Deck Calculator

Deck boards, joists, ledger, beams, posts, footings, hardware, railing and stairs for a rectangular attached or freestanding deck, with a joist-span check. Change any input and the framing plan redraws.

Building a deck off the back of a house A deck under construction against a house wall: a flashed ledger with lag screws and joist hangers, joists at 16 inches on centre resting on a three-ply beam, 6x6 posts on round concrete piers and a rim joist at the front. Composite deck boards are laid from the rim toward the house; a new board drops into place and an impact driver screws it down at every joist. A stack of boards with a circular saw and a speed square sits in the foreground.

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Deck framing reference

JOIST SPAN≤ ¼ SPANDEPTHZ-FLASHING OVER LEDGER, TAPE BEHIND½″ LAGS OR THROUGH-BOLTS, STAGGEREDJOIST HANGER AT THE LEDGERDECKING, ⅛–3/16″ GAPGUARD POST, 2 BOLTSHURRICANE TIE3-PLY BEAM, NOTCH OR CAP6x6 POST ON STANDOFF BASEPIER 2–4″ ABOVE GRADEFROST LINE
Typical attached-deck section, not to scale. Span values are typical IRC table figures (40 psf live, 10 psf dead, wet service); check your local code and the species you buy.

Joist span, southern pine No. 2

Spacing
12″ · 16″ · 24″ o.c.
2x6
9′11″ · 9′0″ · 7′7″
2x8
13′1″ · 11′10″ · 9′8″
2x10
16′2″ · 14′0″ · 11′5″
2x12
18′0″ · 16′6″ · 13′6″
Overhang past beam
≤ ¼ of the span

Ledger and beam

Ledger fasteners
½″ lags or through-bolts
Pattern
2 rows, staggered
Fastened into
house rim joist
Never onto
brick veneer, siding
Flashing
tape + Z-flashing
Beam splices
over a post
Joist to beam
hurricane tie each

Posts and footings

Posts
6x6, ground contact
Footing bottom
below frost line
Footing min.
12″ dia., 6″ thick
Soil, untested
1,500 psf
Design load
40 live + 10 dead psf
Post base
standoff, 1″ above pier
80 lb bag
0.6 ft³

Decking, guards, stairs

Composite joists
16″ o.c., 12″ diagonal
5/4 wood
16″ o.c.
2x6 wood
24″ o.c.
Guard required
> 30″ above grade
Guard height
36″ min.
Openings
< 4″ sphere
Riser · tread
≤ 7¾″ · ≥ 10″
Handrail
4 or more risers

Safety Call 811 before you dig. Most deck collapses start at the ledger: bolt it to the house rim joist, flash it, and never hang it on brick veneer or siding. Wear eye protection with the circular saw and impact driver.

Full notes
  • Joist span is measured from the face of the ledger to the beam, not to the outer rim. The overhang past the beam should be no more than a quarter of that span.
  • Never fasten a ledger to brick veneer, stucco over foam, a cantilevered floor or through siding. Remove the siding, flash the rim board and bolt into the house rim joist, or build a freestanding deck.
  • Footing size depends on the load and the soil: the calculator assumes 50 psf (40 live + 10 dead) and 1,500 psf soil, which is the IRC default when the soil is not tested.
  • Use ground-contact pressure-treated lumber for posts and any framing near the soil, and hot-dip galvanized or stainless fasteners and connectors rated for treated wood.
  • Composite decking usually needs joists at 16 in o.c. or closer (12 in for diagonal boards and stair treads). Read the gap and fastener rules for the exact product.
  • Most US towns require a permit for a deck that is attached to the house, higher than 30 in or larger than 200 sq ft. Call 811 before you dig the footings.

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