How to build deck footings

The footing, in one drawing

A deck footing is a block of concrete that carries one post's share of the deck down to soil that will not sink and will not heave. Everything below is the order you build it in. The size of each footing comes from the footing size calculator, the depth from footing depth, and the positions from footing spacing and layout. This page is the build itself.

1. Call 811 before the first hole

811 is the free national before-you-dig service. Contact it, or your state's 811 centre online, a few business days before you dig. Have the address, the nearest cross street and the exact area you are digging ready. Then wait until every utility has responded and marked its lines. A deck footing goes as deep as buried services run, and an auger finds them faster than a shovel does.

2. Lay out the footing centres

Footing positions are post positions, and post spacing comes from the beam, not from a round number. Size the beam first with the beam span calculator. Then string the layout square off the ledger with batter boards and drop a plumb line to each hole centre. The full method, with the diagonal check, is in how to lay out deck footings. The one rule to bring here: mark the string positions on the batter boards with a saw kerf, because the strings come down to dig and go back up to set the post bases.

3. Dig to depth, onto undisturbed soil

The 2021 IRC puts deck footings at least 12" below undisturbed ground. Where the deck is attached to a frost-protected house, they also go below the frost line your jurisdiction publishes, onto solid rock, or another approved method. Whichever is deeper wins. How to find that number is in deck footing depth, and it is not repeated here.

What matters at the hole: dig every footing to the same bearing depth, flatten the bottom, and clear out loose spoil. If you hit backfill from when the house was dug, keep going until you reach native ground. An auger leaves a clean round hole that suits a form tube. A hand-dug hole is usually wider at the top, so measure it before you pour and allow for it in the concrete calculator.

4. Choose the footing type

Section IRC 2021 R507.3 asks for "concrete footings or other approved structural systems". In practice that means one of four things:

5. Set forms, reinforcing and anchors

Cut the form tube so it sits on the bottom of the hole and stands a few inches above grade. Plumb it, brace it, and backfill around the outside so it cannot float or lean when it fills. Every tube top should sit at the height your post lengths were worked out for.

The code's minimum footing table is written for plain concrete. Its thickness minimum "shall only apply to plain concrete footings". Reinforcing goes in where your approved plan or an engineer calls for it, and if it does, it is tied and supported before the inspection, not pushed in afterwards.

Decide now how each post base will anchor. Simpson Strong-Tie sorts them into two families. Cast-in-place bases (its PBS, PPBZ, CBSQ and MPBZ) are set into the wet concrete at the pour. Post-installed bases (ABU, ABWZ, ABAZ, CPT, RPBZ) bolt to an anchor that is either cast in at the pour or drilled and fixed after the concrete hardens. Simpson also warns that a cast-in base needs enough concrete around its embedded part to reach its published load values. That can mean a bigger footing than the code minimum.

6. Get the footing inspection before you pour

Under the 2021 IRC (R109.1.1), the foundation inspection is made after the piers are dug and any required forms and reinforcing are in place, "prior to the placing of concrete". The inspector checks depth and what the hole bottoms on, and neither can be checked once the hole is full. Book it, have the holes clean and the strings back up, and do not pour ahead of it. Where this inspection sits among the others is in deck permit requirements.

7. Pour

Work out the bags before the day with the concrete calculator for deck footings and have them on site. A footing poured in one go has no cold joint. QUIKRETE's data sheet for its standard mix starts at about 6 pints of water per 80 lb bag, and puts the final water content at about 6 to 9 pints. Stay in that range rather than thinning the mix to make it pour faster.

Fill in lifts and rod or tap the tube to work out air pockets. Strike the top off, and slope it slightly away from where the post will stand. Simpson's post-base detail shows a 1:12 downward slope, drawn to IRC Figure R507.3. Set cast-in bases or anchor bolts now, on the string line, plumb and to height, while the concrete is still workable.

8. Fix the post bases

The post has to be held at its foot, not just stood there. Section R507.4.1 requires lateral restraint by manufactured connectors, or by burying the post at least 12 inches in surrounding soil or a concrete pier. Where soils are expansive, compressible or shifting, the soil cannot be counted on for that restraint. A rated post base is the cleaner answer, because it also keeps the timber out of the wet.

Simpson puts it in one line: preservative-treated wood that is cut must be field treated, or the post must be held 1 inch off the concrete. The standoff bases in both families do exactly that. Match the coating to the lumber. The guide recommends its ZMAX galvanized or hot-dip galvanized finishes on these bases, and offers stainless for some. Then pick the post itself — a 4x4 or a 6x6 — from the code's post height table.

9. Let it cure before you load it

Neither maker publishes a "build the deck on it" figure. What they do publish is how fast the concrete gains strength. At 7 days the standard mix typically has roughly 60 percent of its 28-day strength. Our reading: setting posts and hanging framing on a hardened pier with tight anchors is fine once the concrete is hard. Stacked decking bundles, a full party or a hot tub wait until the curing period is over. In cold weather the clock runs slower, so add time.