Deck Framing Calculator
Size the framing for a deck: the joists, the beam and how many posts it needs, the posts, and the footings, with a framing plan and a list of the lumber, hangers, decking and concrete. Enter the deck's size and height and the calculator works out the rest.
Works: 2x8 joists 16 in apart, a 3-2x8 beam on 3 posts 7′ apart, 6x6 posts and 18 in round or 16 in square footings.
| Member | Size | Used | Limited by |
|---|---|---|---|
| Joists11′ span + 1′ overhang | 2x8 at 16 in | 86% | Bending |
| Beamposts 7′ apart; reaches 9′ 1″ | 3-2x8 | 58% | Bending |
| Posts2,348 lb, 4′ tall | 6x6 No.2 | 17% | Slenderness (height over width, up to 50) |
| Footings36 in deep, 1,500 psf soil | 18 in round or 16 in square, 3 of them | ||
Every check
Joists
| Bending | 86% |
| Shear | 30% |
| Live load deflection (span/360) | 80% |
| Total load deflection (span/240) | 66% |
| Overhang deflection under live load | 2% |
| Overhang deflection under a 220 lb load at the end | 38% |
| Overhang no longer than a quarter of the span | 36% |
Beam
| Bending | 58% |
| Shear | 32% |
| Live load deflection (span/360) | 34% |
| Total load deflection (span/240) | 28% |
| Overhang deflection under live load | 2% |
| Overhang no longer than a quarter of the post spacing | 57% |
Posts
| Compression, with buckling | 15% |
| Slenderness (height over width, up to 50) | 17% |
Loads: 10 psf dead (wood boards and framing) and 40 psf live. The beam carries 6′ 7″ of deck: half the joist span, the overhang, and the overhang's lift off the ledger; that's 65 lb/ft dead and 262 lb/ft live, plus its own weight.
Material list
| Item | Size | Count | Length | |
|---|---|---|---|---|
| Joists | 2x8 | 13 | 12 ft | 16 in apart |
| Ledger | 2x8 | 1 | 16 ft | bolted to the house |
| Rim joist | 2x8 | 1 | 16 ft | across the joist ends |
| Beam | 2x8 | 3 | 16 ft | 3-2x8 |
| Posts | 6x6 | 3 | 8 ft | treated for ground contact |
| Joist hangers | 2x8 | 13 | at the ledger | |
| Decking | 5/4x6 | 26 | 16 ft | 26 rows |
| Concrete | 80 lb bags | 27 | 0.59 cubic yards for 3 footings |
Check the beam in the beam designer Check a joist in the beam designer
Lumber is Southern Pine No.2, treated and wet. IRC and NDS: like the deck beam tables in the International Residential Code (IRC R507.5), a deck beam of two plies takes a 10% bending increase and three or more plies 15%. The National Design Specification (NDS) gives that increase only to joists and other members spaced 24 in or closer, so an NDS check of the same beam allows a shorter span. The beam designer links leave out the overhangs and check the lumber dry, so their results will differ a little.
Allowable stress design (ASD) under the 2018 National Design Specification (NDS) for wood, with the factors for treated lumber that stays wet outdoors. The calculator doesn't design the ledger's attachment to the house, bracing against sideways movement, stairs, railings or hot tubs.
How the calculator sizes a deck
- Joists: the smallest of 2x6 to 2x12 that spans from the ledger to the beam, with the overhang past it. Joists 24 in apart or closer share load, so they take the repetitive member factor of 1.15. Each is checked for bending, shear and deflection (live load to span/360, total to span/240), and the overhang for its deflection and a 220 lb load at its end, the check behind the code's deck joist table. The overhang can be up to a quarter of the span.
- Beam: the joists put half their span, all of the overhang, and a little more (the overhang lifts the joists off the ledger and pushes that load onto the beam) on the beam. Unless you pick one, the calculator takes the smallest beam that reaches 8′ between posts, then spaces the posts evenly as far apart as that beam allows. A freestanding deck gets the same beam at each end.
- Posts: the most heavily loaded post, an inside one or an end one with the beam's overhang, is checked as a column, with buckling over its height. The calculator picks the smallest of 4x4, 6x6 and 8x8 that carries the load and is at least as wide as the beam, so the beam bears across the whole post.
- Footings: the area that keeps the soil pressure under its allowable value, as a round pier or a square pad, counting only the concrete's extra weight over the soil it replaces.
Loads
A home deck is designed for 40 pounds per square foot (psf) of live load: people, furniture and grills. Dead load is the deck itself: 10 psf with wood boards and 12 psf with heavier composite boards. Where the ground snow load is more than the live load, the calculator uses the snow instead, and lets the wood carry 15% more for the shorter time snow stays (the load duration factor).
Without a soil report, 1,500 psf is the usual allowable soil pressure; clay and silt may not take more. The footing depth should reach below the frost line where you live. For bigger footings, see the concrete footing size chart.
How far deck joists span
The longest span for Southern Pine No.2 deck joists, treated and wet, under 10 psf dead and 40 psf live load, without an overhang:
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 9′ 10″ | 8′ 11″ | 7′ 7″ |
| 2x8 | 13′ | 11′ 9″ | 9′ 7″ |
| 2x10 | 16′ 2″ | 14′ | 11′ 5″ |
| 2x12 | 19′ | 16′ 6″ | 13′ 5″ |
The deck tables in the International Residential Code (IRC R507) are close to these; a building department may ask for the code table, and for a deck outside it, a calculation like this one.
Deck framing questions
Can I use 4x4 posts?
For a low deck with a two-ply beam, often yes: the calculator checks the post's buckling for its height. A three-ply beam is 4.5 in wide, wider than a 4x4, so it gets a 6x6 to bear on. Many building departments ask for 6x6 posts anyway.
How many posts does a deck need?
Enough that the beam spans between them. A bigger beam needs fewer posts; pick a beam in the calculator to see how the post count changes. The deck above, 16′ wide, needs 3 posts under a 3-ply 2x8 beam.
What size beam do I need?
It depends on the joist span and the post spacing. To check one beam on its own, use the wood beam span calculator or open it in the free beam designer from the results above.