Snow load

Frequently asked questions

Why is drift the case that governs?

Because wind does not leave snow where it fell. An even blanket is the easy case and rarely the worst one: wind scours snow off open areas and deposits it where the air slows down — against a parapet, in the valley of a pitched roof, on the lower roof of a stepped building, behind a rooftop plant enclosure or a solar array. The drift is a triangular pile whose peak load can be several times the uniform figure, concentrated over a narrow band, and it sits exactly where a lower roof's structure was sized for the general case. That is why a step between two roofs is a structural question rather than an architectural one, why adding a parapet or a screen to an existing roof can overload it without adding any permanent weight, and why the drift surcharge is calculated separately from the balanced load rather than being covered by a margin on it. Roof collapses under snow cluster at these features rather than in the middle of large roofs.

How much does snow actually weigh?

Anything from about a twelfth of the weight of water to nearly half of it, which is why depth is a poor proxy for load. Fresh, dry, cold snow is mostly air and is light. The same snow settles under its own weight over days, and each melt-and-refreeze cycle packs it further, so an old drift can be several times as dense as it was when it fell. Rain falling onto lying snow is the extreme: the snowpack holds the water like a sponge and the load rises sharply with no change in depth at all, which is why Canadian practice carries a separate rain component and why warm-winter regions see snow failures at densities that would seem impossible from the depth. The practical reading is that a roof's remaining capacity cannot be judged by eye from the ground, and that the dangerous moment in a season is often not the heaviest snowfall but the thaw with rain that follows it.

Why do a warm roof and a cold roof carry different snow loads?

Because heat leaking through a roof melts the snow on it, and the codes account for that with a thermal factor. A well-heated building with a poorly insulated roof loses enough heat to melt lying snow from below, so it sheds part of the load — and the standards allow a reduction for it. An unheated structure, a cold-roof design, or a very well insulated modern roof keeps the snow, so it carries more; the factor goes the other way and the load rises. This produces a genuinely counterintuitive result worth stating plainly: INSULATING a roof increases the snow load it must be designed for, because it stops heat doing the work. That matters most on retrofit, where a loft or flat roof is upgraded to current insulation standards on a structure sized decades ago under the old assumption — and it is the same mechanism that stops the melt that used to feed ice dams at the eaves.