Roof Snow Load Calculator

Snow accumulating on a roof can add enormous weight — sometimes hundreds of kilograms per square meter. Understanding the snow load on your roof helps determine whether it is approaching a dangerous threshold and whether action (like removing the snow) is needed to prevent structural damage or collapse.

The Formula Snow load (kg/m²) = Snow depth (m) × Snow density (kg/m³) Snow load (psf) = Snow depth (ft) × Snow density (lb/ft³)

Total weight on roof = Load per unit area × Roof area

Snow Density: Why It Matters So Much Fresh, light snow is mostly air — it weighs only about 50 kg/m³. But as snow settles, compresses, or gets wet, its density increases dramatically:

• Fresh/Light snow: 50 kg/m³ (3.1 lb/ft³) — freshly fallen, fluffy snow
• Settled snow: 200 kg/m³ (12.5 lb/ft³) — snow that has been on the roof a few days
• Wet/Packed snow: 400 kg/m³ (25 lb/ft³) — wet or compacted spring snow. Much heavier than it looks.
• Ice: 700 kg/m³ (43.7 lb/ft³) — if snow has refrozen into a solid layer, the load is extreme

A single inch of wet snow can weigh as much as 10–12 inches of fresh fluffy snow. This is why heavy snowstorms with wet snow cause far more roof collapses than fluffy winter snowfalls.

Typical Roof Load Ratings Building codes in North America generally require residential roofs to support a minimum live load of 20 psf (97 kg/m²). In high-snow regions, roofs are often engineered for 40–60 psf (195–293 kg/m²). Industrial buildings may have higher ratings.

As a practical safety guide:

• Below 20 psf (97 kg/m²): Generally safe for most residential roofs
• 20–25 psf (97–122 kg/m²): Monitor closely, consider removal if more snow is expected
• Above 25 psf (122 kg/m²): Action recommended — remove snow or consult a structural engineer
• Above 40 psf (195 kg/m²): Danger zone for typical residential roofs. Evacuate and seek professional help.

Other Risk Factors The formula gives a uniform snow load, but real roofs have additional considerations: ice dams can concentrate load at eaves, valley sections can accumulate double the depth, and older roofs may have reduced load capacity. Always err on the side of caution.

Worked Example — Metric Snow depth: 40 cm (0.40 m) of wet/packed snow on a 60 m² roof: Load = 0.40 m × 400 kg/m³ = 160 kg/m² Total weight = 160 × 60 = 9,600 kg (about the weight of 6 cars)

Worked Example — Imperial Snow depth: 18 inches (1.5 ft) of settled snow on a 700 sq ft roof: Load = 1.5 × 12.5 = 18.75 psf Total weight = 18.75 × 700 = 13,125 lb — approaching the typical residential limit.