Betty Gerisch was nudging through a dinner-party crowd of Salvation Army members, aiming for a buffet table on the deck of a house in Atlanta one evening three years ago. Just as she and her husband, Robert, stepped through the door, she heard a loud crack and found herself falling through air. The deck had pulled away from the side of the house and collapsed, dumping 60 guests onto a concrete patio 18 feet below and scalding dozens of them in a torrent of overturned grills and chafing dishes. The scene "looked like a battlefield," reported one firefighter. Ambulances jammed the street as rescue workers helped people with broken pelvises, broken backs and third-degree burns. Robert Gerisch, 77, a cardiologist, suffered a head injury that would keep him in the hospital for six months. Betty Gerisch, 74, suffered a broken neck and a severed spinal cord. "I fell on lawn furniture and cement planters, and then everyone fell on top of me," she recalls. "I was conscious, but I dearly wished I hadn't been." She would never walk again. Three years later, she remains partially paralyzed and unable to get around except in a wheelchair. "To think that, in one instant, life can change so dramatically," she says. "And all because of a badly built deck." Decks are phenomenally popular in the United States. The National Association of Home Builders estimates they are included in nearly a third of all new houses, and decks are by far the most popular do-it-yourself construction project. Yet structural defects that can bring decks down are frighteningly common. Although no one keeps statistics, "I'd say as many as 10 percent of the decks I've seen have serious design or construction flaws that could lead to catastrophe," says Bob Fennema, a structural engineering consultant and member of the American Society of Home Inspectors. Moreover, deck collapses tend to occur just when the potential for injury is greatest: when a crowd gathers. Near Kalamazoo, Michigan, a 57-year-old woman was killed when a deck crashed on top of her several years ago while revelers upstairs were singing "Happy Birthday." In 1995, at a campground in suburban St. Louis, more than 100 fans of the Grateful Dead rock group were injured, five of them critically, when a covered deck under which they had sought shelter from a thunderstorm broke away from a lodge. And at the New Jersey shore last summer, an outdoor wedding ceremony was disrupted when the deck collapsed as the bride and groom exchanged vows. (They completed the ceremony in the emergency room, where doctors treated most of the wedding guests, including the mayor, who officiated.) A deck can even handle the rowdiest gang of fraternity brothers as long as the beam that carries the floor joists is properly bolted to the side of the house. By contrast, virtually every deck that collapses has been merely nailed on. Robert Falk, a structural engineer with the U.S. Forest Products Laboratory in Madison, Wisconsin, realized this while researching a deck-building manual five years ago. Falk had heard about the death of the woman in the deck collapse near Kalamazoo, and he wanted to find out the reasons for the failure. Using a database to search five years of newspaper articles from around the country, he found that nearly every collapsed deck had been attached with nails, rather than bolts, and that investigators had pinpointed the nails as the cause of collapse. "On paper, you can calculate that nails will work," Falk says. "In practice, it's a different story." As people gather on a deck, their weight and movement translate not just into a downward force but also into an outward force that acts as a lever prying the deck away from the house. Nails work well to resist the downward force but are no match for the outward force. Held in place only by the friction of bent wood fibers, nails tend to loosen when wood alternately shrinks and swells with changes in moisture content and temperature. Once nails loosen, they offer even less resistance to the prying forces of a crowd. "There is no built-in safety factor with nails, no warning of a coming disaster," Falk says. "When they pull out, they pull out." A screwed-in connector behaves differently. It gains increased frictional strength from the wedging action of wood fibers along the entire length of the shaft. A lag bolt, which looks like a giant screw, has as much as nine times the pullout resistance of a nail for every inch of penetration, Falk says. Better still is the metal-to-metal connection of a true bolt, inserted in a drilled hole and fitted with a nut on the other side. Placing a washer on both sides spreads the pulling force over a larger portion of the beam. "You'd rip the whole structure apart before those bolts would pull out," Falk says. Both of these connectors offer an extra benefit over nails: They don't suddenly pull out as wood shrinks and swells. But they may loosen over time. If the deck is inspected annually, early signs of loosening will show up as a widening gap against the house. "With bolts, you're more likely to see a problem brewing before your deck falls," Falk says. Another crucial step is to keep the connection between deck and house dry by adding flashing to drain water away. This will protect both the deck and the house. Holes made in the side of a house, even if filled with bolts, allow water to seep in. Jim O'Brien— who owned a construction company in Peachtree City, Georgia—recalls tearing a deck off a 7-year-old house and discovering that water flowing in had rotted the house. "We literally had a 5-foot section rotted so badly that you could put your hand through it," he says of the exterior wall. Flashing will prevent this, Falk says. He also advises squirting a durable caulk, such as silicone, into the holes drilled for the bolts: "The connection will be waterproof." Even better, Falk says, is to avoid attaching the deck to the house in the first place. A freestanding deck, built on posts so that a 1-inch gap separates it from the house, allows water to drain with no damage to the house. However, the posts need cross-bracing to keep the deck from wobbling. The bracing gets in the way if the deck extends above a walk-out basement. "But this would be our recommendation in just about every case," Falk says. "From a durability standpoint, freestanding decks are the way to go."