Roof Snow Load Calculator

Roof snow load is the weight of accumulated snow pressing down on your roof structure. In the US, snow-related roof collapses cause millions of dollars in damage annually — particularly during heavy winter storms in the Midwest, Northeast, and Mountain states. Understanding your roof's snow load capacity and monitoring accumulation is critical for structural safety.

The calculator above supports 6 snow types with density values, roof pitch adjustment, and a structural capacity check that compares current snow load against your roof's design load — displaying a clear warning when capacity is exceeded.

Find your pitch with our roof pitch calculator. Estimate roofing with our roofing calculator. Size rafters with our lumber calculator.

Snow Density Table

Snow weight varies dramatically based on moisture content and age. The same depth of fresh powder weighs 4× less than wet heavy snow.

Key insight: Snow on a roof often has multiple layers — fresh on top, packed/ice below. Use the heaviest type present for a conservative estimate, or calculate each layer separately and add them.

Step-by-Step Snow Load Calculation

Step 1: Calculate Snow Volume

Measure the roof footprint (length × width). Apply the roof pitch multiplier to get actual surface area. Multiply by snow depth (in feet) for volume in cubic feet.

Example: 30 × 20 ft at 4/12 pitch = 632 sq ft × 1 ft depth = 632 cu ft of snow.

Step 2: Determine Snow Density

Identify the snow type. Fresh powder: ~1.25 PSF/inch. Packed: ~3.13 PSF/inch. Wet: ~5.2 PSF/inch. When unsure, use packed (3.13) as a conservative middle estimate.

Step 3: Calculate Total Load

Snow Load (PSF) = density (PSF/inch) × depth (inches). Total Weight = PSF × roof area.

Example: Packed snow, 12 inches: 3.13 × 12 = 37.5 PSF × 632 sq ft = 23,700 lbs (11.85 tons).

US Regional Design Snow Loads

Building codes specify ground snow load (Pg) for each region. Roof snow load is typically 70% of ground load (per ASCE 7). Actual design load depends on exposure, terrain, and building importance.

Important: These are approximate ranges. Your specific design load is determined by local building codes, elevation, and exposure category. Check with your building department for the exact value for your location.

Roof Pitch and Snow

Steeper roofs shed snow more easily but have more surface area. The pitch factor affects load calculation in two ways:

• Area increase: A 12/12 pitch has 41% more surface area than a flat roof
• Snow shedding: Roofs steeper than 6/12 shed snow faster, reducing accumulation
• Code adjustment: ASCE 7 allows reduced snow loads for pitches above 30° (7/12) — called the "slope factor" (Cs)

The calculator uses the pitch multiplier for area but does not apply the slope reduction factor — giving you a conservative (worst-case) estimate.

Warning Signs of Snow Overload