3 August 1998
such unbalanced loads are being observed on some large gable roofs with slopes less the 1.
That Commentary suggests that it may be appropriate to consider unbalanced loads for such
roofs with slopes down to 5 (about 1 in./ft.).
6. METAL BUILDINGS. Many metal buildings are built with low-slope gable roofs (single- or
multiple-gable). In cold regions for waterproofing reasons, it is appropriate to require a slope of
at least 1 inch/foot (about 5 ) for metal roofing systems. Metal buildings are designed with little
structural "fat." Many have failed where an unbalanced snow load in one area causepurlins to
fall, initiating progressive collapse. Unbalanced snow loads should be considered on all metal
buildings regardless of their slope.
7. INTERNALLY-DRAINED MEMBRANE ROOFING SYSTEMS. Such roofs usually have
slopes much less than 15 and, thus ASCE Manual 7 does not require unbalanced loads to be
considered. There is always the possibility that some unbalanced loads may develop. To reduce
risks associated with this possibility, the depth of such basins should be as small as possible.
The easiest way to do this is to reduce roof slopes to 1/4 inch/foot. Dead flat roofs are a design
mistake. A l/8 inch/foot design slope can result in as-built flat areas. There is no real evidence
that supports the contention that in cold regions a -inch/foot minimum slope should be used.
Increasing the slope above inch/foot increases costs since higher walls are needed to account
for the greater slope. Slopes of 1 inch/foot not only further increases the risk of unbalanced
snow loads, but these slopes can be more expensive due to the additional attachments needed
to hold roofing components in place on such slopes.
8. BUILDING ORIENTATION. ASCE Manual 7 requires designers to assume that the high
winds which cause snow to drift could come from any direction. Nonetheless, information should
be sought from "locals" on drift orientation. Where such information indicates strong preferential
orientation of snow drifting, give thought to placing drift-prone features (e.g., loading dock roofs
either upwind or alongside the building rather than at its downwind end. Design loads will not
change, but the amount of drifting may be reduced significantly. An example is shown in figure
4. However, changing the orientation of buildings may not be possible. Sloping the roofs on the
loading docks shown in figure 4 would reduce drift loads. However, that may introduce drainage,
ice damming, and sliding snow problems.
9. SLIDING SNOW. The ability of slippery unobstructed roofs to shed snow loads by sliding can
be an advantage and a disadvantage. Loads on a roof can be reduced when snow slides off
(figure 5), but loads will increase on any lower roofs onto which snow slides. If snow drops some
distance, large dynamic loads can be imposed on a lower roof or on an object located below
(figure 6). Snow can creep and glide slowly down slippery surfaces (figure 7), even those with
very shallow slopes. The movement of snow can drag plumbing stacks (figure 8) and other roof
penetrations with it, damaging them and creating holes in the roof (figure 9). If snow slides from
roofs having gutters, they will probably be ripped off. Parapets and scias can also be damaged
(figures 10 and 11). Flow of snow down valleys can bend the standing seams of metal roofing
(figure 12), reducing their strength and violating their waterproofing integrity. Several sliding
snow issues are illustrated in figure 13. Large curling snow cornices can be created at eaves
(figure 7). Such cornices can be quite heavy, and they may curl around enough to damage walls
and windows. When they break off, piles of snow and ice are created on the ground. These
piles may deflect falling snow sideways towards walls, damaging them.Meltwater that drips onto
these piles can enter the building at the base of the wall if that base is not far above the finished
grade outside. Figure 14 illustrates a number of these situations. Electrical service entrance
cables located below eaves can be ripped loose by falling snow or damaged by the weight of ice
that collects on them from roofmeltwater (figure 15). Snow guards may be needed to hold snow