Roof Snow Loads for Structural Design

Lognormal probability distribution functions for both ground snow loads and the ground‐to‐roof conversion factor are used to establish lognormal probability distribution functions for roof snow loads. Site specific values for the conversion factor multiple ${\alpha }_N$ were determined. The conversion factor multiple has the property that product of ${\alpha }_N\text{,}$ the exposure factor EX, the thermal factor T and the N‐year mean recurrence interval (MRI) ground load equals the N‐year (MRI) ground load equals the N‐year MRI roof load. The conversion factor multiples ${\alpha }_N$ for mean recurrence intervals of 25, 50 and 100 years are presented for 128 sites in the United States. The 50 year MRI multiples for all except four sites fell in the range $0.56⩽{\alpha }_{50}⩽0.66\text{.}$ These values were compared with roof snow loads provisions in the 1982 American National Standard. It was found that the use of the ANSI‐1982 ground loads, which have a MRI of about 50 years, in conjunction with the ANSI‐1982 coversion factor multiple of 0.70 results, on average, in a flat roof snow load with a MRI of about 80 years.