DEAR TIM: My wife has instructed me that I no longer can put off the deck project. I’ve got the confidence to do all the work above ground, but I’m worried about the deck foundation. What do I need to know about deck pier forms so my deck posts don’t sink into the ground? I see round and square pieces of concrete under the posts at other decks I look at. It that all you do, just place a precast concrete disc under the post and be done with it? – Eric B., Long Island, New York
DEAR ERIC: Think of your deck as you would any structure. Perhaps you’ve seen houses next to the coast where you live that resemble tables. The houses sit up above the ground on large wooden or concrete posts so the storm surge flows under them. These support columns are not very different than your deck posts. They all transfer the load of the structures to the soil below.
Decks need foundations just like houses, detached garages and skyscrapers do. The concrete deck piers support the concentrated loads that you find at each deck post. Imagine how much weight is pushing down on the few posts you’ll have. By the time the deck is complete and you have lots of people, furniture, snow and so forth on it, the load at each post can exceed several thousand pounds. Properly designed and constructed deck piers ensure the deck doesn’t sink or get uplifted by wind.
The concrete you see at the surface under deck posts ought to continue deep down into the soil. The depth the concrete extends is a function of the frost line in the area and the bearing capacity of the soil. What’s more, there’s a good chance that deep in the ground the concrete is wider than what you see at the top to help spread out the load across more square inches of soil.
I’ve installed all sorts of deck pier forms in my career. It was not unusual to dig a large 2-foot-by-2-foot hole and pour about 6 or 8 inches of concrete in the bottom of the hole. This concrete was a small spread footing.
The next day we’d then lay concrete block on top of the hardened concrete. The hollow cores of the block would be then filled with more concrete. A steel anchor bolt would then be inserted in the wet concrete. This anchor bolt would hold down a metal connector used to fasten the wood deck posts to the new concrete pier.
Many years ago inventors tried to make all this easier. Round concrete forms that look like giant paper towel roll cores were sold to contractors. You can still buy these thick paper tubes. This was set on a similar concrete footer in the ground and acted as a handy form for wet concrete. You no longer had to lay concrete block.
The issue with using one of these giant tubes is that you had to figure out how to brace it so it wouldn’t move as you poured concrete. It’s not as easy as you might think to do this. Then you had to figure out how to get reinforcing steel into the tube in the right position.
A few years ago I stumbled upon a new invention that makes installing deck piers as easy as working with snap-together plastic building blocks children play with. This ingenious deck pier form snaps together in less than a minute with no tools. But wait, it gets better.
The plastic form comes with internal slots and rings that support pieces of precut steel reinforcing bars that come with the form. In years past you had to get long bars of steel and then struggle to cut them. At least two pieces of the steel need to be bent to connect the vertical concrete pier to the wider concrete that forms the footing for the pier. It’s not as easy to bend this reinforcing steel as you might think.
There are several types of plastic deck pier forms. With the ones I prefer, all you need to do is carefully place the form at the correct depth so you meet the local building codes for frost protection and soil bearing capacity. Once you have the form in place and level, you carefully backfill the dirt around the form before you pour the concrete.
What I love about these forms is how much time and effort they save. Everything you need, except for the concrete, is shipped to your home. You don’t have to drive to different places to get all the steel, forms, bracing, etc. You just dig the hole to the correct depth, place the form, carefully backfill so as not to move the form and start mixing concrete. It doesn’t get any easier than that.
Be sure the anchor bolt you install in the wet concrete is hot dipped galvanized. You don’t want the bolt that holds the deck post connector to the pier to rust out over time. Take your time and double check all measurements to ensure you have your deck piers in the correct location. The margin for error is usually less than 2 inches in any given direction.
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