How to work out the waste factor on a residential job
Every estimator adds something for waste. Very few can tell you where the number came from. It gets inherited from the last job, or from the bloke who trained you, and then it sits in the estimate on every job afterwards — on a big job, thousands of dollars either way.
It is worth measuring properly, because it is one of the few numbers in an estimate you can work out exactly from jobs you have already built.
What a waste factor actually is
It is an allowance on top of the measured quantity, not part of it. Take off the work, get your quantity, then add the waste factor so you buy enough to finish.
If the slab needs 100m³ of concrete and you allow 5% waste, you order 105m³. The 100m³ is what ends up in the building. The 5m³ is the cost of reality: offcuts, breakages, overordering because the truck comes in full loads, and the piece somebody cuts wrong.
Keep the two separate. A quantity with the waste already buried in it cannot be checked by anyone, including you in six months.
Get the number from your own jobs
Waste varies by trade, by crew and by how complicated the design is. An average across your last few jobs beats a number from a textbook, because it includes how your people actually work.
Plasterboard is the easiest one to start with, because the sheets are a known size. A 2400 × 1200 sheet is 2.88m².
- Measure the plasterboard area off the drawings.
- Divide by 2.88 to get the number of sheets the job needs.
- Ask the site for how many sheets were actually delivered and used.
- Divide the sheets used by the sheets needed.
| Job | Sheets needed | Sheets used | Waste |
|---|---|---|---|
| Job A | 240 | 258 | 7.5% |
| Job B | 310 | 341 | 10.0% |
| Job C | 185 | 197 | 6.5% |
Three jobs and you have an average — in this example a bit over 8%, which is a far better allowance than a round 10% picked because it sounds safe.
The spread is the interesting part. If one job is 10% and another is 6.5%, the difference is usually the design, the crew, or both. Note which, because next time you price a job with a complicated roof line or a new team, you will know which end of your range to use.
Where it belongs in the estimate
In SmarteBuild, waste is set on the item, not baked into the quantity. Open the quantity calculator on a Bill of Quantities item and enter the wastage percentage. The measured quantity stays visible, the waste is added on top, and the item shows both.
That matters when the drawing changes. Your take-off feeds the measured quantity, the waste percentage rides on top of it, and both update together — instead of you remembering that the 264 in the quantity box was really 240 plus a bit.
Three things worth knowing
Waste is not contingency. Waste is material you will certainly buy and not install. Contingency is for things that might happen. Mixing them hides both.
Some trades have almost none. Windows and doors are ordered to size and delivered to size. Adding 5% to those is money in the bin — or more likely, a price that loses you the job.
Review it once a year. Materials change, crews change, and a factor set in 2019 is describing a business that no longer exists.
Start with the trades where the money is: concrete, framing, plasterboard, tiles and bricks. Five numbers you have measured beat fifty you have guessed.
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