The Effect of Hurricane Winds Upon a House

Image courtesy of NOAA

Image courtesy of NOAA

What follows is an excerpt from a 1998 publication titled AMERICA’S HURRICANE THREAT under the auspices of the South Carolina Sea Grant Consortium.  What I’m showing you is a small part of their sobering account of what can happen to houses in the strong winds of a hurricane.  The house on the left in the image above is mildly damaged compared to many in that neighborhood.  I have been reading the above-mentioned report tonight due to my special interest in Hurricane Andrew.  Those of you who have been following this web-log since its beginning 18 days ago might recall having read somewhere that our home was a total loss in that 1992 hurricane.  In time, I have written about the experience in a recent posting dealing with window protection.  Here is the excerpt from the report:

“Andrew totally destroyed 63,000 homes and partly damaged another 110,000, making 250,000 people homeless. With roofs damaged or blown off, rain following the hurricane poured inside structures, soaking and collapsing Sheetrock and destroying billions of dollars worth of furniture, carpeting, televisions, and other items. The insurance industry estimates that 25-40% of insured losses were due to slipshod construction practices.

Most homeowners do not give a second thought to their roofs-until they leak or disappear. Yet roofs are the Achilles heel of homes in hurricane-prone areas from Maine to Texas.

As strong winds strike a building, their flow is diverted, swirling over and around the structure. Think of a mountain stream roaring against a giant boulder, which deflects the current. The stream flow accelerates around the obstacle, resulting in rapids. In the same way, hurricane winds speed up around corners and edges, creating suction that pulls on building materials like a super-powerful vacuum hose. Fierce gusts and suction pressure make a dangerous combination, especially for roofs. They yank off tiles and shingles, first at the roof edge and then along its slope as you’d peel an orange. During Andrew, huge numbers of tiles were stripped from roofs this way, and carried off by high winds, they crashed through windows by tens of thousands.

If you lose a window or door during a hurricane, you’re in big trouble. Extreme winds push through an opening in a building, increasing air pressure inside like blowing up a balloon beyond its capacity. If you force enough air pressure inside a house, it can break at its weakest point, usually the roof.

As roofs are being pushed off from within, they are being pried loose from the outside. Peel away tiles or shingles and you’ll find a covering of roofing paper, under which is plywood attached to rafters. But a roof won’t stand much of a chance in hurricane-force winds if builders haphazardly tie down plywood to rafters-if they use too few nails or miss the rafters altogether with their nail ‘guns.’ After Andrew, engineers reported that many contractors had routinely missed their marks. ‘With the use of automatic nail guns, the workman lost his feel for the nailing process,’ said Saffir. ‘The result was that many nails went through the sheathing into thin air, not into the truss or rafter below. This was a common occurrence.’

If your plywood sheeting flies away in the wind, you’ve lost more than just a roof covering. You’ve also lost a portion of the house’s structural integrity. That is, plywood sheets are often the sole lateral bracing for the rafters, actually holding the roof together. So with the plywood gone, the rafters are loosely tethered in the wind.

To compound the problem, many contractors fail to tightly fasten wood gable ends-the flat ends of a pitched roof-to walls. So when a powerful gust hits an unbraced gable, the gable end can be pulled loose at the wall, allowing wind to enter the building. If the roof sheathing is pulled off at the gable end, the rafters can fall over.

During hurricane Andrew, tens of thousands of homes were damaged due to such failures in roofs.”

NOTE FROM CLOUDMAN23:  Many of you reading this could be living in dwellings without adequate roof truss tie-downs and/or with poor roof sheathing attachment.  Do you know what the codes were when your house was built?  Do you feel confident that your dwelling was built according to code?  Were the inspections thorough and particular or were they inadequate?  For some, those are hard questions to answer.  In any event, please evacuate if Ike is heading your way.  Heed the warnings.  Please.

Please visit the rest of this web-log at https://cloudman23.wordpress.com/.  If you are interested in weather, there are some tutorials scattered about and more will be added in time.  At the end of this page there is a cue to click to the previous page or the next page.

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