How to build an accurate prebuild

A prebuild is your price for one unit of work: a lineal metre of wall frame, a square metre of decking, one door hung. Measure the plans, multiply by the prebuild, and the quote writes itself. Get the prebuild wrong and every quote that uses it is wrong the same way, job after job, until someone notices the margins.

Here’s how to build one properly, using a square metre of decking as the example.

The five steps

1. 1
   Pick the unit you measure off the plans

   Each, per lineal metre or per square metre. Use the unit you take off in, so the quantity on the plan drops straight onto the price. Decking is measured in m².
2. 2
   Price the materials for one unit, plus waste

   Boards, joists, fixings and finish for one square metre, at what you actually pay, plus your waste allowance. In the example that comes to $140 a square metre including 10% waste.
3. 3
   Get the hours from your own jobs

   Not from a book and not from memory. The last three decks took 52, 43 and 45 hours for 38, 30 and 32 m²: 140 hours for 100 m², so 1.4 hours a square metre.
4. 4
   Cost those hours at what an hour really costs you

   About $86 an hour in our example, not the $45 wage (the real hourly cost guide shows the maths). 1.4 hours × $86 is $120.40 of labour, so the cost is $260.40 a square metre.
5. 5
   Add your margin

   For a real 20% margin, divide by 0.8: $260.40 ÷ 0.8 is $325.50 a square metre. Adding 20% would only give you 16.7%; the markup versus margin guide explains why.

Then every quote is a multiplication

Once the prebuild is right, the price of any deck is the area off the plans times $325.50:

Check it after every job

A prebuild is a prediction. After each job, put the actual hours next to it. If the last deck took 1.6 hours a square metre instead of 1.4, that’s $17.20 a square metre of labour you didn’t price in, on every deck you quote until you fix it. Update the prebuild and the next quote is right.

A few habits that keep prebuilds honest

• Keep supply and install as separate items, so install-only work gets the right price.
• Put fixings, consumables and site clean in the build-up or as their own allowance lines. Don't leave them to the margin.
• Price the hard version separately: working at height, poor access, a second storey. A first-floor wall frame takes longer than a ground-floor one.
• Work in minutes per unit for labour. 84 minutes a square metre is easier to check against a timesheet than 1.4 hours.
• Quote ex GST and show the GST on the quote.