by Tim Carter, www.askthebuilder.com
Q: Tim, you helped me years ago, and now I’m back. I’m going to build a freestanding deck at my house during my vacation. After watching lots of online videos, I’m somewhat confused about how to connect the deck posts to the concrete piers. What is the best way? I’m worried about my freestanding deck swaying back and forth. The top of the deck is going to be about 7 feet up in the air. My goal is to have the deck rock solid. What should I do to make this happen? —Ron S., Richmond, Va.
A: You may have ambitious plans like Ron. A deck could be in your immediate future. Even if you’re going to hire a contractor to build your new deck, you might want to heed the advice I’m about to dole out to Ron. I say this because each week I receive lots of photographs from homeowners like you who have hired contractors to do work. The photos of the mistakes make me shudder!
Ron understands that his deck needs to rest upon concrete footings or piers. That’s a great start. What’s important to realize is a deck foundation is very different from most house foundations. A house foundation typically has a continuous footing and foundation under all the bearing walls. This distributes the weight of the structure over a much larger square-foot area under the house than what’s happening under a deck.
A freestanding deck is not much different from a card table that has four legs. All of the weight of the table and whatever is on it transfers to the floor. This weight is concentrated on just the four small bottom pads of the table legs. A table and all the things on it might just weigh 50 pounds. A deck fully loaded with people attending a party, the deck itself, and all the deck furniture might weigh several thousand pounds.
Do the math as to how much weight is pushing down on each deck post and you see why you need to have concrete piers resting on solid soil. Keep in mind if you live where the soil freezes, the bottom of this concrete pier must be deeper in the soil than the frost penetrates. Your local building department can advise you as to the frost line in your area.
In almost all cases, a 2-foot-diameter pier is sufficient. This pier can get smaller as it comes up to the surface of the ground. A 1-foot-diameter pad at the top of the pier is plenty of space to place a 6×6 or 4×4 wood deck post.
The deck post must be fastened to the concrete pier. It’s best to place a long 1/2-inch anchor bolt in the wet concrete that will be used to bolt down a heavy galvanized metal deck post base. There are quite a few designs; I prefer to use ones that allow for a little adjustment of the post base if the anchor bolt is not in the perfect location.
Many of these post bases are designed so the wood post sits up off the poured concrete pier. This is a good idea, as you don’t want the treated wood to be buried in the soil. I’ve personally pulled treated lumber out of the ground that was eaten up by termites. I’ve seen treated lumber rot that has been in constant contact with wet or damp soil.
Be sure that you use the proper galvanized threaded bolts to attach the wood post to the metal post base. Never use roofing nails. Roofing nails are not structural fasteners. You need high-quality hot-galvanized bolts that can withstand the corrosive environment that’s in play at the bottom of the post base. The copper in the treated wood, when mixed with rainwater, will start to corrode steel or iron that’s not protected by a thick galvanized layer or two of zinc.
The best money Ron can spend on this project — and you should consider it too if you’re building a deck like his — is a consultation and simple plan from a local residential structural engineer. These professionals can develop an easy-to-understand plan that shows all the proper bracing that needs to be installed so the deck will not sway.
The Internet is littered with videos of deck collapses. People have died and been seriously injured when a deck collapses. I know a young woman who fell from a collapsed deck. She broke her neck and ruptured her spleen and yet somehow survived, albeit in bad shape.
Imagine what the combined weight is of 30 adults dancing up on a deck. It can easily be more than two tons. Dancing can create a harmonic motion that starts to get amplified as the people all dance to the same music. It’s a recipe for disaster if the deck doesn’t have abundant diagonal horizontal and vertical bracing.
For starters, think about what corner fence posts look like on a ranch. The corner fence post almost always has two diagonal braces that extend either direction from the top of the corner post down to the base of the adjacent fence post. This brace prevents the top of the corner post from leaning over toward the row of posts as the fencing is stretched. Your deck needs these same braces.
The engineer will not only call out the type of bracing that’s required, but he’ll also call out exactly how you connect the braces to the framing. His plan will show exactly what hardware or fasteners to use. If the plan calls for a half-inch diameter through-bolt, don’t drill a hole larger than 1/2 inch in diameter. You always want structural fasteners to fit snugly in the holes.
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