How the framing is counted
The deck is a ledger along the house (or a second beam, for a freestanding deck), joists running out from it at the spacing you choose, and a beam on posts under the joists, which may overhang it to the rim. The projection is measured from the house wall to the outside of the rim, so each joist is 3 in shorter: the 1½ in ledger and the 1½ in rim.
The outer joists sit flush with the deck’s ends, so the spacing is counted from the first joist’s center, ¾ in in from the end, to the last one’s. Rounding up keeps every space at or under the spacing you chose; the last space may come out shorter.
Worked example: a 16 × 12 ft deck on a ledger
- Joists: 2×8 at 16 in on center. (192 − 1.5) ÷ 16 = 11.9, so 12 spaces and 13 joists, each 144 − 3 = 141 in (11′ 9″), from 12 ft boards.
- Span: with a 2 ft overhang, the joists span 9′ 10½″ from the ledger to the beam’s center.
- Ledger and rim: one 16 ft 2×8 each.
- Beam: two plies of 2×10, 16 ft, on three posts 7 ft apart, the beam running 1 ft past each end post.
- Posts: three 6×6 posts 4 ft tall, cut from two 8 ft posts.
- Blocking over the beam, where the joists overhang it: 12 pieces 14½ in long, from two 8 ft 2×8s.
- Hardware: 13 LUS28 hangers with 130 nails, 13 H2.5A ties with 130 nails, 39 rim nails, 26 beam nails, and three post bases and caps.
Choosing the sizes
The calculator doesn’t pick joist, beam or post sizes: it takes yours and lists what they add up to. Choose them from the span tables first:
- Joists: IRC Table R507.6 or AWC’s DCA 6 give the span for each size, spacing and species. AWC’s free span calculator works out joist spans from the design values in the NDS; it covers joists and rafters only.
- Beams: IRC Tables R507.5(1) to R507.5(4) give the span between posts for each beam size and joist span. A beam may overhang its end posts by up to a quarter of that span.
- Posts: IRC Table R507.4 gives post sizes by height; DCA 6 calls for 6×6 posts.
- Overhang: joists may overhang the beam by up to a quarter of their span, less where DCA 6’s table says so.
DCA 6 also limits joists to 18 ft long, doesn’t let joists frame into opposite sides of the same beam, and covers only decks attached to a house. The calculator flags joists over 18 ft and freestanding decks.
Hangers, ties and nails
| Connection | What the list counts | From |
|---|---|---|
| Joist to ledger | A face-mount hanger per joist: LUS26, LUS28 or LUS210 with the maker’s 10d nails | ESR-2552 |
| Joist to beam | A tie rated at least 100 lb per joist: an H2.5A with ten 0.131 × 2½ in nails | Fairfax County, DCA 6, ESR-2613 |
| Rim to joists | Three 10d nails (or three #10 × 3 in screws) into each joist end | Fairfax County |
| Beam plies | Two rows of 10d (3 × 0.128 in) nails at 16 in along the beam | IRC R507.5 |
| Post to footing and beam | A post base and a post cap per post | IRC R507.4.1, DCA 6 |
Simpson’s single-width LUS series stops at 2×10, so for 2×12 joists the list gives the count and the rating DCA 6 asks for, at least 700 lb, rather than a model. Hangers and blocking should be at least 60% of the joist’s depth (Fairfax County). Blocking goes between the joists over the beam wherever they overhang it; a row at mid-span is optional.
Fasteners in treated lumber
Nails for decks are hot-dip galvanized to ASTM A153 Class D or ASTM A641 Class 3S, or stainless steel (IRC Table R507.2.3).
The ledger calculator spaces the lag screws or bolts that hold the ledger to the house, and the footing calculator sizes the footings under the posts from the same deck layout.
Questions people ask
How many joists does a 12 × 16 ft deck need?
It depends which way the joists run. With the 16 ft side along the house and joists 16 in on center, there are 13: (192 − 1½) ÷ 16 = 11.9, rounded up to 12 spaces. With the 12 ft side along the house, there are 10, which is Fortress Building Products’ count for the same deck: 8 joists between the two outer ones.
How far can deck joists overhang the beam?
AWC’s DCA 6 allows a quarter of the joist’s span, or less where its table says so for the size and spacing. Joists spanning 10 ft can overhang the beam by up to 2½ ft. The calculator warns when the overhang is more than a quarter of the span.
How far can a deck beam extend past its posts?
The IRC lets a beam cantilever past its end posts by up to a quarter of its span between posts (R507.5). With posts 7 ft apart, that’s 21 in. Beam plies are spliced only over posts, and the beam sits on top of the posts, notched in or held by a post cap.
Do deck joists need hurricane ties?
Fairfax County’s deck details and AWC’s DCA 6 ask for a connector rated at least 100 lb at each joist where it crosses a beam, holding it down and sideways. A Simpson H2.5A tie is one; it takes five 0.131 × 2½ in nails into the joist and five into the beam.
How many nails go in a joist hanger?
As many as the maker lists. For Simpson’s LUS hangers that’s 4 nails into the ledger and 4 into the joist for a 2×6 (LUS26), 6 and 4 for a 2×8 (LUS28), and 8 and 4 for a 2×10 (LUS210), all 10d. AWC’s DCA 6 lists the least a hanger must carry: 400 lb for a 2×6 joist, up to 700 lb for a 2×12.
What lengths does deck lumber come in?
Southern pine dimension lumber is made in 8 to 20 ft lengths in two-foot steps. The calculator cuts each joist from the shortest board it fits: a 12 ft projection makes 141 in joists, from 12 ft boards. Rim and ledger boards over 20 ft are split into equal pieces, and beam plies are spliced over posts.
Sources for the numbers on this page
- California Building Standards Commission, via Codebook — 2025 California Residential Code, section R507: exterior decks (2024 IRC text; footings, posts, beams) · checked October 4, 2026
- American Wood Council — DCA 6: Prescriptive residential wood deck construction guide (2015 IRC edition) · checked October 3, 2026
- Fairfax County, Virginia — Typical deck details, version 2021.0 (1/5/2024, PDF) · checked October 4, 2026
- Pennsylvania Housing Research Center, Penn State — Decked out: deck requirements of the 2021 IRC, webinar handout (January 2025, PDF) · checked October 4, 2026
- ICC Evaluation Service — ESR-2552: Simpson Strong-Tie face-mount hangers, LUS series fasteners (reissued April 2026, PDF) · checked October 4, 2026
- ICC Evaluation Service — ESR-2613: Simpson Strong-Tie hurricane ties, H2.5A fasteners (reissued June 2026, PDF) · checked October 4, 2026
- American Wood Council — Span options calculator for wood joists and rafters (free; NDS 2018) · checked October 4, 2026
- Southern Forest Products Association — Southern Pine specification guidelines (standard lengths, 8 to 20 ft in two-foot steps) · checked October 4, 2026
- Fortress Building Products — How do you calculate deck framing? (October 2022) · checked October 4, 2026
Corrections and updates are logged on the sources page.