A deck is rows of boards across a frame of joists, and counting it row by row is what gets the order right. A 12-foot row cut from a 16-foot board leaves 4 feet that no other row can use, while an 8-foot row gets two rows out of every board. The page works out how many rows the deck takes with the gaps between them, whether each row fits in one board or is joined over a joist, and which of the lengths boards come in leaves the least over.
The figures depend on the board, and the page carries both markets’:
- In the US, Trex’s installation guide: composite boards 5.5 inches wide in 12, 16 and 20-foot lengths; joists at 16 inches on centre under 1-inch boards and 24 under 2 × 6, closer for boards at an angle; a gap of at least 3/16 inch between boards; and two screws at every joist.
- In the UK, the Timber Decking and Cladding Association’s code of practice: softwood boards no wider than 145 mm; joists at 400 mm centres under 27 mm boards and 500 mm under 34 mm; a gap of 5 to 8 mm; two screws at every joist crossing — and a platform sized so that no board is cut along its length. When the last row would need ripping, the page gives the two widths either side that avoid it.
Footings, when the design has them, go to the concrete calculator as round pours. The frame itself — the rim, the beams and the posts — is for your plans or the span tables in your local code; this page counts what goes on top of it.
Measured the deck on the calculator? Follow its link here and the area comes with it.