Deck Calculator
Count decking boards, joists, footings and screws for a rectangular deck, with the joist size checked against common span tables.
The Deck Calculator runs entirely in your browser. Your measurements are processed on your device and never uploaded.
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Об инструменте Deck Calculator
A deck is three counting problems stacked on top of each other, and each one is usually out by one. Boards fill the width at board-plus-gap pitch, joists run across the length at their on-centre spacing, and footings sit under every beam line — all three are fencepost counts where the answer is spans plus one, which means one extra board, one extra joist and one extra hole in the ground. This calculator works all three from the deck's dimensions, converts decking into whole boards at the stock length you can actually buy, checks whether the width is within span for common joist sizes, and figures screws at two per board-to-joist crossing, which is the number that surprises people at the till.
Возможности
- Board rows from face width and gap, including hidden-fastener spacing
- Whole boards at 8, 10, 12, 16 or 20 foot stock lengths
- Joist count and linear feet at 12, 16 or 24 inch spacing
- Suggested joist size checked against 40 psf span tables
- Footings and posts from beam span and post spacing
- Deck screws at two per crossing, and balusters at 4 inch clear spacing
- Waste allowance applied to decking only
- Everything computes in your browser
Как использовать Deck Calculator
- Enter the deck length and width in feet
- Choose the board width, gap and stock length you plan to buy
- Pick the joist spacing — 16 inches on centre is standard
- Read the board, joist, footing and screw counts
Пример
Ввод
20 × 12 ft deck · 5.5 in boards, 1/8 in gap · 16 ft stock · joists 16 in OC · 10% waste
Результат
26 board rows → 58 boards · 16 joists (192 linear ft, try 2x10) · 12 footings · about 832 screws
The 26th row exists because the last gap is not needed — division alone gives 25.
Частые ошибки и устранение неполадок
- The board count came out one higher than expected. — A run of N gaps needs N+1 boards, the same off-by-one as fence posts. Dividing width by board-plus-gap and rounding down leaves the last row uncovered.
- Boards ran out before the deck was finished. — Diagonal decking, picture-frame borders and mitred corners waste far more than the default 10%. Allow 15% for diagonals and 20% for herringbone or complex patterns.
- No joist size is suggested for the width entered. — The span is beyond what a 2x12 carries at that spacing. Add a mid-span beam and post line, or close the joist spacing to 12 inches, which buys about a foot of span.
Часто задаваемые вопросы
- How many deck boards do I need for a 20 by 12 foot deck?
- With 5.5 inch boards and a 1/8 inch gap you need 26 rows across the 12 foot width. At 16 foot stock that is two boards per row, so 52 before waste and about 58 with a 10% allowance.
- What gap should I leave between deck boards?
- About 1/8 inch for kiln-dried lumber that will shrink, and 1/4 inch for wet pressure-treated boards that are already at full width. Composite decking follows the maker's spacing, which is usually 3/16 to 1/4 inch for drainage and expansion.
- Should joists be 16 or 24 inches on centre?
- 16 inches is standard and required for most composite decking. 24 inches uses fewer joists but makes the deck feel springy and rules out diagonal boards, which effectively increase the span by about 40%.
- How many footings does a deck need?
- One under every post, with posts spaced along each beam and a beam line every 8 feet or so of deck width. A 20 by 12 foot deck typically needs three beam lines of four posts, so twelve footings.
- How many screws per deck board?
- Two at every joist crossing, so a 16 foot board over joists at 16 inch centres takes about 24. Across a whole deck that runs into the hundreds — buy by weight rather than counting boxes.
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