Every material list needs a little more than the drawing shows. Concrete spills and sticks to the barrow, bricks break, tiles are cut, and boards are trimmed. The extra is the waste allowance, and getting it right is a balance: too little and the job stops while you fetch more; too much and the leftovers eat your profit. This guide sets out the allowances BouWise uses by default, why they differ by trade, and where a maker or industry body publishes its own figure.
Why waste differs by trade
Waste comes from three things: cutting, breakage and losses in handling or mixing. Concrete has no cutting but loses volume to uneven ground and spillage. Bricks are seldom cut but some break in delivery. Tiles and boards are cut at every edge, so the shape of the room and the layout matter more than the area. A small, awkward job always wastes a higher share than a large, simple one.
BouWise’s default allowances
| Trade | Default | Notes |
|---|---|---|
| Concrete: slabs, footings, columns, steps | 10% | Less on a neat, formed slab; more for hand-dug trenches |
| Bricks and blocks | 5% | More for face bricks chosen for colour, or many openings |
| Mortar and plaster | 10% | Droppings and mixing losses |
| Floor screed | 10% | AfriSam’s own allowance for screed |
| Pavers | 10% | The CMA’s allowance for cutting and waste, also for kerbs |
| Tiles | 7 to 20% | By layout: straight 7 to 10%, diagonal 15%, herringbone 20% |
| RhinoBoard partitions | 12% | Offcuts around doors and at the top of the wall |
| Ceilings | 5% | Board lengths chosen to suit the room |
| Roof tiles | 10% | Cuts at hips, valleys and verges, and breakage |
| Cladding planks and decking | 10% | End cuts and splits |
| Skirting and trim | 15% | Mitres and short pieces lost at every corner |
| Sand and topsoil | 10 to 15% | Settling and compaction once spread |
These are starting points, and every BouWise calculator lets you change them. Where a maker or industry body publishes a figure, use it: the CMA asks for 10% on pavers and kerbs, and AfriSam adds 10% to screed materials.
A worked example
A 16.2 m² bathroom floor in 300 × 600 mm tiles needs about 88.7 tiles with no waste. Laid straight with a few cuts, 10% brings it to 98. Laid diagonally at 15%, it is 102; in a herringbone at 20%, 107. The layout changes the order by nearly a tenth, which is why the tile calculator asks for it.
Rounding is a second allowance
Materials come in whole units: bags, sheets, boxes, cubes and tins. Rounding up to whole units adds a little extra on top of the waste allowance, sometimes a lot on small jobs. A slab needing 0.3 m³ of stone still means ordering a half cube. Keep that in mind before adding a big percentage as well; the rounding may already cover you.
When to allow more
- Small jobs, where one broken unit is a larger share of the total.
- Complicated shapes: many corners, curves, openings or angles.
- Diagonal and herringbone layouts.
- Materials you can’t easily match later, such as face bricks and tiles from one batch. A few spare is cheaper than a mismatch.
- Inexperienced crews or difficult access.
When to allow less
- Large, simple, repeated areas.
- Formed concrete on a well-compacted, level base.
- Materials that can be returned or used on the next job.
Waste on the quote
Price the waste into the job, don’t hide it. When you add a calculator result to a BouWise job, the quantities already include the allowance, and the quote prices them at your price-list rates. Keep a note of what was really used on each job; after a few jobs you will know your own waste rates better than any table.
For the trade-specific detail, see estimating concrete, how many bricks do I need? and tile estimating.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.