Deck joist and beam spans, explained
A joist span is the distance a deck joist can safely reach between its supports — from the ledger board at the house to the beam — before it sags or fails under load. A beam span is the distance a beam can carry between posts. A cantilever (or overhang) is the short length a joist is allowed to project past the beam. Get any of the three wrong and the deck feels bouncy at best, or is unsafe at worst.
This calculator answers all three from the same handful of inputs: your lumber species, joist size and on-center spacing. It reads the maximum allowable joist span, the largest cantilever overhang, and the beam size and post spacing straight from the DCA 6 / IRC R507 prescriptive span tables — the same tables a building inspector checks against — so you can plan framing that will pass.
How this calculator works
Pick your framing and read the results live. Every value updates as you change an input, in whichever unit system you choose.
From inputs to a framing plan
- Choose your unitsSwitch between imperial (feet-inches, psf) and metric (metres, kPa). Every span, load and result reflows into the system you pick.
- Set species, size and spacingChoose your lumber species, the joist size (2x6 through 2x12) and the on-center spacing (12, 16 or 24 inches). These three inputs select the table row.
- Read the joist span & cantileverThe maximum allowable joist span and the largest cantilever overhang appear instantly, each tagged as a verified table value or a labelled estimate.
- Size the beam & postsOpen the Beam & posts panel to get the maximum post spacing for a chosen beam, or solve for the smallest beam that reaches a target post spacing.
- Check a planned spanEnter a span you're considering to get a pass/fail verdict against the allowable, with the margin and the binding limit named.
The parts of a deck frame
A ledger-attached deck hangs off the house on one side and rests on a beam carried by posts and footings on the other. Joists run from the ledger to the beam; the beam runs between the posts; the posts carry the load down to concrete footings. The short overhang past the beam is the cantilever.
Each part has its own span limit, and they interact: a longer joist span puts more load on the beam, which then needs to be bigger or carried on more closely-spaced posts.
What drives the span
Four things set how far a joist can reach. Stronger, stiffer species span further — Southern Pine is the strongest group here, and the western cedars and redwood the weakest. A deeper joist — a 2x10 versus a 2x8 — spans much further, because depth drives stiffness. Wider on-center spacing means each joist carries more floor, so it can't reach as far. And heavier load shortens every span.
The published tables are built for one standard case: No. 2 grade lumber, 40 psf live load plus 10 psf dead load, wet-service conditions, and an L/360 deflection limit (the joist may sag no more than its span divided by 360). At that standard, this tool returns the exact table value. Change the load or the deflection limit in the Assumptions panel and it returns a clearly-labelled, deliberately conservative estimate instead — it never quietly interpolates a number the tables don't publish.
Beams and tributary width
A beam doesn't carry the whole deck — it carries the strip of floor that drains onto it. For a ledger-attached deck, roughly half of each joist's load goes back to the ledger and half comes forward to the beam, so the beam's load width (its tributary width) is about half the joist span. That is why the beam table is keyed to the joist span it supports: double the joist span and you roughly double the load the beam carries, so it needs more depth, more plies, or more posts.
The cantilever (overhang) rule
Joists may cantilever a short distance past the beam, but the overhang is capped. The prescriptive limit is the lesser of the tabulated overhang for your species, size and spacing, and one quarter of the actual backspan (the ledger-to-beam distance). So a deck with a 12-foot backspan can overhang at most about 3 feet — and often less, once the table limit is applied. This calculator enforces both caps automatically and reports the smaller one.
Beam sizing and species
Joist spans, cantilevers and span checks in this tool cover all of the listed species groups. Beam sizing is different: this release ships the verified beam table for Southern Pine only. Southern Pine is the strongest of the groups, so its beam spans would overstate what a weaker species can safely carry — publishing them for another species would be unsafe.
If you pick a non-Southern-Pine species, the beam panel says so plainly and shows no number rather than a misleading one. Size that beam directly from IRC Table R507.5 / DCA 6, or frame the beam in Southern Pine.
This is a planning tool, not a stamped design
The prescriptive tables cover ordinary, uniformly-loaded residential decks. Anything outside that — a hot tub, a heavy planter wall, a roof over the deck, unusual snow, or a freestanding (two-beam) frame — falls outside their scope and needs its own design.
Frequently asked questions
How far can a 2x8 deck joist span?
It depends on species and spacing. A Southern Pine 2x8 at 16 inches on center spans up to 11 ft 10 in under standard residential load; at 12 inches on center it reaches 13 ft 1 in, and at 24 inches on center 9 ft 8 in. Weaker species span less — a redwood or western-cedar 2x8 at 16 inches on center reaches about 10 ft 7 in. Pick your exact species and spacing above to read the value.
What size beam do I need for a 12-foot deck?
For a deck whose joists span 12 feet, a Southern Pine built-up beam of three 2x10s spans about 9 ft 2 in between posts, while a double 2x12 spans about 8 ft 7 in. Enter the joist span supported and either choose a beam to see its post spacing, or let the tool solve for the smallest beam that reaches your target post spacing. Beam sizing is verified for Southern Pine in this release.
How do I check whether my planned span is OK?
Open Check my span and enter the joist span (and optional overhang) you're considering. The tool compares it to the allowable and returns a pass or fail, the margin as a percentage, and which limit governs. A 12-foot span against an 11 ft 10 in allowable, for example, fails by 2 inches.
Which lumber species can I choose?
The tables group species by strength: Southern Pine; Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir together; and Redwood, Western Cedars, Ponderosa Pine and Red Pine together. All are No. 2 grade, the standard deck-framing grade. Joist spans and cantilevers cover every group; beam sizing currently covers Southern Pine only.
Why does my span change when I edit the load?
The published tables assume 40 psf live plus 10 psf dead load. When you enter a different load or a stiffer deflection limit in the Assumptions panel, the span is no longer a table value, so the tool scales it by a provably-conservative factor and labels it an estimate. Heavier loads always shorten the span, and the estimate is never longer than the table would allow.
How far can deck joists cantilever past the beam?
The overhang is limited to the lesser of the tabulated cantilever for your species, size and spacing, and one quarter of the backspan (the ledger-to-beam distance). For a 12-foot backspan that is at most about 3 feet, and often less. The calculator applies both caps and shows the smaller.
Does this replace an engineer or the building code?
No. It reads recognised prescriptive tables to help you plan, but it is not a stamped structural design. Local amendments, site conditions and any non-standard load can change the answer. Always confirm with your building department, and use a licensed design professional for anything outside standard prescriptive conditions.
Is the calculator private?
Yes. Everything is computed in your browser — nothing you enter is sent anywhere or stored on a server.