How much waste should I allow for decking?
Last reviewed 1 July 2026
Short answer
10% is a reasonable allowance for a simple rectangular deck built from boards that divide neatly into the run length. Raise it to 15% for angled edges, multiple levels or diagonal laying, and to 20% for herringbone or picture-frame borders. But check the cut plan first: on some deck sizes the off-cut loss is larger than any percentage you would add.
Work it out for your project with the decking calculator →
Two different things get called waste
Cutting loss is timber you buy and cannot use because of how the lengths divide. It is fully predictable from the deck size and the stock length, and it can easily exceed 30%.
Handling waste is damaged boards, bad ends, splits, mistakes and the board you drop off the trestles. It is genuinely random and it is what a percentage allowance is for.
Adding 10% to cover both is what causes second trips to the merchant. Work out the cutting loss explicitly, then add the percentage on top.
Cutting loss by example
| Run length | Pieces per board | Boards for 21 rows | Timber wasted |
|---|---|---|---|
| 1.8 m | 2 | 11 | Nil — the run halves the board exactly |
| 2.0 m | 1 | 21 | 44% — a 1.6 m off-cut from every board |
| 3.6 m | 1 | 21 | Nil |
| 4.0 m | 1 + a 0.4 m piece | 24 | About 12% |
| 5.4 m | 1 + a 1.8 m piece | 32 | About 11% |
The 2.0 m case is the trap: it looks efficient, and it throws away nearly half the timber.
How to reduce it
- Design the deck to a multiple of an available board length wherever you can. A 3.6 m or 1.8 m run from 3.6 m stock wastes nothing.
- Ask which lengths your merchant actually stocks before fixing the deck size. Availability varies far more than catalogues suggest.
- If a run leaves a long off-cut, check whether the off-cuts will make up the fascia, the step treads or the frame noggins. They often will.
- Consider changing the board direction. Swapping span and run can turn an awkward division into a clean one.
When to go above 10%
| Deck type | Allowance |
|---|---|
| Rectangular, boards square to the joists | 10% |
| One angled or curved edge | 12–15% |
| Boards laid at 45° | 15–20% |
| Multiple levels or changes of direction | 15% |
| Picture-frame border or herringbone | 20% |
| Hardwood, where every board is expensive and pre-drilled | 10% but inspect every board on delivery |
Note: Order the frame separately
Safety notice: Material planning only