How to calculate roof pitch
Use rise and horizontal run in the same units. Pitch expressed over 12 is 12 × rise ÷ run. For example, 9 inches of rise over 18 inches of horizontal run is 6:12.
Convert a pitch to degrees with angle = arctan(rise ÷ run). The horizontal run is not the length measured along the roof surface. Confusing those two distances produces the wrong pitch.
Common pitch conversions
Pitch · angle · surface-area factor (rounded): 0:12 · 0° · 1.000; 2:12 · 9.5° · 1.014; 4:12 · 18.4° · 1.054; 5:12 · 22.6° · 1.083; 6:12 · 26.6° · 1.118; 8:12 · 33.7° · 1.202; 12:12 · 45° · 1.414.
These are geometry conversions, not manufacturer-approved installation limits. Check the chosen roofing product instructions and local requirements for the pitch of the actual plane.
Why slope changes roof area
For a single uniform plane, surface area = horizontal projected area × √(1 + (rise ÷ run)²). A projected 1,000-square-foot plane at 6:12 has approximately 1,118 square feet of sloped surface.
Do not apply this factor to an area already corrected for slope. RoofSnap roof surface measurements already account for plane pitch. Multiplying again would double-count the correction.
Mixed pitches need separate planes
A porch, addition or garage may have a different pitch from the main roof. Calculate each plane independently and add the surface areas; applying the main pitch to the whole footprint can distort quantities.
Review unusually flat or steep facets in the satellite results. RoofSnap provides low-confidence facet review, but there is no substitute for checking a suspicious plane against the property itself. Never climb a roof solely to use this guide.
Sources & further reading
Product details reflect RoofSnap’s current measurement and report workflow. Geometry examples are rounded illustrations, not measurements of the pictured property.
Google Solar API documentation