Measure horizontal roof geometry
Use eave-to-eave plan dimensions rather than interior room dimensions.
Treat additions, porches, dormers, and intersecting roof blocks separately when geometry differs.
Convert pitch to slope factor
For rise r per 12 units of run, slope factor = sqrt(12² + r²) ÷ 12.
Multiply horizontal plan area by the applicable slope factor for a simple uniform roof.
Keep overhangs explicit
If footprint dimensions stop at exterior walls, add horizontal eave and rake overhang geometry before slope conversion.
Do not add the same overhang twice when measured dimensions already include it.
Separate geometry from waste
Calculate net roof surface first; then apply waste appropriate to valleys, hips, cuts, product format, and layout.
Starter, ridge, flashing, underlayment, and ventilation are separate takeoff lines.
Verify complex roofs from drawings or field data
A single footprint factor is not adequate for roofs with multiple pitches or irregular intersections.
Use measured plane dimensions, drawings, or a verified roof report when complexity warrants it.