Work across the deck, then along each row
Add the chosen gap to the actual board width to get a planning step across the deck. The last row has no gap after it, so rows = ceil((layout span in inches + gap) ÷ (actual board width + gap)). Each row needs ceil(running span ÷ purchased length) whole stock boards. Multiply these counts, apply the chosen waste percentage, then round up.
This is a conservative straight-row planning model with no assumed offcut reuse. Confirm edge details, joints and the final cutting layout on site.
Worked example: a 16 × 12 ft deck
Assume boards run along the 16 ft side, with an actual width of 5.5 in, a chosen 0.125 in gap, 16 ft purchased boards and 10% waste:
- Across-deck span: 12 ft = 144 in. The planning step is 5.5 + 0.125 = 5.625 in.
- Rows: ceil((144 + 0.125) ÷ 5.625) = 26 rows.
- Length: 26 × 16 ft = 416 linear ft, equivalent to 26 full 16 ft boards before waste.
- Planning order: ceil((416 ÷ 16) × 1.10) = 29 boards.
Why width and direction matter
If the same deck used 3.5 in actual-width boards with the same 0.125 in gap, it would take 40 rows across the 12 ft span. At 16 ft per row and 10% waste, the model suggests 44 boards. Both layouts cover the same 192 sq ft, but their row counts differ.
Changing board direction also changes the running length and the number of rows. A purchased length shorter than a row needs multiple whole stock boards per row in this model. End joints, offcut planning and supports still require a separate layout.
Check the product before ordering
Gap requirements vary by material, product and installation conditions. Use the current instructions for your chosen board and fastening system; for example, Trex advises checking the specific installation guide. This estimate covers surface boards only. It does not design framing, validate structural spans, estimate fasteners, price a project or guarantee an exact purchase list.
Try your layout
The Deck Material Calculator lets you vary board width, gap, orientation, purchased length and waste to see the effect before you order. Recheck dimensions, stock lengths and installation rules with your supplier.
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