Deck Framing Calculator

Joists, beam, posts and hangers for a deck frame.

The frame is the part of a deck that keeps people safe — ledger, beam, posts, and joists. It is the most code-scrutinized stage of the build, and the one place not to improvise.

DifficultyAdvanced

TimeA weekend or two for a typical attached deck, plus footing cure time

SkillsSolid carpentry skills, comfortable working level/square and reading a span table

PermitsDecks almost always require a permit and inspections — especially the ledger attachment and footings. Pull the permit before you build; a failed inspection means tearing work out.

Deck ledger detail (drawn after AWC DCA6 Fig. 10). The flashing tucks up behind the house wrap, laps over the top of the ledger, and kicks out a drip edge — this is what keeps water out of the wall. Bolt the ledger to the band joist with ½″ lags or through-bolts and washers; never nail it.

Tools you'll need

Circular saw and a framing square
Drill/driver and a socket set (for ledger lags/bolts)
Post hole digger or auger
4-ft level and a string line
Joist hangers and structural connectors
Flashing tape and Z-flashing for the ledger

Step by step

Set footings and posts Dig footings below the frost line for your area and set posts on them. Center each post on its footing and stand it on a galvanized post base so the wood never sits on the concrete. Footings that are too shallow heave in winter and rack the whole deck.
Deck footing (drawn after AWC DCA6 Fig. 8). The footing must bear on undisturbed soil below the frost line (commonly 42″ min.). Center the post on the footing and stand it on a galvanized post base/anchor so the wood never sits on the concrete.
Attach and flash the ledger Bolt (don't just nail) the ledger to the house rim joist, then flash it with Z-flashing and tape. A poorly flashed ledger is the #1 cause of deck collapses and rot — do this right. If you can't attach a sound ledger — brick veneer, a cantilever or bay window, or a rim you can't verify — build a free-standing deck instead (a beam on posts at the house side, plus diagonal bracing).
Free-standing deck — the alternative when you can’t attach a sound ledger (brick veneer, a rim you can’t verify, a cantilever or bay). A beam on posts replaces the ledger at the house side, and diagonal bracing gives the deck the lateral stability the house would otherwise provide.
Build and set the beam Assemble the beam (often a doubled 2×8/2×10) and set it on the posts level with the ledger height, minus the joist depth. The beam must bear on the post — notch the post so the beam sits on the shoulder and through-bolt it, or use a post cap. Bolting a beam to the side of a post is prohibited.
Post-to-beam connection (after AWC DCA6 Fig. 7). The beam must bear on the post — either notch the post so the beam sits on the shoulder and through-bolt it, or use a post cap. Bolting a beam to the side of the post (no bearing) is prohibited.
Hang the joists Run joists from ledger to beam at 16" on-center, each in a joist hanger at the ledger. Crown each joist up. Use the calculator above for joist count and lumber.
Add blocking and check square Block between joists at mid-span (and over the beam) to stop twisting, then confirm the frame is square and level before decking goes on.

Common mistakes

Nailing the ledger instead of bolting and flashing it

Use structural lag screws or through-bolts and proper flashing. This single detail is what fails in most deck collapses.

Undersized joists for the span

Check a span table: a 2×8 and a 2×10 reach very different distances. When in doubt, go bigger or shorten the span with another beam.

Footings above the frost line

Dig below your local frost depth so winter heave can't lift and rack the frame.

Skipping joist hangers or blocking

Toe-nailing alone isn't enough. Use rated hangers at the ledger and mid-span blocking to keep joists from twisting.

Safety

Call 811 before digging footings to locate buried utilities.
Braces and temporary supports keep the frame from racking until it's fully fastened.
Lift beams with help — assembled beams are heavy and awkward.

Frequently asked questions

What spacing should deck joists be?

16" on-center is standard and required for most decking. Some composite boards installed at an angle call for 12" — check the board maker's instructions.

How far can a deck joist span?

It depends on lumber size, species, and spacing. As a rough guide a 2×8 SPF at 16" OC spans ~12 ft and a 2×10 ~15 ft, but always verify against your local span table.

How deep do deck footings go?

Below the frost line for your region — often 30–48" in cold climates, as little as 12" in warm ones. Your building department publishes the local figure.

How much framing lumber do I need?

Enter your deck length, projection, and joist spacing above — the calculator returns joists, beam, ledger, posts, and hangers.