Understanding your decking takeoff
The board-row count and linear-length figures are a straight geometric takeoff; the screw count is a planning estimate based on a common fixed-fastening convention, not a manufacturer fastening schedule.
- The 400 mm joist-spacing assumption used for the screw estimate is a common residential decking convention, not a universal standard — check the joist spacing in your actual substructure (see the floor joist calculator) and adjust the screw count accordingly.
- Composite and PVC decking manufacturers often specify hidden-fastener clip systems or wider gaps than solid wood; always check the specific product's installation guide for gap and fastening requirements before finalizing an order.
- This calculator does not account for picture-frame borders, diagonal or herringbone patterns, or breaker boards, all of which increase both cutting waste and linear board length beyond a simple straight-run estimate.
What does a decking calculator do?
A decking calculator converts a deck's overall length and width into a materials takeoff: the number of parallel board rows needed to cover the width, the total linear length of decking board to buy, and the fasteners required to install it. Boards are assumed to run along the deck's length, with each row's effective width equal to the board width plus the gap left for drainage and seasonal movement.
The gap between boards is not cosmetic — most decking manufacturers specify a minimum gap (commonly 3–6 mm for wood, more for composite in humid climates) to let water drain through and to allow the boards to expand and contract with temperature and moisture without buckling or binding against each other.
How to use this decking calculator
- Enter the deck's length and width in meters — length is the direction the boards will run.
- Enter the board width (the face width of a single decking board, typically 9–14 cm) and the gap you plan to leave between boards.
- Enter a waste allowance percentage to cover cutting losses, staggered end joints and mistakes — 10% is a common default.
- Read the number of board rows needed to cover the deck width, the total linear meters of board to order (waste included), and an estimated screw count.
The formula behind decking takeoff
The number of board rows equals the deck width divided by the effective row pitch (board width plus gap), rounded up so a partial final row is still covered. Total linear board length is the row count multiplied by the deck length, with the waste allowance applied on top. The screw estimate assumes two screws per board at every joist crossing, with joists spaced at a fixed 400 mm on-center convention and a 5% rounding margin.
Worked example (calculator defaults): a 5 m × 4 m deck with 14 cm boards and a 6 mm gap gives a row pitch of 0.146 m, so rows = ⌈4 ÷ 0.146⌉ = 28 rows. Total board length = 28 × 5 m × 1.10 (10% waste) = 154 m. With joists at 400 mm centers over the 5 m length there are 13 joist crossings, giving roughly 28 × 13 × 2 × 1.05 ≈ 765 screws.
Common mistakes
- Leaving no gap (or an undersized gap) between boards, which prevents drainage and can cause cupping, splitting or buckling as boards absorb moisture and expand.
- Forgetting that boards are sold in fixed lengths, so the calculated linear meters must be rounded up to whole boards per row, which can add several extra rows of waste beyond the stated percentage.
- Using a joist spacing in the screw estimate that doesn't match the actual substructure, which under- or over-states the fastener count.
- Ignoring picture-frame borders or angled board patterns, both of which increase cutting waste well above a flat 10% allowance.
자주 묻는 질문
How much gap should I leave between deck boards?
Most decking manufacturers recommend a gap of roughly 3–6 mm for wood boards to allow drainage and seasonal movement; composite decking often specifies a similar or slightly wider gap. Always check the specific product's installation instructions, since gap requirements vary by material and climate.
How do I calculate how many deck boards I need?
Divide the deck width by the board width plus the gap to get the number of rows, then multiply by the deck length (plus a waste allowance) to get the total linear board length, which you then divide by the length of the boards you're buying to get a board count.
Why does the screw count assume 400 mm joist spacing?
This calculator uses a common residential joist-spacing convention (400 mm on-center) to estimate how many joists each board crosses. If your actual joist spacing is different, use the floor joist calculator to confirm it and adjust the screw estimate proportionally.
How much waste allowance should I add for decking?
10% is a common default for straight-run decking, covering cut ends and mistakes; picture-frame borders, diagonal patterns or decks with many cutouts (for posts, drains or planters) typically need a higher allowance.
Should deck boards run the length or width of the deck?
There is no single correct answer — boards commonly run the longer dimension to minimize the number of end joints, but design preference, structural framing direction and drainage slope can all influence the choice. This calculator assumes boards run along the entered 'length' dimension.
참고 자료
- American Wood Council (AWC) — general residential deck construction guidance, including board spacing and fastening conventions.
- International Code Council (ICC) — International Residential Code (IRC), deck construction provisions.
- Decking manufacturer installation guidelines (wood and composite) — gap width and fastening-pattern recommendations vary by product and should be confirmed against the specific material chosen.