--- title: "Deck Board & Joist Calculator with Plan | Maple Veranda" description: "Work out deck boards by stock length, joists at 12, 16 or 24 in centres, rim joists and screws or hidden clips, with a plan of your deck. Feet or metres." url: https://mapleveranda.homes/ --- # Deck board & joist calculator Enter your deck's size and boards. You get the boards to buy for each stock length, joists at 12, 16 or 24 in centres, rim joists, screws or clips, and a plan to check it against. ## How to use the deck calculator 1. Enter the deck's length along the house and its depth out from the house, measured to the outside of the frame. If your boards will overhang the frame, add the overhang. 2. Give the board's actual face width (a nominal 6 in board is 5 1/2 in, about 140 mm) and the gap. AWC's DCA 6 guide asks for gaps of about 1/8 in; composite makers set their own. 3. Pick the board direction, joist spacing, how the deck is fixed, and the board lengths your yard sells. ### What the numbers mean **Boards to buy** come from cutting every row out of the lengths you ticked, with joints landing on joists and every piece bearing on at least three joists, as DCA 6 requires. The waste allowance covers split ends, bad boards and staggering the joints; diagonal decks usually need 15% or more. **Joists** are counted from one side of the frame to the other at your spacing, both outer joists included. The rim joist caps the joist ends; the ledger is the board bolted to the house. Boards running out from the house need joists parallel to it, carried on beams. **Fasteners** follow IRC R507.7: two screws or nails per board at every joist. Hidden clips are counted as one per board per joist. ### Worked example A 16 × 12 ft deck with 5 1/2 in boards and 1/8 in gaps needs 26 rows: 26 boards plus 25 gaps make 146 in, just over the 144 in depth, so the last board is ripped. With 16 ft boards that is 26 boards, 29 with 10% waste. At 16 in centres, 192 in ÷ 16 = 12 spaces, so 13 joists, and 26 × 13 × 2 = 676 screws. ## Joist spans, the code and your inspector The span table in the results is AWC's DCA 6 Table 2, the prescriptive guide most US building departments accept for simple single-level decks. It assumes a 40 psf live load and 10 psf dead load, No. 2 lumber in wet service and an L/360 deflection limit, and it measures the span from the face of one support to the face of the other, without the overhang. Its Southern pine figures match the 40 psf column of 2021 IRC Table R507.6. Hot tubs, heavy snow, second-storey decks and anything outside its tables need a design. Joist spacing also depends on the decking: the 2021 IRC (Table R507.7) allows 16 in centres for 1 1/4 in wood boards laid square to the joists but only 12 in at 45°, and 24 in for 2 in boards. Composite and PVC boards follow the maker's installation guide. These tables are a starting point for planning, not an approval. Your area may use another code edition, local amendments or its own frost depth, and outside the US the timber grades and rules are different. The building department and the inspector decide what gets built. ## Sources - American Wood Council, [*DCA 6: Prescriptive Residential Wood Deck Construction Guide*](https://awc.org/resources/dca-6-english-2015-prescriptive-residential-wood-deck-construction-guide/) (2015 IRC edition): Table 2 joist spans and overhangs, decking requirements (2-8d threaded nails or 2-#8 screws per joist, about 1/8 in gaps, each board on at least 3 joists, up to 45°). - International Code Council, [2021 International Residential Code, Section R507 Exterior Decks](https://codes.iccsafe.org/content/IRC2021P2/chapter-5-floors): Table R507.6 deck joist spans and Table R507.7 maximum joist spacing for decking. The values are reproduced in Anne Arundel County's [residential deck guide](https://www.aacounty.org/sites/default/files/2023-03/Deck_Guide.pdf). ## More calculators for the yard Finish the deck, then the patio, the fence and the footings under it all. - [Deck stain calculator](https://mapleveranda.homes/deck-stain-calculator/): Litres or gallons for the floor, railings and stairs, by surface condition, and the cans to buy. - [Patio paver calculator](https://mapleveranda.homes/patio-paver-calculator/): Pavers for any mix of shapes, with joints and pattern waste, plus gravel base and bedding sand. - [Fence post & panel calculator](https://mapleveranda.homes/fence-post-calculator/): Posts, panels or pickets, rails, gates and the concrete for every post hole. - [Concrete footing calculator](https://mapleveranda.homes/concrete-footing-calculator/): Volume and pre-mixed bags for tube or square footings, with DCA 6 footing sizes. ## Recommended Two free services for the online side of a building project. Temporary email · no sign-up ### CleanTempMail Lumber yards, decking brands and stain makers often want an email address before they send a price list, a sample request or a project brochure. CleanTempMail gives you a free disposable inbox straight away, with no sign-up, a random or custom address on hundreds of domains, and messages deleted automatically after 3 days. Your own inbox stays free of follow-up offers. [Open a temporary inbox](https://cleantempmail.com/) Disposable cloud browser · domain lookups ### WebTraceCheck Found composite boards or hidden fasteners cheaper on a shop you have never heard of? WebTraceCheck's free DNS and domain lookup shows the details behind a site's domain, and its disposable cloud browser opens unfamiliar links on an isolated desktop that is deleted after each session. Nothing to install, with free time every day. [Try WebTraceCheck](https://webtracecheck.com/) ## Questions **How many deck boards do I need for a 12 × 12 ft deck?** With 5 1/2 in boards and 1/8 in gaps, 26 rows cover 12 ft, so 26 boards of 12 ft, or 29 with a 10% waste allowance. At 16 in centres the frame has 10 joists. Wider boards or bigger gaps mean fewer rows; enter yours above to check. **Should deck boards run parallel to the house?** Most do, because the joists can then run out from the house and hang from a ledger. Boards running out from the house need joists parallel to it, carried on beams. Diagonal boards look good but take more cutting and closer joists: the 2021 IRC limits 5/4 wood boards laid at 45° to 12 in centres. **What joist spacing do my deck boards need?** The 2021 IRC (Table R507.7) allows joists at up to 16 in centres under 1 1/4 in wood boards laid square to them, 12 in when laid diagonally, and 24 in under 2 in boards either way, as long as each board crosses at least three joists. Composite and PVC boards follow the maker's installation guide. **Is the joist span table enough for a permit?** No. It shows what DCA 6 allows for common US lumber at a 40 psf live load, which helps you plan and price. Your building department decides which code applies, along with local snow loads, frost depth and amendments, and the inspector signs off the work. **Why is a board length marked too short?** Every piece of decking has to reach across at least three joists, with its ends on joists. If a stock length cannot do that at your spacing, especially on a diagonal deck where joists are further apart along the board, the calculator leaves it out.