How much roofing do I need?

Roofing quantities start with a correction most people miss: the roof surface is larger than the building footprint beneath it, because it slopes. Measure the footprint, then multiply by the pitch factor for your roof — 1.118 at a 6/12 pitch, 1.202 at 8/12. A 1,500 square foot footprint at 6/12 is about 1,677 square feet of actual roof, and ordering off the footprint alone leaves you a square and a half short. Divide the corrected area by 100 to get roofing squares, the unit shingles are sold in, then add 10 to 15 percent for waste. Valleys, hips, and the starter course all consume material that a flat area calculation misses, and a complex roof with several dormers can exceed 15 percent. Underlayment, drip edge, and ice-and-water shield are separate line items. Ice-and-water shield along the eaves is required by code in most cold regions and is the single most valuable thing on a roof in a climate that produces ice dams.