TradesHow this works
Enter the deck length and width, the decking material, and the board direction. The tool counts the joists and the deck-board rows, and settles the joist spacing. A straight wood deck sits at 16 inches on center, but composite flexes more and a diagonal layout spans farther, so both step up to 12 inches.
The board figure is total linear feet, since board length is up to the product you buy. Divide by your board length for a count and plan the butt joints on long runs. And treat the manufacturer span table as the authority over any rule of thumb here.
Common questions
What joist spacing does composite decking need?
Most composite and PVC brands require 16 inches on center for a straight run and 12 inches for a 45-degree diagonal. Composite flexes more than wood, so many builders go to 12 inches on center even for straight runs to stiffen the deck, and 12 inches is the rule under a hot tub or heavy load.
Is 16 or 12 inch joist spacing better?
Sixteen inches uses the fewest joists and is the standard for a straight wood deck. Twelve inches costs more joists but makes a stiffer, more solid deck, and it is required for diagonal boards and strongly recommended for composite. The decking manufacturer span table is the final word.
How many deck boards do I need?
Count the rows across the deck width: divide the width by the board face width plus the gap. A 5/4x6 board is 5.5 inches wide with about a 3/16 inch gap, so each row covers about 5.7 inches. Multiply the rows by the length each row runs to get the total linear feet, then divide by your board length.
Why does a diagonal layout need closer joists?
When boards run at 45 degrees, the span between joists along the board is longer than the joist spacing itself, so the board is unsupported over a greater distance. Tightening the joists to 12 inches on center brings that effective span back down, which is why a diagonal deck always steps up to 12 inches.
Does this size the framing?
No. This counts joists and boards. Joist size, beam and footing sizing, ledger attachment, and railings are structural and code items that depend on span, spacing, and load. Follow the decking manufacturer span table and your local code, and pull a permit where one is required.
Sources: Joist spacing: composite/PVC 16 in OC straight, 12 in OC diagonal; wood 16 in straight, 12 in diagonal (Decks & Docks, The Backyard Standard) · Composite flexes more, so 12 in stiffens it and is the call under hot tubs; 5/4x6 board = 5.5 in with ~3/16 in gap. Engine version 1.0.0. Risk tier 2.