How the math works
A roof’s surface is larger than the footprint below it, and the steeper the pitch the bigger the difference. The pitch multiplier is √(1 + (rise ÷ 12)²). An 8/12 roof is about 20% larger than its footprint. A 12/12 roof is about 41% larger.
Northwest roofs
Many Washington homes have deep eave overhangs to shed rain and steeper pitches to handle snow, especially in the Cascade foothills and Spokane. Both add area that a quick estimate can miss.
Checking your estimate
- Find the roof section of the insurer’s estimate and note total squares and the waste percentage.
- Compare those figures with the result here. Differences over 5% to 10% deserve a closer look.
- Check for steep charges, often from 7/12 up, and high-roof charges on two-story sections.
- Confirm ridge cap, starter, drip edge and valley materials are listed separately.
Limits of a footprint calculation
Complex roofs with several pitches, dormers and valleys need measuring plane by plane. An aerial measurement report or field measurement will always beat a footprint estimate. Use this as a check.
Questions about this calculator
What’s a square?
100 square feet of roof surface. A typical two-story Washington home might have 22 to 40 squares.
Why does waste change with roof type?
Hips, valleys and dormers need more cutting, so more material ends up as scrap.
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