Flat Roof Snow Loads - Snow Loads - page 21

snow load. As noted in the Commentary, the average value of the ratio of
25-year to 50-year ground snow loads for more than 200 sites across the
United States is 0.8. For structures in Category IV, the I s factor is 1.2, which
corresponds to a 100-year ground snow load. For snow loading, the return
period used in design is a function of the relative importance of the structure,
based on its intended use and the nature of occupancy.

3.6 Minimum Snow Loads for

Low-Sloped Roofs

The fl at roof snow load (as well as the sloped roof snow load, which is dis-
cussed in Chapter 4 of this guide) incorporates the generalized ground-to-
roof conversion factor of 0.7 as well as the C e and C t factors, which increase
or decrease the roof snow load depending on the specifi c wind exposure and
thermal environment. For most structures, this results in a fl at roof snow
load that is less than the ground snow load.
In low ground snow load areas, the region's design ground snow load
could be the result of a single large storm. If the winds were calm during and
after this single large storm, the snow load on a low sloped roof would be
comparable to the ground snow load because the wind and thermal effects
would not have had suffi cient time to signifi cantly alter the roof snow load.
For instance, in areas with design ground snow loads roughly in the 5 lb/
ft 2 -to-20 lb/ft 2 range (reference Figure 7-1), it is possible that a single storm
could result in both the ground and roof having equivalent loads approaching
the 50-year design snow load, p g . In areas with larger design ground snow
loads ( p g in Figure 7-1 of 25 lb/ft 2 or more), it is still possible for a single
large storm to result in ground and roof loads being equivalent; however, it is
unlikely that these loads would approach the 50-year design load. Expecting
loads greater than 20 lb/ft 2 from a single large storm is unreasonable.
The minimum snow load for low-sloped roofs (defi ned in Section 7.3.4)
is needed for this reason. The minimum load is a function of the ground snow
load, p g , and the importance factor. Specifi cally, when p g is comparatively low
(20 lb/ft 2 or less), a single large storm may result in a roof load approach-
ing p g , and the minimum roof snow load is defi ned as the importance factor
multiplied by p g . On the other hand, when p g is substantial (more than 20 lb/
ft 2 ), a single large storm is unlikely to result in a roof snow load of p g , and
the minimum roof snow load is defi ned as the importance factor multiplied
by 20 lb/ft 2 .
As the name implies, the minimum load in Section 7.3.4 is intended for
low-sloped roofs. Specifi cally, minimum roof snow loads are required for
monoslope, hip, and gable roofs with slopes less than 15° and for curved
roofs where the vertical angle from eave to crown is less than 10°. Histori-
cally, these slope limits for the minimum load have been related to slopes
for which unbalanced loads are required. For example, as shown in Chap-
ter 6, unbalanced loads are not required for curved roofs with a vertical
angle less than 10°. In other words, if the roof slope is steep enough that