How many footings does a deck need?
One under every post, and the posts hold up the beams. An attached deck hangs its house side on a ledger bolted to the house, so it needs footings only under the beam at the far side. A freestanding deck has a beam at each end, and twice the footings.
Along each beam, the posts go no farther apart than the beam can span, often 6 to 8 feet. A 16-foot-wide deck with posts at most 8 feet apart has three posts on each beam: one at each end and one in the middle. A deck that reaches far from the house gets a beam in the middle too, so the joists don't have to span the whole distance. The calculator adds one whenever the joists would run more than 14 feet.
How big each footing should be
A footing spreads the weight of the deck over enough ground that the ground doesn't give way. The American Wood Council's DCA 6 deck guide works it out like this:
- Find the area of deck the footing holds up: the distance between its posts, times half the joist span on each side of the beam, plus any overhang.
- Multiply by 50 pounds per square foot. That's 40 for people and furniture and 10 for the deck itself.
- Divide by what the soil can bear. The answer is the area the bottom of the footing needs, and the calculator rounds up to the next tube size.
Take the deck the calculator opens with: 12 by 16 feet, attached, with the joists running 1 foot past the beam. The beam carries 6½ feet of deck, and with posts 8 feet apart the middle footing holds up 52 square feet. At 50 pounds a square foot that's 2,600 pounds, which needs 1.7 square feet of clay soil. That comes to a footing 18 inches across.
Here is the same sum for decks with the beam at the outer edge, on clay soil:
| Joist span | Posts 6 ft apart | Posts 8 ft apart | Posts 10 ft apart |
|---|---|---|---|
| 8 ft | 14 in | 14 in | 16 in |
| 10 ft | 14 in | 16 in | 18 in |
| 12 ft | 16 in | 18 in | 20 in |
| 14 ft | 18 in | 20 in | 24 in |
What your ground can carry
When nobody has tested the soil, the residential code lets you assume a value by type. Most deck footings are sized at the lowest of them, 1,500 pounds per square foot, because clay is common and it's the safe guess.
- Clay, sandy clay, silty clay and clayey silt: 1,500 lb per sq ft.
- Sand, silty sand, clayey sand, silty gravel and clayey gravel: 2,000 lb per sq ft.
- Sandy gravel and gravel: 3,000 lb per sq ft.
Better ground means smaller footings. Fill dirt, peat or soil that stays wet are another matter, and need a local engineer or the building department.
How deep
At least 12 inches below undisturbed ground, and below the frost line where the ground freezes. Frost depth runs from nothing in the South to 4 feet or more in the northern states. Frozen water in the soil expands, and a footing that sits above the frost line gets pushed up a little every winter.
The calculator counts each footing as a round column of concrete from the bottom of the hole to 2 inches above the ground, the way a cardboard form tube is usually set. Bags are rounded up to whole bags.
Bags, tubes or a truck
A few footings are a job for bags and a wheelbarrow. As the concrete passes about a cubic yard, around 45 bags of 80 lb mix, a ready-mix truck or a short-load delivery starts to make sense, and the calculator says so. Price both before you decide.
Call 811 a few business days before you dig. It's free, and they come out and mark the buried lines.
Fair questions
Can I use deck blocks instead?
Precast blocks sit on the surface instead of going down to the frost line. Some places allow them under a low freestanding deck. An attached deck needs real footings, because a deck that heaves will tear away from its ledger.
Why 6×6 posts?
DCA 6 calls for 6×6 deck posts, so that's what the calculator counts. Each post is about a foot shorter than the height of the deck, the room taken by the beam and joists. Cut them to fit once the footings are in.
Are the end footings smaller?
They carry half the load of a middle one, so they can be, and some plans make them smaller. The calculator sizes every footing for the busiest one, which keeps the shopping list simple and errs on the safe side.
Does this replace my plans?
No. It's for planning and budgeting. Your permit drawings and your building department decide the footings that actually go in. The deck permit guide explains when you need one.
Sources
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide: ledger attachment, joist spans, guards and stairs for wood decks.
- International Residential Code, Table R401.4.1: the load-bearing values the code lets you assume for each kind of soil, from 1,500 lb per square foot for clay to 3,000 for gravel.
- International Residential Code: sections R312 (guards), R311.7.8 (handrails) and R507 (decks). Your city or state adopts and amends it, so check the local version.
- Call 811: the national call-before-you-dig service, for when and how to have buried utilities marked.
- How we calculate: every suggested price behind this calculator, with the date it was last reviewed.
