01
Start with measured roof surface
Use the area along the slopes, including the roof overhangs, rather than the floor area of the house. Break a complicated roof into measurable planes and keep the dimensions on a sketch. Account for dormers and roof openings consistently; very small openings may not reduce the order because surrounding cuts consume material.
02
Calculate the field count
For this illustration only, assume a conventional double-lap roof using 18 × 12 inch slate with a design-approved 3 inch headlap. Exposure is (18 − 3) ÷ 2 = 7.5 inches. Using nominal width and ignoring small joint gaps, each piece covers 12 × 7.5 ÷ 144 = 0.625 square feet. A 2,000 square foot field therefore needs approximately 3,200 pieces before allowances.
03
Keep allowances visible
If the installer selects a 10% field allowance for this example, 3,200 × 1.10 = 3,520 field pieces. Ten percent is an illustration, not a recommended allowance for every roof. List starters, hips, ridges and special cuts separately, and state whether spare stock is already included. Do not add the same waste allowance twice.
04
Reconcile the order with the supplier
Confirm actual dimensions, the headlap assumed in a quoted square, packing quantities and how special pieces are sold. Round the final order to suitable shipping units only after reviewing each roof plane. Different headlap or a proprietary installation system changes the coverage; use the applicable layout before comparing delivered prices.