Unbalanced Loads - Snow Loads - page 47

roof slope at the eave is less than 30°, the leeward unbalanced load increases
from the 0.5 p f value at the crown to a value of 2 p f C s /C e at the eave. As
with the balanced load case, this C s is the slope factor corresponding to the
roof slope at the eave. In a sense, the load at the eave is a multiple of the
sloped roof snow load divided by the exposure factor ( p s /C e ). This properly
accounts for the effects of wind. Less snow is expected on a fully exposed
roof than on a sheltered roof. However, more drifting is expected on a fully
exposed roof than on a sheltered roof. Thus, the two wind effects tend to
counteract each other. This is generally consistent with the unbalanced load
being proportional to p s / C e , which results in a leeward unbalanced load that
is independent of C e .
The trapezoidal load is composed of a uniform load, equal to 0.5 p f , with
a triangular surcharge. The triangular surcharge is maximum at the eave.
This is generally consistent with the curved roof drift shown in Figure G6-5.
That is, moving from crown to eave, the vertical distance from the roof sur-
face to the crown elevation gets larger and consequently there is more space
for drift accumulation.
The unbalanced loads for Cases 2 and 3 (eave slopes 30°) mimics that

outlined for balanced loads. There is a linear variation from 0.5 p f at the
crown to 2 p f C s /C e at the eave or the 70° point. Again, the C s factors used
are those for the roof slope at the point of interest, such as the 30° point, 70°
point, or eave.
Like the gable roof unbalanced load, the curved roof unbalanced load
applies to a specifi c range of roof slopes. However, unlike all the previous
provisions, which were based on the tangent slope, the limits for curved
roofs are based on the secant slope. Specifi cally, unbalanced loads are not
considered if the slope of a straight line from the eave (or 70° point) to the
crown is less than 10° or greater than 60°.

≥

6.3 Sawtooth-Type Roofs

Unbalanced loads on multiple folded-plate, sawtooth, and multiple barrel
vault roofs are presented in Section 7.6.3. The prescribed loading is similar
to that for a curved roof, i.e., 0.5 p f at the high point and 2 p f /C e at the low

Figure G6-5
Typical curved roof drift
formation.