Waste allowance guide
How much extra to order, and — more usefully — why the right number depends on the shape of the job rather than the size of it.
Three different things get called waste
Lumping them together is what produces second trips to the merchant. They behave completely differently and the calculators handle them separately.
1. Cutting loss
Material you buy and cannot use because of how the sizes divide. Entirely predictable, and on decking it can exceed 30% — far more than any percentage you would add.
Handled by the cutting plan, not by the waste percentage.
2. Volume change
Compaction (1.3× for sub-base) and settlement (20% for topsoil). This is not waste at all — the material is still there, it just occupies less space.
Applied before the waste percentage, as a separate factor.
3. Handling waste
Breakage, damaged units, setting-out errors and the odd piece too warped to use. Truly random, and the only one a percentage is genuinely suited to.
Default 10%, range 5–15%.
Suggested allowances
| Material | Simple | Typical | Complex | What drives it |
|---|---|---|---|---|
| Decking boards | 10% | 10–15% | 20% | How the run length divides into the stock length — often larger than the percentage |
| Deck framing | 5% | 5–10% | 10% | Cut to fit, so far less waste than the boards |
| Gravel and aggregate | 5% | 5–10% | 10% | Spillage and depth creep; the compaction allowance is separate |
| Concrete (slab) | 5% | 5–10% | 10% | Uneven sub-base and spillage |
| Concrete (footings) | 10% | 10–15% | 15% | Over-dig — trench sides are never as neat as the drawing |
| Topsoil | 5% | 5% | 5% | Settlement is handled separately at 10–25%, not as waste |
| Paving, straight grid | 5% | 10% | 15% | Perimeter cutting; small patios need proportionally more |
| Paving, herringbone | 15% | 15–20% | 25% | Every perimeter unit is a diagonal cut |
| Bricks | 5% | 5–10% | 15% | Cuts at returns and reveals, breakage, colour rejection |
| Blocks | 5% | 5% | 10% | Larger units mean fewer cuts |
Small areas need more, not less
Cutting happens at the perimeter, and the perimeter is a bigger fraction of a small area than a large one. A 4 × 5 m patio has 0.9 m of edge per square metre; a 2 × 2 m patio has 2 m of edge per square metre — more than twice as much cutting for every square metre laid.
This is why a fixed 10% under-serves small jobs. On anything under about 6 m², add another 5% on top of whatever the table suggests.
Design the waste out where you can
- Set dimensions to whole multiples of the module. A patio 4.27 m long takes seven 600 mm slabs with 10 mm joints and needs no cutting in that direction at all.
- Ask which stock lengths your merchant actually holds before fixing a deck size. Availability varies far more than catalogues suggest.
- Set out dry from the most visible edge, so full units land where they are seen and cuts go against the house or the boundary.
- Keep usable off-cuts. The half unit from one edge often fits the opposite edge.
Always keep spares
Finish with two or three whole units left over and store them somewhere dry. Paving, brick and composite decking all vary between production batches, and a replacement bought in three years’ time will not match.
Note: Over-ordering is usually the cheaper mistake