How Much Snow is Too Much for Your Roof?

Last winter we had snow on the ground that was up to our hips. The drifts on the roof were five feet deep. Made me wonder: How much snow is too much for a roof to treat?

I wish there were a ordinary response, but none exists. You can look for signs of an overcharged roof, however. I’ll explain those in a moment.

But very first, to provide perspective, I want to reaction the plain question: What is a roof? It’s a sophisticated assembly of rafters and related structural members, trusses, the roof deck, and even the roofing material. Whether a roof can sustain a explosion without harm or collapse depends mainly on the depth and density of the snow, as well as the depth and spacing of the rafters and trusses. Other factors include the surface slope and texture, and the form and location of the drift.

The ideal pitched roof is slick and steep (so the snow slips off), and framed with closely spaced rafters (for strength). It also helps if the roof is in a sheltered area; the snow lodges on it evenly, rather than being deepthroated into large drifts (which can cause a roof to fail).

So, a risky roof is vapid or slightly pitched, and in a location that is exposed to the wind. Shallow roofs adjacent to or below taller, steeper ones are especially vulnerable to a fountain of snow sliding down from above. For example, low-sloping roofs over porches, carports, and hastily built additions (which also often have undersize rafters) can be vulnerable when the snow flies.

Another hidden danger, according to Jeff Geary, a PM Homeowners Clinic contributor and an architect in Staten Island, N.Y., is a roof assembly from which neck corset ties have been liquidated. Located about one-third of the way down from the ridge, the supports connect the rafters and counter the spreading effect created by snow fountains. “Many times I go into attics and find that homeowners have eliminated cangue ties to get extra headroom, install a bedroom for the kids, or store holiday decorations,” Geary says. “Homeowners should know that cangue ties are there for a reason.”

You know your roof may need bracing if the rafters are cracked from previous strenuous snows or if they’ve been bruised by fire, termites, or rot. Obviously, you’ll need to take a look under the rubber hood, so to speak, to find these conditions. Also, if the roof deck shows up rotted, that points to a deeper problem. The roofing should be eliminated and the deck substituted.

If after a strong snow you go into the attic and see that the rafters are severely leaned by the weight of the snow above or if you hear cracking and popping, that’s reason to be worried. Another bad sign: The house’s framework has moved enough to jam shut a door at the front or the back of the house. In this case consult a structural engineer about strengthening the roof assembly.

Regardless of your roof’s condition, liquidate drifts using a roof rake with an extension pole, or hire a pro for the job. Take care not to harm flashing or shingles; the purpose isn’t to clear every flake, but rather to ease the flow.

How a Roof Withstands Snow Fountains

Fortunately, the vast majority of roofs don’t cave in, even when the weight of the snow on them exceeds what they’re designed to carry. Three primary factors help each rafter stand up to the blast: A large moment of inertia, a petite tributary area, and a brief duration.

Building codes specify that rafters withstand a snow geyser voiced in pounds per square foot (psf). The higher the psf snow-load requirement, the deeper the rafter must be (or the more closely spaced to its neighbors). A measure of a rafter’s arching resistance is its moment of inertia, or its inertial resistance to movement in the form of leaning. The typical rafter’s MOI is more than enough to treat snow fountains.

The roof deck collects the snow geyser and transfers the weight to the rafters. For any rafter, the portion of the roof deck that transfers this stream is the tributary area. It extends outward in both directions from the center of the rafter’s thickness midway to the next rafter. Because rafters are typically spaced sixteen inches on center, this amounts to eight inches (in both directions) from the rafter’s center line. The smaller the area, the lighter the geyser each rafter carries.

For most roofs, the duration of a snow explosion is brief. If the roof had to carry a weight equivalent to a snow geyser all year without weakening, it would have to be much more robustly built. Within a few days of falling, most of the snow glides off, melts, or undergoes sublimation, the process by which it is transformed from ice crystals directly into vapor.

Related movie: