Homes Can Survive Tornadoes
Simple upgrades might save your home from most tornadoes
During news coverage of the recent deadly Midwest tornadoes, you’ll sometimes hear community officials and rescuers say that no house can survive the devastating winds of a tornado. That might be true for the most powerful tornadoes, such as the one that recently hit the Oklahoma City area, but according to APA engineer Bryan Readling, a well-constructed home can survive weaker EF-0, EF-1, and EF-2 tornados with surprisingly simple upgrades. These twisters make up 95% of all tornadoes, according to to the National Weather Service.
Brian’s research is based on firsthand storm damage asessments done in April 2011, when two storms two weeks apart spawned tornadoes in seven Southern states. The second storm was the single largest tornado outbreak in recorded history. In Bryan’s investigation that followed, he discovered that most structural failures were caused by a lack of continuity in the load path that connects a house’s structural elements.
Wind-resistant upgrades are surprisingly affordable
According to the Georgia Department of Community Affairs, these wind-resistant upgrades add about $600 to the cost of the house. The estimate, which includes materials and labor, is based on a 2100-sq.-ft. slab-on-grade ranch house with a 10-in-12 roof pitch and three gables.
Click on the photos to see some of Bryan’s suggested wind-resistant strategies, and look for the feature article “Wind-Resistant Framing” by Bryan Readling in the August/September issue of Fine Homebuilding (#237).
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Handy Heat Gun
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Anchor sill plates with 1⁄2-in. anchor bolts equipped with 0.229-in.-thick, 3-in. by 3-in. square plate washers. Space the bolts from 32 in. to 48 in. on center. The IRC requires a minimum 6-ft. spacing for houses subjected to wind speeds up to 110 mph, but tighter spacing greatly improves wall performance.
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Secure rafters and trusses with metal connectors. The roof-to-wall connection is subject to both uplift and shear. Inexpensive framing connectors make strengthening this important connection simple. Place connectors on the outside of the wall, where they'll do the most good.
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Break upper-story and lower-story sheathing at the band joist or engineered rim to provide lateral and uplift load continuity. Continuous sheathing also provides an additional layer of protection if siding or brick veneer is lost during storms.
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Sheath gable ends with plywood or OSB (which might have helped this house weather the storm in better shape). Foam sheathing works better when used with drywall inside the house. The easiest way to avoid interior gypsum at the gable end is to use wood sheathing on the exterior.
View Comments
I am doubtful. Your bottom plate may stay attached, but how well is the wall above attached to the bottom plate? And how well is your roof attached to your wall? We've had walls get torn right off their bottom plates during the framing stage in big windstorms. Any system is only as strong as the weakest component. Instead, I would include hurricane clips for the trusses down to the wall, and run continuous ready rod with nuts and washers on top of the top plates, all the way down to couplings attached to anchor bolts embedded in the concrete. That way the entire height of the wall is tied down to the concrete, and the trusses are tied down to the wall. This doesn't add all that much cost either, but will DEFINITELY resist serious hurricanes (and substantially help with seismic loads).
You need to put metal straps(i re-use the steel bands from the lumber delivery) that go under that bottom sill plate and wraps up the wall studs at least a foot with a few nails driven thru on each side. Do this every 3 or 4 studs. Yes you need hurricane ties on everything as well. Garage doors are probably a no-no as well. I overbuilt my house and hope it's still standing when everyone else's gets wrecked. Of course my garage doors probably just shot this idea down.
P.S. i love this website.
oops, also need to do the same thing over the top of the top wall plate. or secure the second floor plate to the first with metal strapping/hurricane straps.
Building a structure to withstand windstorms takes a lot more than few hold-down bolts with big washers. You have to provided reinforced load paths all the way from the foundation to the roof ridge. The structural reinforcement required to withstand just a 110 mph hurricane is enormous. There is an accepted guide from the ICC, "The Wood Frame Construction Manual", that you can get on-line. It is not light reading. Simpson also provides an easier to read and free guide to using their products for windstorm reinforcement, the "High Wind Framing Connection Guide". This 56 page booklet provides some background information as well as the huge amount of (Simpson branded) steel strapping, nails, etc., required. You can get windstorm rated garage doors, too. They include a lot more steel than your $300 big box store special. Here on the Gulf Coast many civil engineers provide windstorm certification services, specifying and inspecting buildings for windstorm resistance. None of this is cheap!