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ARCHIVED - Can your roof survive a windstorm?

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In a project named after the Three Little Pigs, researchers are huffing and puffing and blowing houses down - all to find out how Canadian homes can better stand up to windy weather.

The Three Little Pigs project can simulate the effects of winds up to a Category 5 hurricane - the peak reached by Hurricane Katrina in 2005.
The Three Little Pigs project can simulate the effects of winds up to a Category 5 hurricane - the peak reached by Hurricane Katrina in 2005.

Like many Canadian roofers, Ron McAfee knows that when high winds are coming, he’ll get calls the next day from people needing roof repairs. McAfee is owner of Lion’s Roofing Inc. in Barrie, Ontario, which specializes in repairing and replacing roofs. Like many parts of Canada, Barrie is prone to occasional big wind storms, and along with that, wind-damaged roofs.

After 35 years in business, McAfee has seen it all. He recently saw a barn roof that had been ripped off by the wind on one side, while the other side was left untouched. He has seen homes with close to 100 square feet (9 square metres) of roofing torn away.

“You really don’t want any of the roof missing, or water gets in,” he says. And that’s when even more damage happens.

No place in Canada is immune to windstorms. According to Environment Canada, the country gets an average of 80 tornadoes per year, and there are other “wind events” such as hurricanes, microbursts and squalls that can all reach dangerous levels.

Wind research in Canada

Canada sees a lot of extreme weather, and many Canadian experts are exploring how to help the construction industry protect homes and other buildings from the ravages of wind.

See highlights of Canadian wind research.

According to Dr. Gregory Kopp, Canada Research Chair in Wind Engineering at the University of Western Ontario (UWO), wind damage to homes can be both costly and life-threatening. In 1987, a tornado in Edmonton caused $300 million of damage and killed 27 people. In 2000, a tornado in Pine Lake, Alberta led to 12 deaths and about $13 million of damage. Hurricane Juan in 2003 led to eight deaths.

It's with the goal of preventing property damage and injuries that Dr. Kopp studies how wind affects houses. In a UWO initiative, called the “Three Little Pigs Project,” he and his team use a variety of methods to study how houses behave during windstorms, including mathematical models and wind tunnel tests on reduced-scale models of houses. But most impressively, they test and destroy full-sized houses in a scientifically controlled setting.

Blowing the house down

UWO has been a leader in wind research for more than 40 years. Many wind tunnel studies have been performed on scale models of different types of buildings. "But no one had really looked at houses," Dr. Kopp says.

Bringing down the stage

When a strong wind blows, take cover! That's what music fans discovered during the final concert of the 2011 Ottawa Bluesfest on Sunday July 17. About 20 minutes into a performance by the Canadian band Cheap Trick, the main stage collapsed when a giant gust of wind folded it backward. While the band escaped unscathed, a few people were injured. See a YouTube video of the stage collapsing.

That's why in 2003, a huge structure nicknamed "the Hangar" was built. From the outside, the Hangar looks like a giant garage. But instead of cars, it houses one full-sized four-bedroom house with brick siding. The house was built in compliance with Ontario Building Code standards. It is 167 square metres in size, wired for electricity and even has the plumbing roughed in.

But this house is not for living in ' it's there so researchers can expose it to the varying degrees of pressure changes that occur during windstorms. And it's those changes in pressure that cause damage.

Simulating Katrina-strength hurricanes

Key to the research is a set of devices UWO researchers developed specifically for the Three Little Pigs project: pressure loading actuators (PLAs). The PLAs don't blow wind onto the houses, but rather create areas of low pressure. They actually replicate what wind does when it blows around a house, Dr. Kopp says.

This video shows a wind tunnel simulation of the aerodynamic loads on a roof during a hurricane. The colours represent various wind pressures sucking the roof upwards ' from red (weaker) to blue (stronger). The wind loads are measured as pressure coefficients (CP), which represent the atmospheric pressure divided by the density of the air and the wind speed squared. *

The technology can simulate the effects of wind up to the intensity of a Category 5 hurricane, the strongest level of hurricane around and the peak reached by Hurricane Katrina when it hit New Orleans in 2005.

The test house recently underwent various experiments and the roof was completely destroyed. "We ran a series of tests with increasing mean wind speeds, each wind speed lasting for 15 minutes,” says Dr. Kopp. “One of our main findings is that it takes many gusts to actually cause the roof to fail.” The researchers are now in the process of analyzing that data.

Preventing roof damage

Dr. Kopp is particularly interested in the roofs of houses. In a windstorm, the roof is the most vulnerable structure because it gets the highest wind loads. Worse, "once the roof goes, you can easily lose the rest of the house during a windstorm," says Dr. Kopp. His focus is on the roof sheathing ' the layer that shingles are nailed to.

Dr. Kopp has found that two simple things can help prevent wind damage to roofs: hurricane straps (metal strips that connect the roof to the walls for additional support), and ring shank nails (screw-like nails that can increase the strength of the sheathing).

When shingles are lost, water can get into the attic and down into the house, damaging walls, ceilings and belongings.
When shingles are lost, water can get into the attic and down into the house, damaging walls, ceilings and belongings.

He also found that during wind storms, Canadian homes experience damage sooner than they should. Generally, the threshold for damage to a house in Canada starts with gust speeds of about 100 km/hr. "This is actually below the gust wind speeds that buildings are currently designed for, and reflects problems with installation or maintenance at these speeds," he says.

This doesn't mean homes are collapsing “en masse“, but it's hard to say just how much damage this means across Canada. "Every time we do a damage survey after a storm, we see evidence that some sections were not properly attached," he says.

People often think the integrity of a house relies on the strength of its walls. But many studies have found that what walls are made of is not a predictor of which houses will withstand a windstorm. What’s key is ensuring that the roof is in good shape and that the roof covering (shingles or tiles) is secure and doesn't let water through.

Even if only part of the roof goes, that's still big trouble. If a few shingles or a piece of plywood is lost, water gets into the attic and trickles further down into the house, wrecking walls, ceilings and people's belongings.

“That not only causes thousands of dollars worth of damage, but it puts people's lives at risk,” Dr. Kopp says.

An overall goal of the research is to help improve Canada's national building code. "Mostly our recommendations are small details which can make a substantial difference ' such as adding hurricane straps, and changing the types of nails used in the sheathing," he says. end

Costly wind damage

Predictions from the insurance industry of the cost of damage from big windstorms in large urban areas are dire. One study suggests if an F3 tornado (winds from 254-331 km/h) were to strike Toronto, it could result in $4 billion of damage. Another study estimated that a hurricane moving across Atlantic Canada could cause up to $6 billion in damage.

* Courtesy of the University of Western Ontario.

ISSN 1927-0275 = Dimensions (Ottawa. Online)