Approximately how many squares of asphalt shingles should be ordered for the rear of a two-story house, considering a 10% allowance for starter and waste?

To determine how many squares of asphalt shingles to order, you first need to estimate the total area of the roof that will be covered. A "square" in roofing corresponds to an area of 100 square feet. Once the total area is calculated, it is essential to factor in a waste allowance to account for mistakes, cuts, and overlaps during installation.

If the calculated area for the rear of the two-story house is, for example, 15 squares, adding a 10% allowance for waste and starter material is key. This adjustment is calculated by multiplying the total calculated area by 10% (0.10) and rounding to the nearest square.

In this case, if the calculated area is 15 squares, the total amount after adding the 10% allowance would be:

15 squares + (15 squares * 0.10) = 15 squares + 1.5 squares = 16.5 squares.

Rounding this up results in ordering 17 squares, which makes it practical to ensure adequate coverage without running short.

Thus, given the parameters and necessary calculations for waste allowance, the choice to order 17 squares accurately reflects the need for sufficient material while accommodating for any potential waste during installation.