Q: I’m going to build a free-standing 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 free-standing 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?
A: You’re right that the deck will need to rest upon concrete footings or piers, so that’s a great start. What’s also important to realize is that a deck foundation is very different from most house foundations.
A house’s 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.
Structurally, a free-standing deck is similar to a four-legged card table. All of the weight of the table and whatever is on it transfers to the floor. This weight is concentrated solely on the small bottom pads of the four table legs. The difference, of course, is that the table and all the things on it probably weighs less than 50 pounds. A deck that’s fully loaded with people attending a party, plus all the deck furniture, can weigh several thousand pounds.
With all of that weight pushing down on each deck post, it’s crucial that you have concrete piers that are resting on solid soil. Keep in mind that if you live where the soil freezes, the bottom of each concrete pier must be deeper in the soil than where the frost penetrates. Your local building department can advise you on 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 has plenty of space on which to place a 6-by-6 or 4-by-4 wood deck post.
The deck post must be fastened to the concrete pier. It’s best to place a long half-inch anchor bolt in the wet concrete that will be used to bolt down a 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 a bit above the poured concrete pier. This is a good idea, because you don’t want treated wood to be buried in the soil. I’ve pulled treated lumber out of the ground to discover it had been eaten up by termites. I’ve also seen treated lumber rot because it had been in constant contact with wet or damp soil.
Be sure that you use galvanized, threaded bolts to attach the wood posts to the metal post bases. Never use roofing nails; they are not structural fasteners. You need high-quality bolts that can withstand the corrosive environment of the post base. The copper in the treated wood, when mixed with rainwater, can corrode nails that are not protected by a thick layer of zinc.
The best money you can spend on a DIY deck build is for 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 your deck will not sway.
The internet is littered with videos of deck collapses. People have died and been seriously injured when a deck fails, so getting this project right from the start is extremely important. And that means using plenty of diagonal horizontal and vertical bracing.
Think about what corner fence posts look like on a ranch. The post almost always has two diagonal braces that extend in 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.
Your structural engineer will explain the type of bracing that’s required, and exactly how you connect the braces to the framing. The plan that the engineer provides will show exactly what hardware and fasteners to use. If the plan calls for a half-inch-diameter through bolt, be sure you don’t drill a hole that’s larger than a half-inch in diameter. You always want structural fasteners to fit snugly in their holes.
Tim Carter has worked as a home improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.