Deck code basis: the source, the load model, and where it stops

AWC DCA 6-2015, © 2018 American Wood Council, published free by AWC. Here is the exact file, so you can pull the same one and check any cell we print.

The tables are shipped in our engine as static, transcribed data reviewed against that PDF. The engine does not compute spans from stress equations and then present the result as code — where the guide gives a value, we use the value; where it does not, we say so.

Because 2015 is the edition that carries the reduced Southern Pine design values. In 2013 the Southern Pine Inspection Bureau lowered the published design values for Southern Pine, and span tables printed before that change give longer spans for the same joist.

The tell is one cell. In DCA6-2015, a Southern Pine 2x10 at 16" on center spans 14'-0" — the post-2013 number, and the one our engine returns. The 2012 edition predates the reduction and publishes a longer span for that same joist. We deliberately did not source from it. If you are checking any deck span table against ours, that cell will tell you within seconds which side of 2013 it came from. DCA6-2015 Table 2, p.4

The IRC itself is published on a three-year cycle — 2015, 2018, 2021, 2024 — and jurisdictions adopt on their own schedule, with local amendments, so the newest edition in print is frequently not the one in force where you are building. A tool that names its edition, table and page can be checked against the printed page. A tool that advertises compliance with an edition, but never names a table or a page, cannot be checked at all — and an edition year that is not on the cycle above is a claim about a document that does not exist.

The tables behind joists, beams, posts and footings — 2, 3A and 4 — are published at 50 psf total — 40 psf live load plus 10 psf dead load — for No. 2 grade lumber in wet service conditions, with the joist and beam spans held to L/360 deflection. Table 5 prints the same 40 + 10 loads for the ledger connection, and Table 6 sizes stair treads against a 300 lb concentrated load at L/288. Change any one of a table’s assumptions and that table no longer applies.

• Design load: 40 psf live + 10 psf dead = 50 psf. Snow loads, snow drift loads or sliding snow loads over 40 psf, concentrated loads, hot tubs, planters, outdoor kitchens and roofs are not in this model.

  DCA6-2015 Table 2 fn.1 / Table 3A fn.1 / Table 4 fn.1 / Table 5 header; p.2 item 9

• Grade and service: No. 2 grade, wet service conditions. The published values already account for exterior exposure — do not derate them again.

  DCA6-2015 Table 2 fn.1, p.4

• Deflection: L/360 for joists and simple-span beams; cantilever length/180 with a 220 lb point load for overhangs.

  DCA6-2015 Table 2 fn.2–3 / Table 3A fn.1

• Incising is assumed for Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir. The tabulated spans already embody it.

  DCA6-2015 Table 2 fn.4, p.4

• Footings: 1,500 psf assumed soil bearing capacity, 150 pcf concrete, 2,500 psi compressive strength. Corner post values may be multiplied by 0.9. Soil that bears less than the assumption needs a bigger footing than the table gives.

  DCA6-2015 Table 4 fn.2 + fn.4, p.12

• Posts: the prescriptive minimum deck post is 6x6 nominal, not 4x4. An 8x8 may substitute anywhere to a maximum height of 14 feet.

  DCA6-2015 p.10 + Table 4 fn.5, p.12

Southern Pine reads its own column. Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir share a single span column in Tables 2 and 3A — so a Douglas Fir-Larch design and a Hem-Fir design return the same span from us.

That is the guide’s own grouping, not a shortcut of ours, and the shared column is the governing minimum of the group — which makes it the conservative read for any of the three. Table 4 is the exception: for post heights the guide splits the group into three separate columns. Douglas Fir-Larch has its own. Hem-Fir shares a lower one with Western Cedars. Spruce-Pine-Fir shares the lowest with Ponderosa Pine and Red Pine, where 4 combinations are marked Not Permitted outright. Our engine reads each species’ own Table 4 column, so a Hem-Fir or Spruce-Pine-Fir post is never checked against the Douglas Fir-Larch column, which allows a taller post than the guide does for either. DCA6-2015 Table 4, p.12 Species outside those four are outside our engine, not outside the guide: the printed tables do cover cedar, redwood and the other northern species, but no page here publishes a span for them. And any grade other than No. 2 is outside the tables’ stated basis.

Ten places in one 44-page document produce every number this site reads from the guide. The stair rise, run and uniformity limits are the exception — structural numbers cited to the IRC here, not to a DCA 6 table.

One caution worth carrying: DCA6-2015 renumbered its tables relative to older editions. Table 1 here is the species list, not the joist spans. If a source cites “DCA 6 Table 1” for a joist span, it is quoting an older edition.

Three of those rows are republished elsewhere on this site, cell for cell out of the same engine — for the columns the engine carries. Table 2 is on the deck joist span chart, with a block per nominal size — its Southern Pine and grouped Douglas Fir-Larch / Hem-Fir / Spruce-Pine-Fir columns, not the printed page’s third species column. Tables 3A and 4 are on the beam and footing calculator linked below, for the species the engine carries. The stair rules on p.20 are DCA 6 body text rather than a table, and some of them cite the IRC by section while others — the three-stringer minimum, the guard-post requirements — are the guide’s own. The deck stair calculator screens a flight against the IRC’s rise, run and uniformity limits — it does not republish Table 6.

These are the boundaries our engine detects and refuses to guess past. Each one is a place the guide simply stops publishing values, so past it we stop publishing one too — and we name the footnote that ended the road rather than extending it.

A deck needs a licensed design professional whenever it falls outside the prescriptive tables — and the tables are narrower than most people expect.

• →A joist span longer than 18'-0", or a span the table cannot cover even at 2x12.
• →An interior beam line with unequal joist spans on its two sides, or equal spans whose doubled length passes 18'-0". The Commentary sizes a two-sided interior beam, and its posts and footings, at twice the joist span (p.C5).
• →A post and footing combination the guide marks Not Permitted, or a post taller than its tabulated maximum.
• →A ledger on brick or stone veneer, hollow masonry, or a cantilevered floor overhang.
• →Any load the tables do not assume: a hot tub, a planter wall, an outdoor kitchen, a roof structure, or snow, snow drift or sliding snow loads over 40 psf (p.2, item 9).
• →A deck the house does not brace laterally, a deck of more than one level, or a deck deeper (away from the house) than it is wide. The guide covers single-level decks attached to the house, no deeper than wide (p.2, items 1 and 2).
• →Species or grade outside Southern Pine, Douglas Fir-Larch, Hem-Fir or Spruce-Pine-Fir at No. 2 grade.
• →Soil that bears less than the assumed 1,500 psf, or any footing condition your soil report contradicts.
• →Anything your building official asks to see stamped — which they may, even on a deck that passes every table.

Passing every table is not the same as being approved. The prescriptive method is a safe-harbour path within one code edition at one load basis; the code your jurisdiction has adopted, its local amendments, and the official reviewing your permit set are what actually govern.

When does a deck need an engineer?

A deck needs a licensed design professional whenever it falls outside the prescriptive tables. In practice that means: a joist span over 18'-0"; a required span longer than the table allows even at 2x12; an interior beam line carrying unequal joist spans from its two sides, or equal spans whose doubled length passes 18'-0" (the AWC DCA 6-2015 Commentary sizes an interior beam at twice the joist span, p.C5); a post-and-footing combination the guide marks Not Permitted; a ledger on brick or stone veneer, hollow masonry, or a cantilevered floor overhang; and any load the tables do not assume — a hot tub, a planter, an outdoor kitchen, a roof, or snow, snow drift or sliding snow loads over 40 psf (p.2, item 9). The guide itself also only covers single-level decks attached to the house to resist lateral forces (p.2, item 1), and decks no deeper than they are wide (p.2, item 2). Your Authority Having Jurisdiction can also require a stamped design for a deck that passes every table.

What are deck span tables based on?

The deck span tables in common use come from AWC DCA 6, the American Wood Council's Prescriptive Residential Wood Deck Construction Guide, which is written against a specific edition of the International Residential Code. We publish from DCA6-2015, based on the 2015 IRC, © 2018 AWC. Its tables assume 40 psf live load and 10 psf dead load, No. 2 grade lumber, wet service conditions, and L/360 deflection.

What load do deck span tables assume?

40 psf live load plus 10 psf dead load — 50 psf total — is the stated load basis of every DCA6-2015 table we use (Table 2 fn.1, Table 3A fn.1, Table 4 fn.1, and the Table 5 header). Snow loads, snow drift loads or sliding snow loads over 40 psf (DCA6-2015 p.2, item 9), concentrated loads, hot tubs, planters, and roofs are not in that model. A deck carrying any of them is outside the prescriptive tables.

Which edition of DCA 6 should I use?

Use an edition published after 2013, because the Southern Pine design values were reduced by the Southern Pine Inspection Bureau in 2013 and older tables publish longer spans for the same joist. DCA6-2015 carries the reduced values: Southern Pine 2x10 at 16" on center spans 14'-0". We deliberately did not source from the 2012 edition, which predates the reduction. Whichever edition you use, your jurisdiction adopts a specific code edition and its amendments — that adoption governs, not the guide.

Is a deck designed from span tables code compliant?

Meeting a prescriptive table is a screen, not a certification. The tables are a safe-harbour path within one code edition, at one load basis, for one grade and service condition. Compliance is determined by the code your jurisdiction has adopted, its local amendments, and the building official who reviews the permit set. Our tools screen a design against the DCA6-2015 / IRC prescriptive baseline and tell you where it falls outside; they do not produce stamped engineering.

Why cite an older code edition instead of the newest one?

Because the edition is the fact, and a specific dated source is more useful than a vague newer-sounding one. The IRC is published on a three-year cycle — 2015, 2018, 2021, 2024 — and jurisdictions adopt on their own schedule, so the newest edition is very often not the one in force where you are building. Naming the edition, the table, and the page lets you check our number against the printed page. A compliance claim that names no edition, table, or page cannot be checked at all.