Tornado Damage

On June 17, 2014, an EF-2 tornado ripped through the small town of Angus, Ontario, about 100 km north of Toronto. It caused damage to more than 100 homes, tearing the roofs off of 11 of them, including seven in a row down one street. The worst of the damage was confined to two streets. In fact, the tornado appeared to travel down the backyards between the houses over a stretch where about 70% of the damage occurred. Events of this magnitude seem to happen every couple of years or so in Ontario.

Most people would say there isn’t much that we can do about these events, particularly when they look at the photographs of chaotic damage in the aftermath of the storm. However, when one digs into the details, patterns of damage emerge that suggest measures can be taken to reduce the number of people who lose their homes and have to re-build their lives. The most important mitigation measure is inexpensive – in the range of $100 to $200 per house.

In Angus, much of the observed damage was to the cladding (vinyl siding – 30% of the damaged houses), roof cover (asphalt shingles – 50%), and fascia, soffits and eaves troughs (40%). This is fairly typical of such storms, and while the damage can add up, our analysis suggests that these costs are not the main drivers of overall losses in severe tornadoes.

Of greater concern are structural failures, such as roof sheathing (10% of the houses) and complete roof failure (10% of the houses), which is both a life safety issue and a much more expensive type of failure. It has been shown that the loss of one sheet of plywood from the roof can lead to substantial contents loss due to rain entry. Complete roof failure obviously leads to substantial re-building costs along with a high proportion of contents loss.

Roofs in wood-frame houses are fastened to the walls with toe-nails – a set of three nails connecting each roof truss to the top of the wall. In convective storms in particular, the wind creates pressure, which acts to lift roofs up. So, in a severe wind storm, we need to think about holding the roof down, rather than having walls that just hold it up. The toe-nailed connections have this job.

Engineers tend to dislike toe-nailed connections because it is difficult to control their quality and, even when properly installed, their capacity to withstand uplift is relatively small. Often, the lumber will crack when the nails are put in; and it is easy to miss nails. As a result, the toe-nailed connections are often the weak link in severe wind. In other words, how the roof is fastened to the walls tends to be the weakest structural component in wood-frame houses, even when they are installed correctly.

As a result, one usually finds complete roof trusses on the ground after the storm, with the toe-nails protruding out. In Angus, as part of the research we did after the storm, we found many such toe-nailed connections during our survey. However, most of these were clearly faulty, often with only a single nail, instead of the three required by the building codes in Canada. Nailing problems were prevalent in Angus – we didn’t find one failed toe-nail connection that was correct (although in the piles of lumber, we may have missed some).

Once a roof fails, it will fly through the air and could potentially impact adjacent houses that may have been otherwise undamaged. We saw several circumstances where roofs had landed on adjacent houses, and we also saw roof trusses that had penetrated adjacent walls like spears protruding through windows. About 40% of the houses in Angus were damaged by debris impacts. While shingles flying through the air have enough momentum to break common window glass, much of the damage due to debris impacts was caused by structural roofing material. Thus, if one can keep the roof structure attached to the walls, there will be significantly reduced damage to surrounding houses.

So, how can we reduce the number of roof failures? We need two things. First, builders need ensure that the building code is followed and the correct nails installed. Missing nails can be disastrous. It appears that improved inspection practices are needed in this regard.

Second, in order to significantly increase the strength of the weakest link, roof straps should be required instead of toe-nails. These are thin metal straps that replace the toe-nails. The simplest and cheapest of these are far stronger than toe-nailed connections, and they cost less than $1 a piece. In an average house, like those in Angus, they would cost $100 – $150 per house to install. Such straps would make inspection easier since they can easily be seen from the floor.

Severe tornadoes are relatively rare in Canada, at least compared to the numbers in the southern United States.

Nevertheless, for EF-2 tornadoes like that in Angus this year, or in Vaughan five years ago, much of the severe damage can be mitigated with roof clips holding the roofs onto the walls, and a few more nails holding the roof sheathing in place.

Our analysis, based on full-scale testing at the Insurance Research Lab for Better Homes, suggests that the use of roof straps would have eliminated the roof failures in Angus, substantially reducing the overall losses. Just because it is a tornado does not mean we have to accept the damage – we can actually reduce the worst of it!

Dr. Gregory A. Kopp is a Professor in the Department of Civil & Environmental Engineering at the University of Western Ontario. Dr. Kopp is currently also a Research Director and a member of the Board of Directors of the Boundary Layer Wind Tunnel Laboratory. He can be reached at gakopp@uwo.ca; @gregoryalankopp; or 519-661-3338.