A proper evaluation of snow loads on roofs is crucial for structural design especially to guarantee an adequate
reliability level of lightweight roof structures. The definition of roof snow load in structural codes is based on both
the evaluation of ground snow loads and conversion factors from ground to roof load, which are function of the
roof’s geometry, its exposure to wind and its thermal properties. However, reference values of roof snow loads are
based only on an extreme value analysis carried out to derive characteristic values of ground snow load, while
conversion factors are considered as deterministic quantities due to the lack of the data.
In this paper, first a methodology to evaluate the reference value of roof snow load is presented based on the
definition of probability density functions for ground snow loads and conversion factors accounting for roof’s
geometry and its exposure to wind. The results lead to the definition of a design conversion factor which depend
on the coefficient of variation of ground snow loads and are compared with the constant values provided by the
Eurocode models, in EN1991-1-3:2003. Then, structural reliability is assessed for reference steel and timber
structures located in different sites. Considering different proportions between variable and permanent loads, the
reliability of flat roofs designed according to Eurocode provisions, provided by the current version and the new
draft, is finally compared with the required target reliability levels
