Footings & Posts
How many footings, how big, and what each one is actually carrying.
Every load on a deck ends up in the soil. Footings are the part of the build that is invisible when finished, impossible to fix afterwards, and priced by the hole.
How many
Not a number you pick — a number that falls out of the beam span. Posts go where the beam table allows them to go, and each post gets a footing. Shorten the beam span and you buy more holes; lengthen it and you buy more beam.
For a freestanding deck, double the count: there is a second beam line where the ledger would have been.
How big
Footing size is tributary area × design load ÷ soil bearing capacity.
A post carrying 6 ft of beam and 12 ft of joist supports a tributary area of 6 × 6 = 36 ft². At the IRC residential deck load of 50 psf (40 live + 10 dead), that is 1,800 lb. On 1,500 psf soil — the presumptive value for silty clay — the footing needs 1.2 ft², so a 15 in diameter pier. On 2,000 psf sandy gravel, a 12 in pier does it.
Presumptive soil values are in IRC table R401.4.1. They are conservative on purpose, and a local inspector may hold you to something lower.
How deep
Below the local frost line, and that is a jurisdiction number rather than a state one — 12 in on the Gulf, 42 in through much of the upper Midwest, 60 in and more in northern Minnesota. A footing that stops short of it will lift in the first hard winter and keep lifting, and the deck racks a little more each year.
Depth is measured to the bottom of the footing, not the top of the pier.
Concrete per hole
A 12 in diameter pier 42 in deep is 2.75 ft³ — about four 60 lb bags. A 16 in pier the same depth is 4.9 ft³, or seven bags. Doubling nothing but diameter nearly doubles the concrete, which is the whole argument for getting the soil number before ordering.
Piles instead of concrete
Helical piles are drilled rather than dug, install in an hour, carry a certified capacity, and cost three to five times a poured pier. They earn it on sites with poor soil, deep frost, or no access for a mixer — and nowhere else.
What to check before digging
Where the utilities are. It is a free phone call and the single cheapest step in the entire build.
1 article
How Many Footings Does a Deck Need? Work Backwards From Tributary AreaNobody decides footing count directly. It falls out of how far the beam can span, and how much deck each post is holding up.