Snow Load Calculator

Under EN 1991-1-3 the roof load is s = mu1 x Ce x Ct x sk, and the three coefficients do three different jobs. mu1 handles pitch: 0.8 below 30 degrees, falling linearly to zero between 30 and 60, and zero above 60 because the snow slides off. Ce handles exposure, and EN 1991-1-3 cl. 5.3.3 gives the recommended value as 1.0; the banding of 0.9 fully exposed, 1.0 partly exposed and 1.2 sheltered comes from ASCE 7 Table 7.3-1 and some national annexes, so do not mix the two. Ct handles thermal and is also 1.0 by the EN recommendation, rising only for cold roofs that never melt, 1.1 to 1.2 under ASCE. One counter-intuitive point: snow load is applied to the plan projection, not the sloped area. Snow is measured over its horizontal footprint on the ground, and spreading it over a slope thins it without changing that footprint, so do not multiply by the roof factor the way you would for tiles — that double counts. A steep roof is not automatically safe either: above 60 degrees the coefficient is zero, yet valleys, parapets and the lower slope of a stepped roof are drift zones where local load can be more than double, and those must be checked separately. Light steel roofs also need the unbalanced case — one slope melted and the other did not — because half-span loading can produce the same moment as full span.