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.
The structural-opt-out is the most important thing on this page. 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. This calculator handles the cedar that goes on top of that frame; it doesn't pretend to handle the frame itself. That boundary is what keeps SiteworkMath honest and out of the “DIY engineer” territory that gets decks failing inspections.
Where this number breaks down
A few traps that put the math on the wrong side of the order:
- Composite warranty needs hidden clips.Trex, TimberTech, AZEK all require hidden-fastener clip systems for warranty coverage. Face-screws on composite void the warranty in writing — even though the deck performs identically for the first few seasons. Material → fastener-type isn't a preference; it's a contract.
- 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. Composite warranty often requires 12″ oc for flexural performance — confirm the manufacturer spec. 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-cluster 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?
For a 16 ft × 12 ft deck with 5.5″ actual-width boards (standard 6″ nominal) at 16 ft length and 10% parallel-to-house waste cushion: 27 board rows, 432 LF exact, 476 LF practical, 30 boards total. With 16″ on-center joists you have 13 joists across the deck length, giving 351 board-joist intersections. Face-screws at 2 per intersection = 702 screws; hidden clips at 1 per intersection = 351 clips. Composite decks (Trex, TimberTech) require clips for warranty.
What's the difference between cedar, pressure-treated, and composite decking for the cost math?
Cedar runs ~$3-4 per linear foot (varies by grade + region), pressure-treated ~$1.50-2/LF, composite ~$3-7/LF (Trex / TimberTech / AZEK premium tiers can run higher). The board-count math is identical across materials; the per-board cost differs by 2-4× from PT to premium composite. The calculator's optional price input lets you plug in your local supplier's quote for the cost output. PT is cheapest upfront; composite wins on lifetime cost in north-facing or shaded conditions where wood rots fast.
What does this calculator NOT estimate?
Anything structural. Joists, beams, posts, footings, ledger flashing, railings, stairs, permits, labor, and structural-design review are all explicitly out of scope for v1. Pretending a Cycle 1 calc handles structural design is the kind of overreach that turns trust-credible into spammy. The surface-material count is independent of frame requirements — your frame goes through AWC DCA-6 + IRC 2021 R5 + a qualified contractor or engineer. Structural deck calc is deferred to Cycle 2+.
Why are 20 ft deck boards a bad idea?
16 ft is SiteworkMath field preference — 20 ft boards warp + cup over the first 1-2 winters in wet outdoor service across cedar, PT, and composite. AWC DCA-6 is a prescriptive structural deck guide (joist spans, ledger, beam, footings, fastener requirements); it does NOT publish deck-board length max or warp behavior. The 16 ft cap is operator + manufacturer-handling experience, not a code citation. Plan staggered runs across 12-16 ft boards instead of single-piece 20 ft spans; the calculator surfaces a warning when board length exceeds 16 ft.
Should I use hidden clips or face-screws for my deck?
Material decides. Composite (Trex, TimberTech, AZEK) requires hidden clips for warranty — face-screws void most composite warranties. Cedar and pressure-treated take face-screws as standard, with clips as an optional upgrade for the cleaner look. Clips run 1 per joist per board (~2× per linear foot of board); face-screws run 2 per joist per board (~4× per linear foot). The calculator's fastener-count math handles both; pick the system from your warranty + finish requirements.
Does the calculator account for diagonal layouts or only parallel-to-house?
Both, via the layout segmented control. Parallel-to-house defaults to 10% waste (boards run perpendicular to the house, butt joints land on side rims, minimal cuts). Diagonal layout (45° rotation) bumps to 15% waste because every board edge needs an angle cut and the off-cuts are usually too short to reuse. The waste % is the only thing that changes between layouts in v1; the structural opt-out applies regardless.
Related guides
- Cedar vs pressure-treated vs composite deck →
- Deck board spacing + joist layout →
- Concrete yardage calculator (footings live here) →
- Tile calculator (sister cluster) →
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.