This winter’s near-record snowfall blanketed Anchorage houses and buildings with tons of snow, equivalent to the weight of several pickup trucks parked on every roof, according to structural engineers.
Two roof collapses in the last week — one at the Palmer Public Library and another at the Turnagain CrossFit gym in South Anchorage that killed one woman and trapped two others — led to widespread concern about snow piled a few feet high on area rooftops.
But for the vast majority of residences, current snow loads are typically well within the safe zone for design and construction standards in Anchorage, experts say.
Engineers also cautioned that residents wanting to clear their own roofs can face greater risk of hurting themselves or damaging their roofs than from it collapsing from the snow.
Roofs across the city generally are not at risk of buckling under the snow, except perhaps for rare cases with unique circumstances, such as homes with extensive ice-damming or windblown snow that’s overloading a section of a roof, they say.
“It’s too early to panic,” said Scott Hamel, chair of the Civil Engineering Department at the University of Alaska Anchorage and a structural engineer.
“If we get another two feet of snow, we may potentially need to take some mitigation measures,” especially if there are no warm temperatures to melt the new snow quickly, he said.
Roofs are currently at only about two-thirds of their design capacity when it comes to snow loads, said Hamel, who is on the eight-member Snow Load Committee for the Structural Engineers Association of Alaska.
Other engineers agreed.
“Based on snow levels we’ve seen falling, there’s not a cause for concern, and we are within what’s expected for normal design limits,” said Sterling Strait, a structural engineer who’s also on the Snow Load committee.
Strait said the roof collapses at the Palmer Public Library and the Turnagain gym should warrant people taking a second, close look at their roofs.
[Anchorage gym collapses during event, killing 1 and rattling tightknit community]
But the cause of those collapses are still under investigation and unique circumstances may be at play, including the age of the library and a design that allowed snow drifts to pile up near the area that buckled, according to people familiar with that building. As for the gym, the building’s roof was flat, and it appears that “tremendous” ice build-up due to potential structural issues may have been at play, though more analysis must be done before a conclusion can be drawn, said Ross Noffsinger, the acting building official for the municipality.
Alex Boyd, assistant chief in the Anchorage Fire Department, said the only roof collapse in Anchorage that he’s aware of this winter has been the CrossFit gym and a related, adjacent part of the roof in that same building on Sunday.
Boyd said structures have collapsed very occasionally during heavy snow years in the past — usually the result of multiple issues, such as an overloaded section of wind-drifted snow combined with poor construction or drainage problems that cause ice-damming.
But, he said, collapses are “super rare.” In his 24 years with the department, he said he recalls only two failures at residences where snow loads contributed. They were partial collapses involving modified garages.
One had a deck built atop it, compromising the original structure, he said. Another was a garage that had been converted into a living space in the Muldoon area that he remembers failing in 2006. No one was hurt in those collapses, he said.
“And there are so many causal factors that having a snow load is part of a constellation of other problems that often lead to a collapse,” he said.
A ‘margin of safety’
Anchorage is on track to have one of its snowiest seasons on record — more than 7 feet has fallen so far, with weeks of wintry conditions still anticipated. To date, snowfall is pacing about 1.5 feet behind the record winter of 2012, when more than 11 feet fell by season’s end. According to the National Weather Service, the official snow depth on Wednesday was 33 inches at the service’s headquarters near the Anchorage airport.
Hamel said, “if your house is built up to code, and you don’t have a lot of extenuating circumstances, I would not be too worried about (the snow load), especially if you made it through 2012 when we had a lot more snow.”
Anchorage, with its heavy snow plus powerful earthquakes, has long had strong design standards, and they’ve gotten beefier over time, Boyd said.
“Those two things come together so that we have pretty strong buildings,” he said.
Hamel said Anchorage code typically requires that houses are designed to handle a maximum of 40 pounds of snow for every square foot.
He said less than 30 pounds a square foot are typically sitting on houses right now, based on measurements of snow he and other engineers have taken this winter, and melting that occurred during the warm streaks.
Snow loads can vary by each house as wind blows snow around, and melting and draining can reduce snow loads, engineers say.
Hamel and other experts said that even if snow loads this year approach the design limit, that typically will not be a problem for houses that are properly designed and built, which encompasses most houses in Anchorage, including old ones.
That’s because the design standards are conservative, Hamel said.
“We don’t design it so the wood breaks at exactly 40 pounds,” he said. “There’s an additional margin of safety.”
Hamel said he’s only shoveled a small area of the roof at his South Anchorage house, after snow from a higher section dropped onto a lower section, creating an enormous, worrisome load in one spot. He donned a safety harness and anchored into structural members on the side of his house, he said.
He didn’t clear any more snow than that, he said. This year’s snow has generally not been dense or heavy enough to warrant the risk of getting hurt on the roof.
What to watch for
Ice-damming is something to keep an extra close eye on, Hamel and other engineers said.
After December’s snowfalls, a couple of warm-ups caused the thick snow on roofs to melt. That caused ice dams in some cases, often at a roof’s edge where vents release household warmth, Hamel said. Drains can also become frozen shut or damaged, causing dangerous ice buildup on the roof.
Eight inches of ice is the point at which the 40-pound design limit is reached.
So if that thick ice extends across much of the roof, a licensed, bonded contractor should remove it, he said.
“If you have 8 inches of ice and it’s only a foot-and-half wide you’re probably fine,” he said. “But if that’s 30 feet long, then you have a serious problem.”
Boyd, with the fire department, said most structures in Anchorage are built with wood frames, which often provide warning signs before a structure fails, such as cracking and popping sounds. In fact, firefighters entering a compromised building will often shore it up with wood beams for that reason, he said.
Property owners should also be on the lookout for other clues, including newly cracked walls, walls with nailheads that have suddenly popped out, doors that no longer open or close properly, and a sagging ceiling or roof, he said.
People experiencing those problems who are worried about snow loads can reach out to municipal engineers and officials at the Anchorage Development Services department, Boyd said.
Department officials will work with homeowners to determine if they’re experiencing signs of a potentially dangerous snow load, he said.
Property owners can also contact building engineers or hire a structural engineer to evaluate snow-load risk, he said.
The department is hesitant about having people shovel their roofs, he said. When situations require roof-clearing, property owners should call a licensed and bonded contractor. They can get hurt if they do it themselves, or damage the roof, he said.
“It’s more than just the weight of the person, it’s how they move around and how they move that snow,” he said.
Noffsinger, the acting building official for the municipality, said he agrees with Hamel and other engineers that snow loads in the Anchorage area are in a safe range, but some houses concern him.
Older houses, including those that are poorly ventilated and constructed, might have problems inside the roof cavity that makes the roof weaker than normal. Houses with flat roofs can also have ice-damming in low areas that get covered in snow so it’s not easily visible, unlike houses with sloped roofs where damming is often more visible at the edges.
“So these are the types of buildings that are more concerning,” Noffsinger said.
Daily News reporter Tess Williams contributed.