Snow Load

Snow load is the downward force that accumulated snow, ice and meltwater place on a roof. In cold climates it is often the largest load a roof structure carries, and excessive snow is a recognised cause of roof failures.

From ground to roof

Design standards start with a ground snow load for the location, based on climate records, and then adjust it to estimate the load on a specific roof. In the United States, ASCE 7 considers the ground snow load, the roof's exposure to wind, its thermal condition (heated or unheated), roof shape and slope, drifting, and the importance of the building. Europe uses Eurocode 1, Part 1-3 (snow loads), which sets out characteristic ground snow loads for regions of Europe and shape coefficients for different roofs. Canada's National Building Code and other national codes use similar approaches.

Snow is variable

Snow density changes a great deal. Fresh, dry snow is light, but a settled snowpack may reach about 30 percent of the density of water, and around 50 percent by late spring. Rain falling on snow, or snow that melts and refreezes, can add weight quickly. Ice, near the density of water, is far heavier than the same depth of fresh snow, so depth alone is a poor guide to load.

Drifts and unbalanced loads

Wind moves snow from one part of a roof to another. Drifts form:

• against higher walls or parapets on lower roofs;
• in valleys between roof planes;
• on the leeward side of ridges, creating unbalanced loads;
• around rooftop units and penetrations.

Wind-blown snow can form deposits several metres deep in sheltered spots, so drift loads often control the design of lower roofs next to taller buildings.

Ponding and freezing

On flat and low-slope roofs, meltwater that cannot drain adds load, and decking that sags under the weight can collect even more water. In cold climates, ponded water can freeze and damage the roof surface.

Practical guidance

Signs of overload in buildings include sagging, cracking sounds, and doors that stick. Removing snow can be dangerous for people and can damage roofing; guidance generally recommends roof rakes from the ground or qualified contractors. Snow guards on steep roofs keep snow from sliding off suddenly but also retain the load on the roof, which the structure must be designed to carry.