How I'd actually use this on a flip
The cedar deck I built at the Chicago-suburb gut rehab in summer 2020 was a 16×12 ft surface with 5.5″ actual-width cedar boards, 16 ft lengths, parallel-to-house layout, 16″ on-center joists, face-screws (cedar takes screws fine; clips are an option for the cleaner look but didn't make sense on the budget). 27 board rows × 16 ft = 432 LF exact, 476 LF practical with the 10% cushion, 30 boards ordered. 13 joists × 27 rows = 351 intersections; 702 screws.
The cushion did its job — a few boards came in with bow that wouldn't flatten under fasteners, three got cut into trim pieces, one got returned because the supplier shipped a different grade by mistake. Net result: 27 boards installed, 3 spares left for the railing-cap rip (which is actually a different scope, but the spares saved me a trip). 10% on cedar parallel-to-house is the right number; bump to 15% only if you're running diagonal or if the supplier's stock is inconsistent.
What the calculator does NOT do: the frame. The frame for that deck was 2×8 PT joists 16″ oc on a 6×6 PT post-and-beam, ledger-attached to the rim joist with through-bolts + flashing. I didn't design that — I followed the AWC DCA-6 prescriptive guide. The calc handles the cedar that goes on top of that frame; joist sizing, beam spans, post footings, ledger flashing all live with a structural engineer or your local building inspector.
Where this number breaks down
A few traps that put the math on the wrong side of the order:
- Composite fasteners follow the board profile. Grooved-edge composite boards (Trex, TimberTech, AZEK and similar) commonly require hidden-fastener clip systems for warranty coverage. Square-edge boards in the same product lines often have an approved face-fastener system — but the right screw is spec’d in the manufacturer’s install guide and varies by board family. Read the install guide for the exact board profile before you buy fasteners; the warranty hinges on following it.
- 20 ft boards warp.SiteworkMath field preference caps practical residential board length at 16 ft — 20 ft boards cup + warp under outdoor wet service across cedar, PT, and composite. DCA-6 doesn't publish a board-length max; this is operator + manufacturer-handling experience. Plan staggered runs across 12-16 ft boards.
- PT ships wet + variable.Pressure-treated lumber ships saturated; a 16 ft 5/4×6 PT board can vary 1/4″ in straightness from rack to rack. Hand-select at the lumber yard — don't let them pull from the back. The calculator assumes uniform stock; the field doesn't.
- Joist spacing varies by load. 16″ oc is residential code default. Most composite product lines specify 12″ oc for the flexural deflection their boards are rated to — confirm the install guide for your specific board profile. Heavier loads (hot tub, outdoor kitchen) may require 12″ oc + heavier joists. Defer to AWC DCA-6 + your contractor.
- Diagonal layout cuts more than the 15% suggests. On unusually angled decks (octagonal, multi-tier with a 45° offset), the 15% diagonal default is light. Bump to 18-20% and accept the surplus.
Methodology
Every number on this page traces to one of three layers — site arithmetic for the count math, manufacturer data sheets for board dimensions + fastener specs, and AWC DCA-6 for structural conventions (16″ on-center joist spacing default + structural-frame opt-out only — DCA-6 covers joist spans, ledger, beam, footings, NOT deck-board length). The 16 ft practical board-length cap is SiteworkMath field preference, not a DCA-6 publication. The per-calculator sourcing tier in methodology spells out which sources back which kinds of claims. AWC DCA-6 has a free PDF mirror; IRC 2021 Ch.5 is paywalled at ICC and cited by name when this page touches structural conventions (without claiming the calculator enforces them).
Show the formulas
- Board rows:
ceil(deck_width_ft / (board_width_in / 12)). - Linear feet exact:
board_rows × deck_length_ft. - Linear feet practical:
ceil(LF_exact × (100 + waste_pct) / 100)— integer formulation avoids the JavaScript float trap. - Board count:
ceil(LF_practical / board_length_ft). - Joist count:
ceil(deck_length_ft / (joist_spacing_in / 12)) + 1. - Fastener count:
board_rows × joist_count × (clip:1 | screw:2). - Material cost (optional):
boards × board_length × price_per_LF.
Frequently asked
How many deck boards do I need for a 16×12 ft surface?
30 boards (16 ft × 5.5″ wide, the standard 6″ nominal), plus 702 face-screws or 351 hidden clips. The math: 27 board rows running 16 ft each = 432 linear feet exact. Add a 10% waste cushion (parallel-to-house layout) = 476 LF practical. Rounded up to whole 16 ft boards = 30 boards. Fasteners with 16″ on-center joists: 13 joists across the 16 ft deck length × 27 boards = 351 board-joist intersections. Face-screws at 2 per intersection = 702. Hidden clips at 1 per intersection = 351. Which fastener: For composite, board profile decides — grooved-edge boards usually need the manufacturer's hidden-clip system; square-edge boards in the same line often have an approved face-fastener. Read the install guide before you buy.
What's the difference between cedar, pressure-treated, and composite decking for the cost math?
The board count is identical across materials. The per-board price varies 2-4×, so the cost output is driven by the linear-foot price you plug in. Typical price bands (varies by region + grade): • Pressure-treated — ~$1.50-$2 per linear foot. Cheapest upfront. • Cedar — ~$3-$4 per linear foot. • Composite (Trex / TimberTech / AZEK) — ~$3-$7 per linear foot, with premium tiers running higher. Which to pick: PT is cheapest upfront. Composite wins on lifetime cost in north-facing or shaded conditions where wood rots fast. What the calc does: Plug your local supplier's per-LF quote into the optional price input; the calc does the rest.
What does this calculator NOT estimate?
Anything structural. This is a surface-material calc only. Explicitly out of scope: Joists, beams, posts, footings, ledger flashing, railings, stairs, permits, labor, structural-design review. Why the line is drawn here: Pretending a surface calc handles structural design is the kind of overreach that turns a trust-credible tool into spam. The board count is independent of frame requirements. Where your frame design comes from: The prescriptive deck-framing code (joist spans, ledger, beam, footings, fastener requirements) plus a qualified contractor or engineer. A structural deck calc may ship later; this one stays deliberately surface-only.
Why are 20 ft deck boards a bad idea?
They warp and cup over the first one or two winters in wet outdoor service — across cedar, PT, and composite alike. 16 ft is the practical cap. Where the cap comes from: Operator field experience plus manufacturer handling guidance. Not a code citation — the prescriptive deck-framing standard covers spans and fastener requirements, not maximum board length or warp behavior. What to do instead: Plan staggered runs across 12 ft and 16 ft boards rather than single-piece 20 ft spans. Looks intentional, performs better, easier to source. Where the calculator warns you: Set board length above 16 ft and the warning surfaces above the order.
Should I use hidden clips or face-screws for my deck?
Material + board profile decide. Composite is split between the two; cedar and PT default to face-screws. Composite (Trex, TimberTech, AZEK): Grooved-edge boards usually require the manufacturer's hidden-fastener clip system. Square-edge boards in the same product line often have an approved face-fastener system. The warranty is profile-specific — read the install guide for your exact board profile before buying fasteners. Cedar and pressure-treated: Face-screws standard. Hidden clips work as an optional upgrade for the cleaner look. Quantity math: Clips run 1 per joist per board (~2× per linear foot). Face-screws run 2 per joist per board (~4×). The calculator handles both.
Does the calculator account for diagonal layouts or only parallel-to-house?
Both. Pick the layout in the segmented control; the waste percentage adjusts automatically. Parallel-to-house — 10% waste: Boards run perpendicular to the house, butt joints land on side rims, minimal cuts. Diagonal (45° rotation) — 15% waste: Every board edge needs an angle cut and the off-cuts are usually too short to reuse, so the cushion is wider. What else changes between layouts: In this version, only the waste percentage. The surface-only opt-out (no frame design) applies regardless.
What I'd do next
- Pick the deck material first
The math changes by ~30% across cedar, PT, and composite. Material choice drives the whole estimate.
- Plan joists + spacing before ordering boards
Joist on-center spacing drives screw count, board run direction, and the gap between boards you'll see daily.
- Concrete for the footings
Out of scope for the surface calc, in scope for your weekend. Cylinder mode covers post-hole footings.
By James Wu. Surface-material count is site arithmetic, formulas shown above. Joist-spacing default (16″ on center) + structural-frame opt-out come from American Wood Council DCA-6 — Prescriptive Residential Wood Deck Construction Guide; the 16 ft practical board-length cap is SiteworkMath field preference, not a DCA-6 publication (the standard covers joist spans, ledger, beam, footings, fastener requirements, NOT deck-board length or warp behavior). IRC 2021 Chapter 5 (Floors) is paywalled at ICC and cited by name. Manufacturer fastener-system requirements (composite warranty conditions) follow Trex installation guidance and TimberTech installation guidance. Engine logic in lib/sitework/deck.ts. Surface- material only — frame design (joists, beams, posts, footings, ledger flashing, railings, stairs) requires AWC DCA-6 + IRC 2021 R5 + a qualified contractor or engineer. Not structural- engineering advice. Full methodology.