How Many Footings Does a Deck Need?

Digging footings is the slowest, most expensive part of framing a deck, so it's worth asking how many footings a deck actually needs before you rent an auger. The honest answer is: fewer than most people guess, because footing count comes down to your beam's post spacing, not just the deck's overall width. A beam that reaches farther needs fewer posts, and since every post needs a footing, beam size and footing count are directly linked.

This guide walks through the maths, a worked example, and the trade-offs between fewer big footings and more small ones.

Footing Count Starts With Your Beam

Every footing under a deck exists to support a post, and every post exists to support a beam. So the number of footings you need isn't really about the deck's square footage — it's about how far apart your beam can carry its posts. A larger beam, like a 3-ply 2x10 in Southern Pine, can span roughly 13 feet between posts when it's only carrying 6-foot joists, but drops to about 10 feet of post spacing when carrying 12-foot joists, per IRC Table R507.5. Upsize the beam and you buy yourself wider post spacing — and fewer holes to dig. Run your own joist span and beam size through the beam span calculator before you plan your footing layout.

How Many Footings Does a 16-Foot Deck Need? A Worked Example

Take a 16-foot-wide attached deck with a beam sized to allow 8-foot post spacing. Divide the beam's length — which matches the deck's width, 16 feet — by the 8-foot maximum spacing, and you get 2 spans. Spans always need one more post than the number of spans, so that's 3 posts, and 3 footings, along that beam line. Widen the deck to 20 feet on the same beam and you'd need 3 spans (20 ÷ 8 rounded up), meaning 4 posts and 4 footings. The formula holds everywhere: footings = (deck width ÷ max post spacing, rounded up) + 1.

Post Spacing vs Deck Width: The Numbers

The table below shows how footing count shifts with both deck width and beam post spacing, using the formula above. Notice how a wider allowable post spacing — from a bigger beam — consistently pulls the footing count down for the same deck width.

A 32-foot-wide deck framed for 6-foot post spacing needs 7 footings, but the same deck on a beam rated for 10-foot spacing needs only 5. That's two fewer holes, two fewer footings poured, and two fewer posts to set plumb — plug your own numbers into the footing calculator to see the effect on your layout.

Don't Forget Ledger-Side and Stair-Landing Posts

The formula above covers one beam line, but most decks need footings elsewhere too. An attached deck doesn't need footings along the house side — the ledger board carries that load into the home's foundation instead. But a free-standing deck needs a second beam and its own row of footings on the house side, roughly doubling the count compared with an attached design — see floating deck vs attached deck for the full comparison. Stair landings almost always need their own footing too, since a stringer resting directly on soil or a paver will settle unevenly over a few seasons. Multi-level decks or wraparound layouts add further beam lines, each with its own footing count calculated the same way.

Does Poor Soil Mean You Need More Footings?

Yes, often. Footing size and spacing are both tied to the soil's bearing capacity — the IRC's presumptive value of 1,500 psf for typical sand, silt or clay assumes reasonably competent soil. If a site test or your local building department flags weaker soil, each footing carries less load safely, which can force either bigger footing pads or tighter post spacing to keep the load per footing within limits. That tighter spacing pushes your footing count up even with an unchanged beam and deck width. It's worth checking soil conditions early, since redesigning the footing layout after the beam is already sized wastes time and lumber.

Fewer, Bigger Footings vs More, Smaller Footings

Digging and pouring a footing has a largely fixed labour cost regardless of its exact size — the auger has to go in, the concrete truck or bags still need to show up, and the post base still needs setting. That's why fewer, larger footings paired with a bigger beam usually save money over more, smaller ones, even though the beam itself costs more per linear foot. Labour typically runs $15-35 per square foot of a deck build, and footing work is a meaningful chunk of that on a deck with a complex or oversized footprint. The trade-off flattens out once beams get large enough that handling and lifting them costs more than the footings saved — for most residential decks, that ceiling sits around a 3 or 4-ply beam.

A Worked Scenario: Adding a Corner Post

Take a homeowner planning an L-shaped deck wrapping two sides of a garden room. The straight run along the back is 22 feet, which on a beam rated for 8-foot post spacing needs 3 spans, rounded up from 22 ÷ 8, so 4 posts and 4 footings. Where the deck turns the corner to run another 12 feet down the side, that leg needs its own footing count, worked out the same way against whatever beam carries it — in this case 2 spans, or 3 posts, one of which is the shared corner post already counted on the first run. The corner post itself often carries a larger tributary area than a straight-run post, since it picks up load from two directions at once, so it's worth sizing that particular footing to the higher of the two loads rather than assuming it matches its neighbours. Skipping this step is a common reason L-shaped and wraparound decks come in slightly over budget on footing work compared with the homeowner's first back-of-envelope count.

“We see homeowners default to footings every 6 feet out of habit, when the beam they've already bought could easily handle 8 or 10 feet of post spacing. Size the beam first, then let the footing count follow — not the other way round.”

Size your beam and post spacing first, then run the numbers through the footing calculator to get an exact footing count and depth for your soil and load before you book the auger.