Deck Joist Span Calculator

How this is calculated

⚠️ THIS IS A TABLE LOOKUP, NOT A STRUCTURAL DESIGN. The figures come from Table 2 of the American Wood Council's DCA 6, Prescriptive Residential Wood Deck Construction Guide - a free, published document written to be used exactly this way. A prescriptive table is a set of pre-approved answers for ordinary cases. It is not an engineer, and it does not know about your site, your soil, a hot tub, a roof over the deck, or snow. ⚠️ YOUR BUILDING DEPARTMENT HAS THE FINAL SAY, ALWAYS. Decks need a permit and an inspection in most jurisdictions, and local amendments to the code are common. If your inspector's table differs from this one, theirs is the one that counts. Deck collapses hurt people, and they overwhelmingly happen at the LEDGER - the connection to the house - rather than in the middle of a joist. WHAT THE TABLE ASSUMES, and every one of these matters: 40 psf live load and 10 psf dead load, which is the ordinary residential figure. No. 2 grade lumber. Wet service conditions. A deflection limit of L/360. Incising is assumed for Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir; the redwood and cedar group assumes northern species with no incising. If your deck will carry a hot tub, a heavy planter wall, or a roof, none of this applies and you need an engineer. THE SPAN IS MEASURED FACE OF SUPPORT TO FACE OF SUPPORT and does NOT include the overhang. That is a common misreading - people measure the whole deck and conclude they are over the limit, or measure to the beam centreline and conclude they are under it. SPECIES MATTERS MORE THAN PEOPLE EXPECT. A Southern Pine 2x10 at 16 in centres spans 14 ft; the same size in Douglas Fir-Larch, Hem-Fir or SPF spans 13 ft 7 in, and in redwood or western cedar 13 ft 0 in. That is a foot of difference from the sticker on the lumber, at no visible difference in the timber. SPACING IS THE CHEAPEST LEVER YOU HAVE. Going from 24 in centres to 16 in takes that same Southern Pine 2x10 from 11 ft 5 in to 14 ft - two and a half feet of span for a few more joists, which is far cheaper than moving up a lumber size across the whole deck. ⚠️ THE OVERHANG IS NOT ALWAYS LARGER AT CLOSER SPACING, which looks like a misprint and is not. A Southern Pine 2x10 is allowed 3 ft 1 in of overhang at 12 in centres but 3 ft 5 in at 16 in. The AWC has a published FAQ about this. The overhang is governed by more than one criterion - deflection under load and a 220 lb point load at the tip - and which one binds changes with spacing. Read the number off the table rather than reasoning about what it ought to be. AND THE OVERHANG IS ALSO CAPPED AT ONE QUARTER OF THE SPAN, whichever is smaller. On a 12 ft span that is 3 ft, so the quarter-span rule binds before the table figure does. The calculator applies both and reports the smaller. JOISTS ARE PRESCRIPTIVELY LIMITED TO 18 FT in DCA 6, for footing design reasons - which is why the 2x12 entries stop there rather than continuing. A LEDGER MUST BE AT LEAST A 2x8, and so must any joist or rim joist that a guard post attaches to, regardless of what the span table would otherwise allow.

Common questions

How far can a deck joist span?

A Southern Pine 2x10 at 16 in on centre spans 14 ft 0 in, according to Table 2 of the American Wood Council's DCA 6 prescriptive deck guide. The same joist in Douglas Fir-Larch, Hem-Fir or SPF spans 13 ft 7 in, and in redwood or western cedar 13 ft 0 in. The span is measured face of support to face of support and does not include any overhang.

Is this a substitute for a permit?

No, and please do not treat it as one. Decks require a permit and inspection in most jurisdictions, and local amendments to the code are common - if your inspector's table differs from this one, theirs is the one that counts. This is a lookup of a published prescriptive table, which is a set of pre-approved answers for ordinary cases. It is not an engineer. Deck collapses injure people every year, and they overwhelmingly happen at the ledger connection to the house rather than in the middle of a joist.

What does the table assume?

A great deal, and all of it matters: 40 psf live load plus 10 psf dead load, No. 2 grade lumber, wet service conditions, and an L/360 deflection limit. Incising is assumed for Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir; the redwood and cedar group assumes northern species with no incising. If your deck will carry a hot tub, a heavy planter wall, a roof, or significant snow, none of these figures apply and you need an engineer rather than a table.

Where exactly do I measure the span from?

Face of support to face of support - and it does NOT include the overhang. This is misread constantly in both directions. People measure the whole deck including the cantilever and conclude they are over the limit when they are not; or they measure to the centreline of the beam and conclude they are under it when they are not. The overhang is a separate figure with its own separate limit.

Should I go up a size or reduce the spacing?

Reduce the spacing first - it is almost always the cheaper lever. Taking a Southern Pine 2x10 from 24 in centres to 16 in takes the allowable span from 11 ft 5 in to 14 ft 0 in, which is two and a half extra feet for the cost of a few more joists. Moving up a nominal size means buying deeper lumber for the entire deck, and often means deeper beams and a different ledger detail as well. The calculator shows what one size up would give you so you can compare the two directly.

Why is the overhang bigger at wider spacing?

It genuinely is, in some rows, and it looks like a misprint. A Southern Pine 2x10 is allowed 3 ft 1 in of overhang at 12 in centres but 3 ft 5 in at 16 in - the AWC has a published FAQ addressing exactly this question. The overhang is governed by more than one criterion: deflection under distributed load, and a 220 lb point load applied at the tip. Which criterion binds changes with spacing, so the result is not monotonic. Read the figure off the table rather than reasoning about what it ought to be.

Is the overhang limited by anything else?

Yes - it can never exceed one quarter of the actual span, whichever is smaller. On a 12 ft span that is 3 ft, which on most rows binds before the table figure does. The calculator applies both limits and reports the smaller of the two, which is what the guide requires. There is also a prescriptive ceiling of 18 ft on joist length in DCA 6, for footing design reasons, which is why the 2x12 entries stop there instead of continuing.

Does the species really matter that much?

More than most people expect, and it is invisible in the timber. Southern Pine, Douglas Fir-Larch and western cedar look broadly similar on a rack but differ by up to a foot of allowable span at the same size and spacing. Check the grade stamp rather than assuming - and note that the table also assumes No. 2 grade, so a lower grade is not covered by these figures at all. One more rule that overrides everything above: a ledger must be at least a 2x8, as must any joist a guard post attaches to.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Deck Joist Span Calculator at https://www.bfcbrilliance.com/tools/deck-joist-span-calculator.

What I entered:
- Species group: ___
- Joist size: ___
- Joist spacing: ___
- Span you need (ft): ___
- Deck width (ft): ___

What it calculated:
- Maximum allowable span: ___
- Span to spare: ___
- Maximum overhang: ___
- Overhang from the table: ___
- Quarter-span limit: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

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