Snow Load

Snow load is the gravitational load on a roof surface from accumulated snow, calculated per IS 875 Part 4:1987. Although India is not a snow-dominated country, snow loads are critical for roof design in Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, Arunachal Pradesh, and high-altitude regions of Nepal/Bhutan border. The basic ground snow load Sg ranges from 0.5 to 5+ kN/m² depending on elevation and location, with values from IS 875 Part 4 Annex A providing data for major Indian hill stations.

The design snow load on roof Sd = μ × Sg, where μ is the shape coefficient depending on roof type, slope, and exposure. Flat roofs (slope ≤ 15°): μ = 0.8 for unobstructed, exposed roofs; 1.0 for normal roofs. Sloped roofs (15-60°): μ decreases linearly from 0.8 to 0 at 60° slope (steep roofs shed snow). Sloped roofs above 60°: μ = 0. Drift snow accumulation behind parapets, against walls, or in valleys can reach μ = 1.5-3.0 — IS 875 Part 4 Annex B provides drift coefficients for typical Indian roof geometries.

For a typical hilltop residential roof in Manali (Sg ≈ 1.5 kN/m²) with 30° slope and obstructed access (μ = 0.6): design snow load = 0.9 kN/m². For Shimla (Sg ≈ 2.0) on a flat utility roof: design snow = 1.6 kN/m². These values are added to dead load, live load, and wind load per IS 875 load combinations. The most-overlooked aspect is sliding snow — when snow accumulates and then melts/slides off, it can damage adjacent roofs, gutters, and architectural projections. IS 875 Part 4 provides guidance for sliding-snow design but is rarely fully implemented in routine practice.