Taking detailed measurements on-site during the initial inspection is critical to producing accurate cost estimates. Tight measurements are crucial, but what about the waste that is produced during the installation of materials such as roofing and siding? The contractor has paid for the waste and the labor that produced it. Shouldn't she be properly compensated for this expense?
So how should we handle waste in our cost estimate? My suggestion is to add waste into the quantity takeoff. For example, the actual measurement of coverage for a roof replacement is 1,500 sf. If this is a simple roof (gables only-no hips or valleys), and normally pitched (between 4-6/12), then add 10% to the quantity takeoff, making the bid quantity 1,650 sf (16.5 squares). If the roof is more complex, add 15%.The most complicated roof should warrant an additional 20%.
If waste is not factored into the quantity takeoffs, the contractor is forced to reflect waste in the cost/sf price of her bid. There is also the possibility that she will not realize that waste was not factored in, and underbid the project altogether. Another bidding contractor may add the waste into the cost/sf bid price, making a comparison between bidders impossible. Problems will definitely show up during a project when the contractor realizes her mistake and asks for more money.
The bottom line is to make your contractors aware of your takeoff practices. They should feel more comfortable bidding the project more tightly if they trust your quantity takeoffs. And after the work begins, fewer arguments about reimbursement should take place as well.