4-04 Building for the Big Storms

PATH - A Public Private Partnership for Advancing Housing Technology

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Building for Big Storms

Homes can be built to stand up to powerful storms. Good builders know that choosing the right building materials and paying attention to construction details are keys to building a storm-resistant home. Prospective homeowners should also know enough to be able to talk to their builders about the costs and benefits of a more weatherproof home.

Preventing damage in the vortex of a tornado is virtually impossible, but mitigation measures for hurricanes and tornadoes share some similarities.

Hurricane Andrew's 165-mph gusts in 1992 inflicted moderate to severe damage to roofing in 77% of the homes in its path. More than 60% of the homes lost one or more roof sheathing panels. Few escaped without water penetration; damage to interior finishes was reported in 85% of the homes.

Most of the observed failures from the category F4 tornado that hit La Plata, Maryland in 2002 related to roof, wall and foundation connections and their resistance to wind uplift loads.

While researchers still are working on design improvements and models that accurately predict how light-frame homes behave during extreme wind events, here are a few key recommendations from the PATH report Durability by Design:

Continuous uplift load path and anchorage: In hurricane-prone areas, use hurricane ties or clips to connect the roof to the wall system. Line up bracing and truss tie-down points, and anchor each level to the level below. Use light-gauge steel straps to anchor the first story to the foundation. (During Hurricane Andrew, hurricane ties had a 92% success rate.)
Wall bracing: Proper diaphragm and shear wall construction using a rated bracing material such as oriented strand board or plywood is critical.
Construction practices with durability in mind: Material durability is key to future disaster resistance. Buy and install materials that will last. Also, attention to small details - lapping wall top plates at intersections with interior walls and attaching sheathing to a common stud in corner construction - can make all the difference, according to full-scale shear wall testing done by the NAHB Research Center.
Proper nailing schedule: Size, type and placement are all factors. Inadequate nailing schedules in older building codes were implicated in the widespread roof sheathing damage in Hurricane Andrew. Preliminary findings of an NAHB Research Center study suggest that nail edge distance tolerance is quite tight (acceptable nail edge distance should be 1/4 to 3/4 inch) and that nail overdrive can reduce uplift strength of the connection significantly. Thicker sheathing can offset sensitivity to overdriving. When inspecting roof sheathing nails, attention to a gable end truss is especially important.
Ring-shank nails: To secure sheathing panels in hurricane-prone areas with basic wind speed of 110 mph or greater, ring-shank nails are recommended for their higher withdrawal capacity. Costs are comparable, and with a slightly smaller diameter, they offer the added benefits of less frequent reloading of pneumatic nail guns and more forgiving edge distance tolerances.
Wind-borne debris protection: Reduce forces on the structure and minimize water- and wind-related damages to the interior by protecting windows with approved shutters or properly fastened wood structural panels in coastal homes that might experience a hurricane.

A key to minimizing water damage during severe storms is to keep the roof covering intact. PATH team members working in Florida have found two practical ways to do that: 1) rubber or asphalt sheets with peel-and-stick undersides ("self-adhering polymer modified bitumen underlayment" instead of standard roofing felt) beneath standard shingles, costing $700-1,000 more than standard asphalt roofing for a 2,400 sf house, or 2) a new Owens Corning roof shingle with a 130 mph wind rating, costing approximately twice the price of standard roof shingles.

Storm-resistant shutters for a 2,400-square-foot, single-story home with 312 square feet of windows cost about $700.

At a site demonstration in Melbourne, Florida, a PATH team is examining how to exploit the strength of poured concrete walls to enable homes to weather a major storm. Poured concrete walls are quicker to construct and better resist wind loads than concrete block walls.

Surprisingly small additional investments in the concrete wall system fortify the standard concrete home enough to withstand a 125-mph wind-force: extra tie-downs and alignment of the bearing walls with the truss tie-down points to create a direct load path configuration. Essentially a planning issue, these adjustments represent less than $10 in additional costs. Similarly, additional steel costs average about $13, primarily for reinforcing around windows and sliding glass doors. And with another nominal cost increase to cover a deeper reinforcing member, the team found that the wall system could reach 135-mph wind resistance.

See PATH tools and resources related to disaster resistance. For more on results from PATH's research into high wind-resistant home construction see Technology vs. Nature, a PATH article in the August 2003 issue of Professional Builder.

Content updated on 3/31/2004

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