Reducing the Risk of Wind Damage by Building Stronger Homes

Responding the ever-present and only-increasing threat of tornadoes, in 2016 Dufferin County, Ontario implemented a hurricane-clip rebate program to help protect homes against tornadoes and other severe wind events. Southern Ontario is one of the most tornado-prone regions of the country and even the world. There have been multiple severe tornado outbreaks in the past several decades, with tornadoes occurring in Vaughan in 2009, Leamington in 2010, Goderich in 2011, Angus in 2014, and Dunrobin in 2018. Roofs are a common point of failure when a house is struck by a tornado, or even high winds, as they are subjected to strong upward lift forces that can damage or even outright remove a roof. The use of hurricane clips on roof rafters or trusses can substantially reduce the risk of this kind of damage.

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Understanding and Assessing Impacts

Tornadoes are a common event in Canada. The most common sites of known tornado formation are all across the Prairie Provinces and also in Southern Ontario. The tornado season in Canada runs roughly from April through September with tornado formation peaking in the Summer months. While most tornadoes are on the lower-end of the wind speed scale, powerful F3 tornadoes occur about once every eight years in Southern Ontario, and F4 and the extremely powerful F5 tornadoes have been recorded in Canada (Environment Canada switched to the Enhanced-Fujita scale in 2013 – the old system is used here as it is deemed likely to be more familiar to users). In the event that a tornado strikes a home, there is a high chance that the roof will sustain damage or even be removed altogether. When a home is struck by a tornado, substantial uplifting forces are generated on the roof and the toenails that hold the roof to the roof rafters are often insufficiently strong to resist these forces, resulting in sections of the roof (or the entire roof) lifting of the house. In addition to damage directly to the home, these roof debris are then accelerated by the tornado to very high speeds and pose significant danger to people and other structures. Hurricane clips are basically sturdy metal plates with pre-drilled holes for screws or nails that are used to strongly bind wooden frames together. These cheap and simple additions to a home will substantially increase its resistance to the uplifting force of a tornado.

Identifying Actions

This project came about on the initiative of Dufferin County’s Chief Building Official and Director of Facilities and Property. He had been present for at least three tornadoes in the area, including a major one that went through Grand Valley in 1985, and had long considered these events a threat in his county. After seeing an article in the Toronto Star on the subject of reducing tornado damage, he contacted one of the interviewees, who was the Director of the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario, who in turn put the Chief Building Official in touch with the Institute for Catastrophic Loss Reduction, which had published the report on hurricane ties and tornadoes that went on to inform this rebate program. Officially, neither the Ontario Building Code nor the Building Code of Canada mandate that new homes be constructed with hurricane ties. Since Dufferin County did not have the legal authority to make hurricane ties mandatory, it was instead decided that offering a rebate program was the best method for promoting hurricane tie use in new homes. The costs are very small, and even more so when considered relative to the total cost of building a new home. The ties themselves only cost roughly a dollar and installing them is simply a matter of driving in a handful of nails. Dufferin County decided to offer a rebate of $4.50 per hurricane clip, with Dufferin County putting forth $3.00 of that and ICLR providing the remaining $1.50.

Implementation

Officially, neither the Ontario Building Code nor the Building Code of Canada mandate that new homes be constructed with hurricane ties. Since Dufferin County did not have the legal authority to make hurricane ties mandatory, it was instead decided that offering a rebate program was the best method for promoting hurricane tie use in new homes. The costs are very small, and even more so when considered relative to the total cost of building a new home. The ties themselves only cost roughly a dollar and installing them is simply a matter of driving in a handful of nails. Dufferin County decided to offer a rebate of $4.50 per hurricane clip, with Dufferin County putting forth $3.00 of that and ICLR providing the remaining $1.50.

Outcomes and Monitoring Progress

As of June 1, 2018, approximately 2,200 clips had been installed in the area serviced by Dufferin County Building Department.

Resources

Link to Full Case Study

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