A deck is a stack of simple decisions wearing the costume of one big project. The surface boards sit on joists, the joists sit on a beam, the beam sits on posts, and the posts sit on footings in the ground. Every board in that stack answers a question you can compute: how far a joist can span, how many joists fit along the house, how far apart posts can stand under the beam, and how many deck boards cover the square footage you just framed. Get each answer from its table and its formula, and the lumber order writes itself.
What the order won't forgive is guesswork in the two directions. Joists running the wrong way, or a width that quietly exceeds the span a 2x8 is allowed to carry, turns a plan into a re-order — and pressure-treated lumber bought in two trips is lumber bought at two prices. This guide sizes every member of a basic rectangular deck using the same tables your building department uses, works one 20 × 12 example from footprint to screw count, and the deck lumber calculator runs the whole takeoff live as you type.
A 20 × 12 ft attached deck, 2×10 joists at 16 in on center and a doubled 2×10 beam, needs about 29 framing boards: 16 joists (18 boards with 10% waste), a 20-ft ledger and rim (2 boards each), 2 end joists (3 boards), and a 2-ply beam (4 boards) on 4 posts at 6 ft 8 in spacing. The surface takes 35 deck boards at 16 ft (5.5-in face + 1/4-in gap, 10% waste) and about 832 screws — two at every joist crossing.
Deck Framing from First Principles
Five members, one load path. The decking carries your feet to the joists. The joists are the closely spaced beams running perpendicular to the house — each one hands its share of the load to the supports at both ends. On an attached deck, the joist's house end hangs in a metal joist hanger nailed to the ledger, the board bolted to the house framing; the far end sits on the beam, which is usually two 2× boards nailed together (a "2-ply" or "doubled" beam). The beam concentrates a whole strip of joists onto posts, and each post ends in a footing of concrete below frost depth. A rim joist closes the far end of the joist run, and end joists close the two sides.
A freestanding deck swaps the ledger for a second beam and its own posts at the house side — same members, one more line of them, and no dependence on the house's framing for support. Either way, the takeoff is the same five questions: how far can the joists span, how many are there, how long are the rim and ledger, how much beam, and how many posts can the beam ask for. The order in which you answer them matters, because joist span drives the beam, and the beam drives the posts.
The rules below come from the prescriptive tables of the International Residential Code — the span figures your inspector will check. They assume Southern Pine No. 2, the benchmark pressure-treated framing lumber sold across most of the country, carrying the code's standard 40 psf of people plus 10 psf of deck weight. Douglas Fir-Larch and Hem-Fir, common in the West, span slightly less at the same size, and cedar or redwood framing less still: when in doubt, pull the species off the grade stamp on the board and use the shorter number.
Step 1 — Size the Joists Before You Count Them
Joist size answers to span — the clear distance between supports, ledger to beam — and to spacing, and the code table prices the trade exactly. At 16 in on center, the workhorse spacing:
| Joist size | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 2×6 | 9 ft 11 in | 9 ft 0 in | 7 ft 7 in |
| 2×8 | 13 ft 1 in | 11 ft 10 in | 9 ft 8 in |
| 2×10 | 16 ft 2 in | 14 ft 0 in | 11 ft 5 in |
| 2×12 | 18 ft 0 in | 16 ft 6 in | 13 ft 6 in |
IRC Table R507.6, Southern Pine No. 2, 40 psf live + 10 psf dead. Span is the clear distance between supports, not total board length.
Read it as a menu, not a challenge. The 20 × 12 example spans 12 ft, so 2×10s at 16 in carry it with 2 ft of headroom; 2×8s at 16 in top out at 11 ft 10 in and would be over span by 2 in — an inspector's red tag, not a judgment call. When a deck is just short, the three honest fixes are upsizing the lumber, tightening the spacing (which buys span — the same 2×8 reaches 13 ft 1 in at 12 in o.c.), or adding a mid-span beam that splits the span in two. Sometimes the extra post is cheaper than the bigger joists; price all three.
Two footnotes live under this table. A joist may cantilever — run past the beam — but only by one quarter of its actual back-span: a joist bearing 12 ft between ledger and beam may overhang 3 ft beyond the beam, no more. And the table assumes decking laid perpendicular to the joists; diagonal or picture-frame patterns need the joists tightened to 12 in o.c. under most products, because a board crossing the framing at 45 degrees spans farther between joists.
Step 2 — Count the Joists
Joists are distributed along the dimension parallel to the house, and the count is spacing arithmetic, not guesswork: divide the deck length by the on-center spacing, round up to a whole number of spaces, and add one joist for the far end of the first space. In inches, for a 20-ft deck at 16 in on center: 240 ÷ 16 = 15 spaces, plus one = 16 joists. The last space is allowed to run a little short when the length doesn't divide evenly — the rim joist closes whatever remains. Deck length that lands on an even multiple (12 ft at 16 in, 12 ft at 24 in, 18 ft at 12 in) divides perfectly and hides no remainder at all.
Each joist runs the full span, so joist length equals the deck width — 12 ft boards for the example deck. The takeoff line is therefore 16 joists × 12 ft = 192 lineal feet, and at a 12-ft stock length with 10% cut waste that is ceil(192 × 1.1 ÷ 12) = 18 boards. Buy the stock length that minimizes offcuts: for a 12-ft joist, 12-ft stock is exact; for a 13-ft span you're into 14s or 16s, and the calculator prices whichever length you set.
Step 3 — Ledger, Rim, End Joists and the Beam
The perimeter members close the frame. On an attached deck, the ledger runs the deck's length against the house (20 ft — two 12-ft boards with the splice landed on blocking), the rim joist closes the far end over the beam (another 20 ft, 2 boards), and two end joists close the sides (2 × 12 ft, 3 boards with waste). A freestanding deck replaces the ledger with a second rim and a second beam. One hanger per joist at the ledger — 16 for the example — belongs on the hardware list, not the lumber list, and the ledger's lag screws or structural screws are specified separately from the joist count.
The beam is where decks earn their posts. A doubled 2×10 beam spanning between posts can carry much less than a 2×10 joist can, because it gathers load from the whole joist span behind it — which is why the beam table is read by the joist span it supports:
| 2-ply beam (SP No. 2) | 6-ft joists | 8-ft joists | 10-ft joists | 12-ft joists |
|---|---|---|---|---|
| 2×8 | 7 ft 7 in | 6 ft 8 in | 6 ft 0 in | 5 ft 6 in |
| 2×10 | 9 ft 8 in | 8 ft 6 in | 7 ft 7 in | 6 ft 11 in |
| 2×12 | 11 ft 9 in | 10 ft 3 in | 9 ft 3 in | 8 ft 5 in |
IRC Table R507.5, 2-ply Southern Pine No. 2. Maximum post spacing (center to center) by the joist span the beam carries; a beam may cantilever past its end post by up to one quarter of the adjacent span.
The example deck's 12-ft joist span sends a 2-ply 2×10 beam to the table's 12-ft column: posts no more than 6 ft 11 in apart. A 20-ft beam therefore needs 3 spaces between posts — ceil(20 ÷ 6.92) — which is 4 posts at an even 6 ft 8 in, and 4 footings under them. Posts land at both beam ends and between, and the beam itself is 2 plies × 20 ft = 40 lineal feet: 4 boards at 12-ft stock with waste, their splices staggered so no two plies break over the same post bay. Footings below frost depth, sized by your soil, are a concrete order — the concrete calculator turns 4 footing tubes into bag counts or ready-mix fractions.
Joist spans past 12 ft simply stop at the table's last column — the 12-ft figure carries forward as the cap, which is conservative and honest. A freestanding deck runs this same math twice, once per beam, and doubles the post count: 8 posts for the 20 × 12 freestanding version, 4 per beam. What the tables never cover is hardware: the hangers, the hurricane ties that pin joists to beams in wind country, the post bases that keep concrete and post apart, and the beam's nailing schedule — the code prescribes it, the lumberyard sells it by the box, and the calculator deliberately counts only what a saw can cut.
Step 4 — The Decking Itself
Decking is an area-to-lineal-feet conversion, and the only trap is the board's real width. A "6-inch" deck board is 5½ inches of actual face; add the gap you'll leave between boards — 1/4 in is the default for pressure-treated wood, which is still shrinking on the rack — and each board covers 5.75 inches of deck. The formula: square feet × 12 ÷ coverage inches = lineal feet of decking. The 240 sq ft example needs 240 × 12 ÷ 5.75 ≈ 501 lineal feet, which at 16-ft boards with 10% waste is ceil(501 × 1.1 ÷ 16) = 35 boards.
Composite boards are the same math with different constants: the face is still 5½ inches, and hidden clips set the side gap — Trex's fasteners hold boards at 3/16 in, which the calculator fills in for you when you switch fastening methods. What composites change is structure: 16 in o.c. is the residential maximum under most brands, 12 in for diagonal or picture-frame layouts, and stair treads often demand 12 in even when the field doesn't. Check the installation sheet for the specific product — the span rating printed there is the manufacturer's tested number, and warranty claims live or die on it.
The board count is a total, not a layout: 16-ft boards on a 20-ft deck must break somewhere, and pros stagger those butt joints across mid-joists, using each board's offcut to start a later row. That's why the waste allowance is 10% rather than 2% — it absorbs both the stagger and the ripping of the final row, whose last board is usually a narrow strip. Order the whole surface at once. Lot numbers color-match, and a second trip for three boards is a second delivery fee and a visible shade line.
Step 5 — Fasteners: Two Screws per Crossing
Screw counts are crossing arithmetic. Face-screwed decking takes two screws wherever a board crosses a joist — one per edge, a thumb-width in from each side — so the count is rows × joists × 2. The example deck runs 26 rows of 5.75-in coverage across 16 joists: 26 × 16 × 2 = 832 screws, which works out to about 347 per 100 sq ft — right on the published rule of thumb of 350. Buying by the pound is buying blind — box counts vary by brand and screw size, so divide 832 by the count printed on the box you are actually holding and round up to the next whole box.
Hidden clips run one per crossing instead of two, because each stainless screw angles through the clip into the joist while the clip grips the groove of the board above: 26 × 16 ≈ 416 clips for the example at 16 in o.c. Manufacturer packaging anticipates exactly this arithmetic — Trex's 90-count Hideaway box is labeled as covering 50 sq ft at 16-in spacing, so the 240 sq ft example orders 5 boxes and has a handful of spares for the breaker boards a picture-frame border demands. Add the starter clips for the first row and face screws for the last board and any perimeter breaking the pattern, and buy stainless fasteners within 15 miles of salt water, always.
Waste, Grade Stamps and the Delivery
Two disciplines keep a deck lumber order honest. The first is waste as a percentage with a reason: 10% across the framing absorbs the checking, the crown and the one board you'll mismeasure; 10% across the decking absorbs the staggered joints and the ripped last row. Straight-walled rectangular decks can lean to 5%, and diagonal layouts or lots of notches should lean to 15% — the percentage is an input, not a constant, precisely because it's a judgment.
The second is species discipline. The span tables above are Southern Pine No. 2, and the takeoff arithmetic is species-blind — but the span capacity is not. Douglas Fir-Larch and Hem-Fir boards carry shorter spans at the same size, which means the same deck may need 2×12s where a Southern Pine deck framed happily on 2×10s. The grade stamp on each board's face says exactly what you bought; if the stamp and the plan disagree, the lumber loses and the inspector wins. While you're reading stamps, reject the badly checked and the heavy warp — a crown you can see across a 12-ft run is a deck surface that waves forever.
On delivery day, count what the truck actually carried against the takeoff you printed, sticker the stack off the ground under cover, and let pressure-treated lumber dry a few days before it meets fasteners sealed for wet wood. And keep the list: every deck changes a rail height or a stair width between plan and build, and the delta between what you ordered and what you used is the calibration for the next one.
Where to Read Next
- Run: Deck Lumber Calculator Footprint, joist size and spacing in — joists, rim, beam, posts, boards and screws out, with the span verdict and a plan-view diagram.
- Run: Concrete Slab & Footing Calculator Those deck footing tubes are a concrete order — cubic yards or 60-lb bags for the tubes and a shed-slab extra.
- Explore: Garden & Outdoor The outdoor category — deck framing here, beds and ground cover a page away.
The bottom line: size the joists from the span table before counting anything, let the joist span pick the beam and the beam pick the posts, convert the square footage through the board's true 5½-in face, and budget two screws at every crossing. A lumber takeoff that shows its arithmetic survives the lumberyard, the inspection, and the second trip you won't have to make.