Offcut WorksDiagonal decking calculator
Work out boards, angled cuts, joists and fixings for decking laid at 45 degrees, with a realistic waste allowance.
How the numbers are worked out
Linear metres of board = deck area ÷ (board width + gap). The angle does not change how much board covers the area. What changes is the waste, because every board end at the edge is cut at an angle and the triangular offcuts rarely get reused.
Boards = linear metres × (1 + waste) ÷ board length, rounded up. Rows are counted across the diagonal, which is (length + width) ÷ 1.414 for a rectangle. Angled cuts = two per row.
Joists run along the width and are spaced along the length. Fixings are counted where each board crosses a joist, which is every joist spacing × 1.414 along the board.
Why the waste allowance is higher
Straight boards often finish square with the joists and the offcut from one row can start the next. At 45 degrees, every board is cut at both ends, so offcuts are short triangles. Guides we checked quote 15 to 20% waste for diagonal layouts, compared with around 10% for straight boards. At least one online calculator we found multiplies the board length by 1.41 for 45 degree decking. That looks too high, because the area covered is the same and the extra material is waste at the edges.
Why the joists are closer
Boards laid at an angle cross the joists at an angle, so the span between supports along the board is longer than the joist spacing. For that reason joist makers often recommend closer joists for diagonal layouts. One joist maker suggests 250 mm centres for diagonal boards, and some board guides say the same. Check yours.
Working out the longest board you need
The longest diagonal run on a rectangle is the shorter side × 1.414. If the boards you buy are shorter than that, some rows will need joints, which need support. The calculator shows the longest board required, so you can decide on a board length.
Borders and breaker boards
Many diagonal decks have a picture-frame border or a breaker board, where straight boards run around the edge. These add cuts and need extra support. Add to the waste allowance, or calculate the border separately with the decking calculator.
Questions
How much extra decking does a diagonal layout need?
Guides we checked quote 15 to 20% waste, compared with around 10% for straight boards. The default here is 20%.
Do diagonal decks need closer joists?
Often, yes. One joist maker suggests 250 mm centres for diagonal boards. Check the installation guide for your boards.
Is the board length multiplied by 1.41 for diagonal decking?
No. The area covered is the same. The extra material is the waste from angled cuts at the edges.
Can I use offcuts?
Sometimes, for the corners, but offcuts from diagonal cuts are triangular and rarely fit elsewhere. That is why the waste allowance is higher.
Where the defaults come from
- Composite decking joist listing (PBSL Group). Joist centres no greater than 300 mm, 250 mm for diagonal boards or above ground, and double joists under butt joints with an 8 mm end gap.
- eDecks composite decking installation guide (PDF). Joists at 300 mm centres, hidden clips, and a 5 mm gap against solid structures.
- The 20% default waste is the top of the 15 to 20% range seen in the guides we checked.
- The 300 mm default spacing is a common composite value, and 250 mm is one maker's advice for diagonal layouts. Use your board maker's figure.
- The fixings count is an estimate that does not include extra fixings for edges or a border.