- Q. I've always thought that building codes leave a pretty generous margin for error when specifying allowances for snow loads, but a series of recent storms and a lot of drifting have resulted in some collapsed roofs in my area. I've seen some roofs with large loads on one side and virtually none on the other. Is a roof that has been unequally loaded due to blowing and drifting snow more prone to failure?
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 the rafters.
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 more.
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.