Massachusetts Flat Roof Snow Load Requirements

Roof snow load and ground snow load are different quantities, and Massachusetts treats them separately. Table R301.2(2) of the Massachusetts Residential Code gives each municipality a minimum flat roof snow load, Pf, alongside its ground snow load, Pg. The operative rule is in Note 1 to that table: the design flat roof snow load is the larger of the value calculated from Pg by the normal code method and the Pf figure listed for that municipality. In other words the listed number is a floor, not a substitute for the calculation. Listed minimum flat roof loads across the state run from 25 psf to 40 psf, with 35 psf the most common. The listed roof figure is always at or below that municipality's ground snow load, which is what you would expect: a roof sheds and a flat field does not.

Like the ground snow load, this is enforced at plan review rather than by citation. Structural drawings submitted for a building permit must show the roof designed for the greater of the calculated load and the listed minimum for that municipality; drawings that use only the lower of the two will be rejected. Where an existing roof is in question after heavy snowfall, assessment is a matter for a registered design professional, not the code table alone.