Synopsis: Build your deck on concrete piers, and you’ll have a solid foundation that won’t heave or twist as a result of frost or poor soil bearing capacity. Veteran builder Rick Arnold walks us through the how-tos of building deck piers. Rick explains how to size piers for a basic deck and shares his method for a precise layout. You’ll also learn techniques for digging and pouring piers (which include valuable tool tips), when to use spread footings, and how to deal with poor soil bearing capacity.

Dig a hole and fill it with concrete. How hard can that be? I’ve seen old decks built on top of little more than a shovelful of concrete, cinder blocks up on end, and even 8-in. by 12-in. patio blocks. I’ve also seen old decks—not to mention a couple of new ones—sink and pull away from a house, heave up with the same results, and even both sink and heave from one end to the other.

An insufficient design or a bad installation of this simple foundation system can have disastrous consequences in terms of safety, aesthetics, and a builder’s reputation. That’s why I approach piers with the same care as I do a house or addition foundation.

Soil conditions and load requirements determine pier size and spacing

Because piers perform the same job for the deck that the foundation does for the house, it’s critical to size and space them properly.

I begin by figuring out how many piers I’m going to need. This decision depends mostly on deck design. For this project, I was building a simple 12-ft. by 16-ft. rectangular deck with a double rim joist to act as a beam that could span about 8 ft., with posts running down from the beam to the piers. In this type of application, I start with two piers on the corners and divide the 16-ft. double rim joist until I get a figure of 6 ft. or less. Here, I found that dividing the rim joist into three sections gave me a span of roughly 5 ft. 4 in., which came out to four piers. Even though my double rim joist could span 8 ft., I chose to use a 5-ft. 4-in. pier spacing to minimize the pier diameter.

After calculating the number of piers I need, I determine the size they need to be. The size of builder’s tube dictates the size of the bottom of the pier, which is the area that will be in contact with soil at the bottom of the excavation. To figure this out, I calculate the maximum weight each pier must be designed to bear (by code).

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