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Decks take a regular beating. Your favorite outdoor entertaining space is exposed year-round to sunlight, rain, snow, and other elements that could add up to accident-causing damage. According to the Consumer Product Safety Commission (CPSC), an average of 33,000 people per year are injured because of the structural failure or collapse of a deck, porch, railing, or staircase. Six thousand of them suffer traumatic injuries. Frank Lesh, executive director of the American Society of Home Inspectors (ASHI), tells you how to identify potentially dangerous deck problems. So before you invite the block over for a barbecue, hunt down these troubling signs and call an official inspector to check them out.

Rusted fasteners and connectors

Take a look under your deck to see if your fasteners and connectors are sound. Items that hold your deck together, including nails, bolts, screws, and hangers, are made of galvanized steel, but this material still needs a close inspection. "Even good-quality galvanized steel will rust," says Lesh. "If you see rust or something looks loose, you need to have an inspector look at it."

Damage from bugs

Modern decks—those built in the 1980s or since then—are made with lumber treated with chemicals that bugs don't like. (Before then, decks were often built with rot-resistant redwood.) According to the North American Deck and Railing Association (NADRA), there are about 50 million decks in North America; 40 million are residential and 10 million are commercial. Twenty million to twenty-five million of these decks are 30 years old. "If your deck is more than 30 years old, it's a reason to be concerned," he says. Typically, decks aren't meant to be around for that long.

House construction materials generally aren't treated with insect-resistant chemicals, so carpenter ants or termites can get into areas where a new deck made with treated wood attaches to the house. When that happens, even an otherwise structurally sound deck can pull away from the house and cause an accident.

Check for this problem by paying attention when you first step out onto your deck. "There should be no movement," says Lesh. "It should be in the same position as when it was built." If it has any give or you notice it has shifted from the original connection to the house, call an inspector to go over it with a fresh set of eyes. Get a better understanding of how a deck is attached to a house with This Old House TV general contractor Tom Silva.

Cracks

Wood tends to split a bit with wear, but certain kinds of cracks signal problems. Lesh says that tiny cracks are no worry, but you should still keep an eye on them. "If you notice a crack getting bigger or if it's around fasteners, that's a sign of weakness," he says. Water can get into smaller cracks, freeze, and cause the cracks to expand, weakening the board. Lesh also suggests paying particular attention to a crack in the middle of a deck-joist span. Decks are typically built on joists spaced 16 or 24 inches apart, depending on local regulations. The midpoint between joists can be a weak spot, and cracks there can be especially dangerous; a deck board with a crack in that location needs to be replaced so that a guest won't step on the weak spot and go crashing through the deck.

Because of a deck's "tremendous" exposure to the elements, Lesh recommends staining it at least every few years to prevent cracks and other problems. He advises against power-washing. "Power-washing rips into the fibers of the wood and weakens it," he says. "Even if you stain it afterward, the wood surface is not as good." A mild brushing at low pressure is fine for cleaning decks, but he says that people often crank up the pressure to see quicker cleaning results, which is harmful to deck surfaces.

Rotted wood

A little rot may seem harmless enough, but that's a no-brainer that needs attention. "If you see rotted wood, there's something wrong," he says. "It's like looking at bald tires; if one part looks great but there's one bad spot, that's a problem." His suggestion for staining it every few years will also help prevent wood rot.

Loose Railing

Deck railing is an essential safety feature for the gathering spot. It might also seem like the perfect place to take a seat during a crowded gathering, but it's not. "It's designed to give people a place to lean a little bit and give the deck a border, like painted lines on a highway," he says. Avoid sitting on or pushing against it. Lesh says the rule of thumb is that an average-size person should be able to lean against a deck railing and feel a little give. (Railings are supposed to withstand about 200 pounds of lateral force.) If it sways when you push on it, have an inspector check it out.

Just like any stairway, deck stairs require a railing. But while a deck railing encourages leaning, deck-stair railing is for grasping. "It's got to be firm enough so that if one person slips toward the railing and grabs it for support, it will hold him up," he says. As a result, deck-stair railing isn't required to withstand as much lateral force, but any swaying or wobbling is cause for concern. Additionally, the stair steps should be sturdy.

Mold and mildew growth

A green tinge on a deck in the shadows is nothing to bat an eye at, but if the discoloration bothers you, Lesh suggests cleaning it off with a bleach-and-water solution. Growths like mushrooms or fuzzy mold, however, require attention. "Mold eats away at the structure," he says. "What you see on the surface could be the tip of the iceberg." Stairs are particularly susceptible to dangerously slick surfaces from mold or mildew growth.

Pops, creaks, and unusual give

For any change in your deck that seems to happen overnight, get an inspector's opinion. Lesh says that when you walk across the deck to your favorite chair and hear a creak that wasn't there before, the noise needs to be checked out. The same goes for a deck surface that feels spongey or springy.

Call a pro

Like many parts of your home, your deck can fade into your everyday landscape, causing you to overlook dangerous defects that an outsider could spot. A railing's wobbly corner or a cracked deck board may be something you've grown used to, but a neighbor over for a party may not know there's a problem and could get hurt. "When I walk up to a house with a deck," Lesh says, "I'm going to see a defect, and not just because I'm a professional."

You should have your deck inspected every few years, he says. "Your deck is in weather all year long, and there's no protection. There's no 'check engine light' on a deck." He recommends hiring an official inspector, whose concern is safety. Then you can reach out to a deck expert to make any repairs the inspector suggests. You can find an inspector at ASHI's website or the North American Deck and Railing Association.