Why especially flat roofs collapse under snow, and what to do (and not do) about it
Suburban property owners might have bigger concerns than clearing mounds of snow off driveways and sidewalks as snowloads begin to weigh down rooftops.
Structural experts say repeated heavy snowfalls on flat roofs could pose a hazard, and they offer some tips to head off collapses like those that occurred at two buildings in Elgin since last weekend.
Experts say it's best to immediately remove snow after a heavy snowfall to help prevent the buildup of ice dams. But using a snowblower to remove snow or an ax to chip away at an ice dam might end up damaging the roof, and it might not be safe, according to EnergyStar.gov.
Instead, the National Weather Service recommends using a broom with stiff bristles to remove snow from flat and low-slope roofs, and a roof rake on sloped roofs. Keeping the attic well ventilated and adding extra insulation results in less melting and refreezing on the roof, according to the weather service.
Jon Lewis, a structural engineer from Arlington Heights, warns homeowners to take care when removing snow from rooftops because using heavy equipment, or improperly piling snow in an area or against building walls before it's removed, could result in a weight imbalance and cause more damage.
For bigger jobs, he and other experts urge hiring professionals.
"Most building codes in this part of the country require roofs to be able to take a snowload of 25 to 30 pounds per square foot," Lewis said. "What that basically equates to (is) it could be anywhere from a foot or more of really heavy, wet snow (and) up to two perhaps three feet of fluffy snow."
Monday's storm and other recent ones didn't dump the kind of heavy, wet snow that often causes roof collapses, but without a thaw, the potential for another storm could pile on and add to the problem.
Several factors contribute to roof collapses: successive storms, heavy dumping of snow, subzero temperatures preventing melting, and existing structural defects.
"It usually takes a combination of factors to lead to a failure," Lewis said. "If you get a heavy rainstorm and the snow has clogged your drains, and the water adds on to the snow, (it) can cause an issue. The risk increases the longer we go without a thaw and the more successive snows we pile on.
"Usually, the structures that we live in or the garages will start to show some signs. They will start to creak or crack."
For more expert tips on preventing roof collapses, visit amfam.com.