How the math works
Roof surface is larger than the footprint underneath it. The pitch multiplier is √(1 + (rise ÷ 12)²): a 5/12 roof is about 8.3% larger than its footprint, a 12/12 roof about 41.4% larger. Squares are surface area divided by 100.
The eave ice-barrier estimate
Cold-climate building codes commonly require a self-adhering ice barrier from the eave edge to a point at least 24 inches inside the exterior wall. The calculator measures that distance along the slope (overhang plus the inside distance, times the pitch multiplier), divides it by a roughly three-foot roll width to get the number of courses, and multiplies by the eave length of a simple gable roof. Hip roofs have eaves on all four sides, so adjust if yours does.
Checking the carrier’s roof estimate
- Compare total squares and the waste percentage with the result here; a gap over 5–10% deserves a closer look.
- Look for separate line items for ice and water barrier, drip edge, starter, ridge cap and step flashing.
- Check for steep charges (often starting around 7/12) and high-roof charges on two-story sections.
Complex roofs with several planes need measuring plane by plane; treat this as a sanity check, not a measurement report.
Questions about this tool
What is a roofing “square”?
One hundred square feet of roof surface. Waste is added on top because hips, valleys and cuts turn some material into scrap.
Is ice barrier required on every North Dakota roof?
Requirements depend on the code your city or county has adopted and when the roof was installed. Ask the local building department, and check whether ordinance-or-law coverage applies to code upgrades.