Unbalanced Snow Load on Gable Roofs

When wind blows across a gable roof, it scours snow off the windward side and deposits it on the leeward side. The result is a roof carrying unequal loads: one side lightened, the other loaded above the balanced value. ASCE 7 §7.6 requires an unbalanced check for gable and hip roofs above a minimum slope threshold.

When ASCE 7 requires an unbalanced check

An unbalanced snow load check is required for gable roofs with a slope between 2.38° (1/2:12) and 30.2° (7:12). Slopes below that range are essentially flat and treated differently; slopes above 30.2° are steep enough that wind loading matters but the balanced load is already low from Cs.

The unbalanced load distribution

On the windward side, ASCE 7 allows a reduced load of 0.3 × Pf. On the leeward side, the load increases above Pf. For a symmetric gable the leeward load is Pf + 0.3 × Pf = 1.3 × Pf on the leeward slope projection. The ridge still carries the full balanced load. This asymmetric load creates asymmetric bending in the ridgeboard, purlins and ridge beam that the balanced case does not produce.

Why it matters for design

Many roof framing members, particularly ridgeboards in older construction that rely on ridge support rather than collar ties or ridge beams, can be overstressed by an unbalanced case that would not cause failure under a balanced load. An unbalanced snow event in 2010 caused hundreds of roof collapses in New England, many of them on roofs that were within their balanced-load capacity.

Running the check

RoofHelm's unbalanced load calculator takes your Pf, roof slope and half-span and returns the design leeward and windward loads for the §7.6 case. Check your ridge and purlin members against these numbers in addition to the balanced case.