure - exposed roofs lose snow to wind; sheltered areas accumulate drifts. Roof obstructions - chimneys, vents, and valleys create drifting. Building importance - hospitals and emergency shelters have higher safety factors. Nearby buildings - taller buildings can drift snow onto lower roofs. Trees and terrain - wind patterns affect deposition. Regional climate - frequency of snow vs rain-on-snow events increases load significantly.