Deck footing layout

Arithmetic on your numbers. Footing size and depth, frost protection, ledger fastening, guards and stairs are for your plans and your inspector, not this page.

Type the deck’s width and projection and the two spans your plans allow. The page works out how many beams the joists need, how many posts each beam needs, where every footing centre lands measured from the house and from one end, the diagonal that proves the layout is square, and — if you give the hole size — the concrete. The plan and the table print with the Print button.

Worked example

A 16 × 12 ft deck on a ledger, joists cantilevered 1 ft past the beam and the beam running 1 ft past each end post. Say the plans allow the joists 12 ft and the beam 8 ft between posts (your numbers come from your own table — these are only for the sum).

The joists reach 12 − 1 = 11 ft from the ledger, within 12, so one beam whose centre line is 11 ft out. The beam runs 16 − 2 = 14 ft between end posts: 14 ÷ 8 = 1.75, so 2 bays and 3 posts, 7 ft on centre — at 1, 8 and 15 ft from the left end. Three footings. Square it with the diagonal: √(14² + 11²) = 17′ 9 5/8″, from each end-post line at the house to the far end post. Freestanding, the house side takes a beam too: six footings.

Holes 12 in across and 48 in deep hold π ÷ 4 × 1² × 4 = 3.14 ft³ each, 9.42 ft³ for the three — 0.35 yd³. In metric: 4.8 × 3.6 m with 0.3 m cantilever and overhang, joists 3.6 m and posts 1.4 m: one beam 3.3 m out, 4.2 ÷ 1.4 = exactly 3 bays, so 4 posts at 0.3, 1.7, 3.1 and 4.5 m, and a diagonal of √(4.2² + 3.3²) = 5,341 mm.

Try one

The formula

beams = ⌈(projection − cantilever) ÷ joist span⌉  ·  posts = ⌈(width − 2 × overhang) ÷ beam span⌉ + 1  ·  footings = beams × posts

Questions

How many footings does a deck need?

Beams times posts per beam. The beams come from how far the joists may span: the run from the ledger (or the house-side beam) out to the last beam, divided by the longest joist span your table allows, rounded up. The posts on each beam come from how far the beam may span between posts: the beam length between end posts divided by that span, rounded up, plus one. Type the two spans from your span table or plans and the page counts and places the rest.

Why does this page not suggest a span?

Because a span is a structural value. How far a joist or a beam may reach depends on its size, species, grade, the load and your code — that lives in span tables, your plans or an engineer, and it changes from place to place. The page lays out whatever spans you give it; it never makes one up.

Attached or freestanding — what changes?

An attached deck hangs its inner joist ends on a ledger fixed to the house, so there is no beam and no footings along the house. A freestanding deck needs a beam on posts at the house side too, which adds a whole row of footings. Pick one and the count follows.

How do I get the footings square to the house?

Mark the two end-post lines on the house wall, run string lines out square, and check the diagonal: from one end-post line at the house to the far end post of the outer beam, and the same the other way. When both diagonals match the number this page gives, the layout is square. On a freestanding deck measure from the house-side beam line instead.

How much concrete do the footings take?

Type the hole diameter and depth and the page gives each footing’s volume and the total — a round footing is π ÷ 4 × diameter² × depth. For bags net of the post, or a flared footing, use the post hole concrete calculator.

Related tools

• Post hole concrete calculator bags per hole, net of the post.
• Joist spacing calculator how many joists across the deck.
• Deck board calculator boards and fasteners on top.
• 3-4-5 square calculator square the string lines.
• Stair stringer layout the steps down off the deck.