Recent events show the importance of a shelter standard or, at minimum, compliance with NSSA specifications, he adds. In late April, in the first shelter fatalities, two people were killed when projectile debris penetrated the door. "We're almost certain it was a lightweight door that wasn't up to [FEMA 320 specifications]," says Kiesling. The lab will soon verify that.

The door, which can cost as much as $1,200 more than an $800 non-NSSA-compliant door, is the most critical element of a shelter, he adds. For a production homebuilder or shelter maker, attaining an NSSA seal and membership requires independent verification by an engineer that a shelter's drawings—created by a licensed architect or engineer—comply with NSSA standards. The shelter also must be debris-impact tested by firing a 15-lb 2x4 at 100 mph at the shelter. Finally, the NSSA member must promise to install only NSSA-compliant shelters.

While shelters can minimize fatalities, minimizing property damage requires retrofits. "For one dollar per square foot, you can make an ordinary house go from withstanding 90-mph winds to 135-mph winds," says Ramseyer. "We build to 135-mph winds because [beyond that] the cost goes up significantly," he adds.

And winds up to 135 mph account for more than 80% of tornadoes and 85% of the damage caused by an EF-5 tornado, says Ramseyer.

Reserve Strength