A.Christopher DeBlois, a
structural engineer with Palmer Engineering in Tucker, Ga.,
responds: Most roof collapses from heavy snow loads are
caused by connection failures, not failure of framing members.
In fact, actual breakage of rafters is fairly uncommon except
for with flat roofs.
To help visualize loading-connection issues, picture two
extension ladders leaning against each other. If two people of
about the same weight want to climb them, one on each side, the
system is stable so long as the bottom of each ladder is braced
and can't kick out. This is a balanced loading situation, and
illustrates the importance of a good connection at the base of
For an unbalanced situation, picture the same two ladders, but
imagine only one person climbing up one side (again with the
bases braced). If the ladders aren't too steep, the system
might still be stable. But as the ladders get steeper, the
ladder opposite the climber will likely be pushed over as the
climber ascends to the top — a ridge-connection failure.
So to ensure good roof stability when there is unbalanced
loading, it's important to make sure there is also a good
connection at the ridge.
In fact, if you think about the ladder analogy, with a good
connection at the top and the two sides tied together, you've
just imagined an ordinary stepladder. Without the top
connection and the cross ties to keep the two sides from
pushing out, a stepladder would be unstable for the unbalanced
load of a single climber.
Likewise, unbalanced snow loads — whether the result of
wind drifts, uneven melting effects from the sun, or uneven
snowfall based on variable protection (usually from trees)
— aren't necessarily more likely to cause a roof
collapse, but they do stress a roof and its connections in
places that balanced loads do not.
Snow design loads are based on figures published by the
ASCE; estimates for the actual weight of snow range from 1 to
1.5 psf per inch of depth. Note that the density of snow
increases as depth increases. In a 6-inch snowfall, an inch of
snow has a design density of 1.25 psf per inch, and a
real-world density closer to 1 psf per inch; in 48-inch-deep
snow, the design density is more than 2.4 psf per inch while
the actual density is probably 1.5 psf per inch or
Nevertheless, if rafters, valleys, ridges, and hips are
properly sized for the balanced snow condition, and if
connections between members, at ridges, and at the attic or
ceiling are sound, most residential roofs should be able to
handle both balanced and unbalanced snow loads.
There are of course special cases — like deep drift loads
on large flat roofs; and sliding loads, where snow slipping off
a higher roof inundates a lower one — that should be
looked at carefully by a structural designer. The building
codes don't attempt to account for all possible snow load
scenarios. Instead they reference ASCE-7, a specification
published by the American Society of Civil Engineers and the
Structural Engineering Institute called "Minimum Design Loads
for Buildings and Other Structures"; it includes formulas for
calculating snow design loads for various locations and roof
configurations. Generally, loads get higher where more snow
falls, when roofs are shallower rather than steeper, and where
drift loads can accumulate.
ASCE-7 includes a formula that can be used to convert snow
depth to weight, which was used to create the chart on page 30.
Note that while a 30-inch-deep snowfall corresponds to a design
load of about 52 psf, the actual weight of that snow is
probably somewhat less, in the neighborhood of 40 psf. (For
obvious reasons, engineers err on the high side when
calculating design loads.) Just the same, that's a lot of
weight — engineers use a design live load of 50 psf for
parking decks — so it's no surprise that an improperly
designed or built roof can fail in a big snowstorm.