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Calculate boards needed by deck size and board width.
With these inputs:
| Result | Value |
|---|---|
| Boards needed | 28 boards |
| Total linear footage | 448 lft |
| Material cost | $2,285 |
| Board length each | 16 ft |
Includes a 10% waste factor for cuts and defects.
Board count is where deck estimates most often go wrong, because the number depends on the gap, the board width and how you handle the run lengths. Ordering by area alone always comes up short. This works the count from the real board width plus the gap, adds a realistic waste allowance, and shows the linear footage to take to the counter.
Divide the deck width in inches by the board's actual width plus the gap, then multiply by how many board lengths it takes to cross the deck. A 5/4x6 board is 5 1/2 inches actual, not 6, so a 12 ft wide deck at a 1/8 inch gap needs 26 rows — not 24. Getting the actual width right is where most estimates go astray.
Ten percent on a plain rectangular deck with runs that match stock lengths, and 15 percent once you add a diagonal layout, a picture-frame border or an awkward shape. Buying one extra board is far cheaper than a second trip, and boards from a later batch rarely match the first in colour — particularly with composite.
Whatever crosses the deck with the fewest joints. A 16 ft deck is best in 16 ft boards; buying 12 ft boards means a butt joint in every row, and every joint needs blocking beneath it and becomes a place water sits. Where a joint is unavoidable, stagger them across the deck rather than lining them up.
Perpendicular to the joists, which in the usual framing layout means parallel to the house. Running the boards the other way means rotating the whole frame, because joists must always cross the boards at right angles. A diagonal layout at 45 degrees is the compromise, and it needs the joist spacing tightened to compensate.
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