WARNING: This article is an account of another deck failure. Please check your own deck for rot, wood destroying insects, and inadequate securment to the home.
EDITOR'S NOTE: The following are excepts from a well-written lengthy article. I encourage all of you to purchase the July/August 98 issue of This Old House magazine on your newstand and read the full account. In fact, I encourage you to get your 2 year subscription..... It is a good magazine. I originally reprinted this article in the Monday Morning Marketer, a newsletter specifically written for Home Inspectors.
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 the 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.........
Decks are phenomenally popular in the United States......... 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.
.........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 a woman in a 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 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 the deck and the house dry by adding flashing to drain the water away.......... Jim O'Brien,...... recalls tearing a deck off a 7-year old house and discovering the 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."
..........Of the three main building codes in the United States--all of which are scheduled to be consolidated into a uniform code in the year 2000--not one deals with fastening decks to houses. "You're not going to find any prescriptive details relevant to the structure of a deck," concedes Mike Pfeiffer of the Building Officials and Code Administrators, International, which publishes the BOCA National Building Code.
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