Section 7.10/Rain-on-Snow Surcharge
ASCE 7-22 Section 7.10 requires a rain-on-snow surcharge load of 5 psf for locations where the ground snow load pg is 20 psf or less and the roof slope is less than W/50.
For locations where the ground snow load pg is 20 psf or less, a rain-on-snow surcharge of 5 psf must be added to the flat roof snow load pf, provided the roof slope is less than W/50 (where W is the horizontal distance from eave to ridge in feet). This surcharge accounts for rainfall that saturates the snow pack on the roof before the snow melts. The combined rain-on-snow load replaces the flat roof snow load in all load combinations, including those with drift loads and sliding snow. The surcharge does not apply where pg exceeds 20 psf because in heavy snow regions the snow is typically dry and powder-like during the period of peak accumulation, and rain events during snow cover are less likely to coincide with peak loads.
Why this section exists
In regions with light snow (20 psf or less), the snowfall is often wet and partially melted. Rain falling on this saturated snow pack is absorbed and retained by the snow, significantly increasing the roof load. Several roof collapses have been attributed to rain-on- snow events where the combined load exceeded the design snow load. The 5 psf surcharge represents approximately 1 inch of retained rainwater in the snow matrix. The slope threshold (W/50) excludes steep roofs where rain drains quickly through the snow and does not accumulate.
What plan reviewers look for
Plan reviewers check the ground snow load for the project location. If pg is 20 psf or less, they verify that the structural calculations include the 5 psf rain-on-snow surcharge added to the flat roof snow load. They check the roof slope against the W/50 threshold to confirm the surcharge applies. They verify the surcharge is included in all snow load combinations, not just the balanced load case.