Two questions decide a deck's framing: how far apart can the joists sit, and how far can they span between supports. Both are answered by AWC DCA 6 (the Prescriptive Residential Wood Deck Construction Guide) and IRC R507. Get either wrong and you've either built a bouncy deck nobody wants to walk on or — far more often — built a deck that fails inspection.

Joist span — the cheat sheet

Allowable spans for 40 psf live + 10 psf dead load (residential decks per IRC), Southern Pine #2 or Douglas Fir-Larch #2, single span between supports:

Joist size    16" oc        12" oc        24" oc
─────────────────────────────────────────────────────
2x6           9'-11"        10'-9"        8'-8"
2x8           13'-1"        14'-2"       11'-5"
2x10          16'-2"        17'-9"       13'-7"
2x12          18'-0"+       18'-0"+      15'-9"

Source: AWC DCA 6 Table 3 + IRC R507.6 (2021 IRC).
SPF #2 spans are roughly 10% shorter — drop one size or
tighten one OC step.

The 2021 IRC retired the old "cantilever ≤ L/4" rule — cantilevers are now governed by the same R507.6 table that limits them to roughly 1/4 of the back-span (with hard ceilings at 4'-0" for 2x10 and 2x12). If your deck has cantilevers, look up the actual table — don't free-hand it.

Joist spacing — the modern call

• 16" oc — the historical default for 5/4 PT or cedar deck boards. Still fine for wood today.
• 12" oc — required for almost all composite (Trex, TimberTech, Fiberon, Azek, MoistureShield) and for any diagonal or herringbone pattern in wood.
• 19.2" oc — almost no one uses it on decks. Saves a few joists; saves you nothing in the joist count once you account for hangers.
• 24" oc — only with engineered boards rated for it (e.g. TimberTech AZEK 24"-oc) and never for stairs.

The ledger — where 90% of deck collapses start

The single most-cited failure mode in deck-collapse investigations is the ledger pulling away from the house. The fix is two-part:

• Flash religiously. Peel-and-stick on the housewrap, butyl over the ledger location, then a metal Z-flashing kicked out at the top. Belt + suspenders, every time.
• Use staggered structural screws, not single-row lag bolts. FastenMaster LedgerLOK or Simpson SDWS at code-required spacing (typically 16–24" oc, staggered top to bottom, with end distances ≥2" from board ends).
• Add lateral load tension ties. IRC R507.9.2 requires Simpson DTT2Z (or equivalent) at minimum two locations along the ledger, attaching to a band of the floor framing inside the house.

Fastener tier — what to put where

• Ledger to band joist: FastenMaster LedgerLOK 3-5/8" or Simpson SDWS Timber 1/4" × 5" (code-listed, IAPMO ER-192). Staggered pattern.
• Joist hangers: Simpson ZMAX (G185 galvanized) for above-grade work. Stainless for any bracket touching soil. Use Simpson SD9 or SDS structural-connector screws — every hole filled. Generic deck screws void the load rating.
• Built-up beams + LVL: FastenMaster HeadLOK 3-1/4" or larger. ~900 lb shear, low-profile head, no through-bolts.
• Post bases: Simpson ABU66Z or ABA66 with a 1" standoff above the pier. Never bury the bracket — even "galvanized" rusts through in 3–5 years in soil.
• Decking face screws: Stainless for hardwoods like ipe and cumaru; ceramic-coated deck screws for treated and cedar. Composite uses hidden fasteners (CAMO Edge for grooved boards, Cortex for face-screwed) — never raw deck screws on composite.

Joist tape — the cheap insurance

The single most cost-effective thing you can do on a new deck didn't exist 20 years ago. Roll butyl joist tape across the top of every joist, every beam, and the rim before laying decking. Trex Protect, G-Tape, CAMO Joist + Ledger Tape, or Vycor — they're all the same butyl chemistry. About $30 for an average residential deck. It keeps water from sitting in the screw holes and slowly rotting the joist tops from above.

Worked example: 14 × 20 deck off the back of the house

Plan:          Composite decking, 14 ft from house × 20 ft long
Joists:        Span 14 ft from ledger to a single beam at the far edge
Loading:       40 psf live + 10 psf dead (residential)
Decking:       12" oc required (Trex Enhance Naturals)

From DCA 6 Table 3:
  - 2x10 SPF #2 @ 12" oc spans 16'-9" (single)
  - 2x10 SYP #2 @ 12" oc spans 17'-9" (single)
  ✓ Both work for the 14 ft clear span.

Joist count (12" oc, 20 ft long):
  20 × 12 + 1 = 21 joists
  (one per foot plus the closer at each end)

Ledger fasteners (LedgerLOK, staggered top + bottom @ 16" oc):
  20 ft × 12 / 16 × 2 = 30 LedgerLOK screws
  Add 4 more for the doubled ends → 34 total

Lateral load ties:
  2× Simpson DTT2Z (one at each third-point along the ledger).

Joist tape:
  21 joists × 14 ft + the beam top + the rim = ~330 lf of tape.
  One 75 ft roll covers about 4–5 joists (overlap + waste);
  buy 4 rolls.

Common mistakes

• Sizing the joist for wood spacing, then deciding to upgrade to composite. Composite needs 12" oc — you'll either waste joists or need to rip out and re-frame.
• Cantilevering past the new R507.6 limits. The L/4 rule was retired in the 2021 IRC; modern limits are tighter and depend on joist size.
• Single-row 3/8" lag bolts on the ledger. Splits the rim, pulls loose under the year-five spring thaw. Stagger LedgerLOK or SDWS instead.
• Skipping the lateral load tension ties. R507.9.2 inspection failure, every time.
• Using "galvanized" hardware in soil contact. ACQ-treated lumber eats standard galvanized. Stainless is required where the bracket touches dirt.
• Buried 6×6 posts "because grandpa did it." Wood + soil = rot in 5–8 years. Use a Simpson ABU standoff base on top of a Bigfoot footing.

Run the numbers on your phone

The BuilderCalc app handles joist spacing for span and load (live + dead aware), the deck materials takeoff (boards, joists, fasteners, with the joist tape called out), and the concrete pier calc for footings — all offline, free on the App Store and Google Play. The Build a deck project guide walks through ledger flashing, post bases, joist tape, and the modern fastener picks step by step.

Related

• BuilderCalc — DIY project guide + 14 calculators
• How much concrete for a slab, footing, or pier

Note: Spans and fastener specs above are starting points pulled from AWC DCA 6 and the 2021 IRC. Always verify against your local code, your inspector's notes, and the actual lumber grade and species you're using. Cantilevers, snow loads, multi-span joists, and elevated decks (over 30") have additional rules not covered here.