Every deck, porch, and pier-and-beam structure stands on a handful of concrete columns doing quiet, critical work: collecting the load from posts and delivering it to soil that can carry it, below the depth where frost can shove it around. Concrete piers are the simplest foundation system a homeowner can build — a hole, a tube, some concrete — and also one of the easiest to get wrong in ways that show up as a heaved deck or a sagging beam three winters later. This guide covers sizing, frost depth, bell versus straight shafts, cardboard tube forms, precast pier blocks, and the code rules inspectors actually check.
How a Pier Carries Load
The math is simple bearing arithmetic. A deck corner post might carry 2,500-5,000 pounds. Soil bearing capacity ranges from about 1,500 psf (soft clay, the default assumption in code) to 3,000+ psf (compact gravel). The pier’s job is to spread the post load over enough soil area: a 5,000-lb load on 1,500-psf soil needs 3.33 square feet of bearing — roughly a 21″ x 21″ footing. That’s why the bottom of a pier matters more than its shaft, and why code (IRC R507 for decks) sizes footings by tributary load, not by whatever tube was on the shelf.
Frost Line: The Depth Rule That Isn’t Optional
Water in soil expands about 9% when it freezes, and freezing soil grips and lifts anything embedded in it — frost heave forces can exceed 10,000 psf. The fix is to bear below the local frost line, where soil never freezes:
- Gulf Coast and Deep South: 0-12″
- Mid-Atlantic and lower Midwest: 24-36″
- Upper Midwest, New England, mountain West: 42-60″+
Your building department publishes the exact number — it’s the first question to ask when you pull the permit. Two supporting details: dig to undisturbed native soil (never bear on backfill), and keep pier sides smooth and straight or slightly tapered so freezing soil can’t grab the shaft and lift it anyway (“adfreeze”). This is also why the classic mistake — a pier ending at 18″ in a 42″ frost zone — works fine until the first hard winter, then jacks one corner of the deck 2″ up.
Bell vs Straight Piers
Straight piers are uniform cylinders — simplest to dig and form, fine when the shaft diameter alone provides enough bearing area or when a separate spread footing is poured beneath.
Belled piers flare at the bottom into a cone or “bell,” typically twice the shaft diameter. The bell multiplies bearing area without the cost of a full-width shaft: a 10″ shaft belled to 20″ quadruples the bearing surface. Bells also anchor against frost and uplift because the flare locks into soil below the frost line. Manufactured bell forms (like the Bigfoot Systems BF-series, $30-50 each) attach to the bottom of a standard tube; hand-digging a bell at the bottom of a hole works too, in cohesive soil that holds the shape. In loose sand, bells collapse while you dig — use a bigger straight footing instead.
The common workhorse detail for decks combines both ideas: a wide poured footing pad (e.g., 20″ x 20″ x 8-10″) at depth, with a tube-formed shaft rising from it, tied together with rebar.
Sizing: Tubes and Footings
| Tube diameter | Concrete per ft of depth | Typical use |
|---|---|---|
| 8″ | 0.35 cu ft | Light loads — stair landings, small landings |
| 10″ | 0.55 cu ft | Small deck interior posts |
| 12″ | 0.79 cu ft | Standard deck piers — the default |
| 16″ | 1.40 cu ft | Heavy beams, hot tub corners, tall posts |
Example: a 12″ tube, 42″ deep, takes 2.75 cu ft — about five 80-lb bags of concrete. High-early-strength bags like Quikrete 5000 or Sakrete 5000 Plus are the right grade for structural piers; the full product rundown is in our Quikrete concrete mix guide. Past a half-dozen deep piers, price a short-load ready-mix delivery — mixing 30 bags by hand is a decision you only make once.
Cardboard tube forms (Sonotube and equivalents) cost roughly $10-25 per 4-ft length depending on diameter. They keep the pour clean, give a straight shaft that resists frost grip, and let you extend the pier a uniform 6″ above grade to keep post bases dry. Cut them with a jigsaw or handsaw, brace plumb with 2×4 kickers, and backfill around them before pouring so they don’t float or bulge.
Building a Deck Pier, Step by Step
- Layout. Set pier centers from the ledger with string lines and batter boards; diagonal-measure to square the grid.
- Dig. Post-hole digger and digging bar for a few holes; a rented one-man or towable auger ($100-300/day) past that. Dig to frost depth plus footing thickness, down to undisturbed soil. Call 811 before any digging — it’s free and it’s the law.
- Bearing surface. Flatten the bottom, tamp it, and add nothing — no gravel bed under a footing unless your inspector directs it (gravel under a footing can hold water and isn’t compacted bearing).
- Form. Set the bell form or pour the spread footing, then the tube, extending 6″+ above grade. Level the top, plumb the tube, brace it.
- Reinforce. Typical residential detail: two to four vertical #4 bars in the tube, hooked into the footing, with 3″ of cover — plus ties on taller piers. Why steel matters and how laps work is covered in reinforced concrete.
- Pour and consolidate. Fill in lifts, rodding or vibrating each to chase out voids — honeycombed piers are weak piers.
- Set the connection. Wet-set a galvanized post base anchor (Simpson ABU or similar) or embed an anchor bolt while the concrete is plastic. Code requires a positive post-to-pier connection — a 4×4 sitting in a shallow socket doesn’t count.
- Cure and strip. Wait 24-48 hours before light loads, 7 days before building on them. Tear away exposed cardboard whenever convenient.
Precast Pier Blocks: Where They’re Legal and Where They’re Not
The tapered precast blocks with a slot on top ($10-20 each) are tempting: no digging, no mixing. Understand their limits. They bear on the surface, so they’re only appropriate where frost depth is minimal or the structure is allowed to float — which is why codes generally restrict them to freestanding, ground-level decks not attached to the house, sheds, and similar light structures. A ledger-attached deck on pier blocks is a red-tag in most jurisdictions: the house side can’t move but the block side will. Even where legal, set blocks on 4-6″ of compacted gravel, keep post heights short, and expect some seasonal movement.
Code Basics Inspectors Check
- Footing bearing below frost depth, on undisturbed soil.
- Footing size matched to tributary load and assumed 1,500-psf soil (IRC deck tables), or per an engineer’s letter.
- Minimum footing thickness — commonly 6-8″ for spread footings.
- Positive connections: post base at the pier, post cap at the beam. Uplift and lateral loads are real.
- Pier top at least 6″ above grade so wood posts stay out of splash and soil.
- Inspection before the pour — open holes with forms and steel visible. Schedule it; concrete is unforgiving of “we’ll show them after.”
Piers for porches and additions that tie into the house foundation are a step up in stakes — those interact with the existing footing system, covered in our concrete foundation guide, and usually deserve engineering.
Concrete Piers FAQ
How many piers does my deck need?
Divide the beam spans by allowable post spacing (commonly 6-8 ft between posts) and check the IRC deck tables for footing size at each. A typical 12′ x 16′ attached deck lands on 4-6 piers.
Can I pour piers and set posts in the same concrete?
Embedding wood posts in concrete invites rot at the concrete line — the pocket holds water against the grain. Pour full-concrete piers with a standoff post base instead; the hardware costs $10-15 and doubles post life.
Bagged concrete or ready-mix for piers?
Up to roughly 6-8 average piers, bags are practical. Beyond that — or with 48″+ frost depth — a short-load truck saves your day and gives more consistent concrete.
A pier is thirty years of silent service purchased in one afternoon of digging. Get the depth from your building department, the footing size from the load tables, and the connection hardware into wet concrete — and the deck above will never remind you it exists.
