Understand rise and run before using pitch
Roof pitch describes vertical rise relative to horizontal run. A common U.S. notation expresses rise for 12 units of horizontal run, but any measured rise and run can be used if both values use the same unit.
Do not use the sloped rafter length as the run. The run is horizontal. Mixing those dimensions changes the angle and causes the area multiplier to be wrong before any roofing material calculation begins.
Use the pitch multiplier to convert plan area to slope area
A sloped roof plane has more surface area than its horizontal projection. The pitch multiplier captures that geometric increase and can be derived from the rise-to-run relationship. The steeper the roof, the greater the surface area for the same horizontal footprint.
For a simple symmetrical gable roof, the building footprint can be a useful starting point. For more complex roofs, calculate individual planes because different sections may have different pitches, lengths, overhangs, or interruptions.
Add overhangs and secondary roof sections deliberately
If the footprint dimensions are measured at the walls but the roof extends beyond the walls, the overhang needs to be represented in the roof-plane geometry. The same applies to porch roofs, attached garages, dormers, and other sections that are not included in the main rectangle.
Breaking the roof into measured planes makes the estimate easier to audit. It also prevents a waste factor from being used to cover missing geometry, which can hide the real reason the material quantity increased.
Do not confuse roof area with roofing purchase quantity
Roof area is only the first step. Shingles, metal panels, underlayment, sheathing, ridge materials, starter, flashing, and fasteners each have their own effective coverage and packaging rules.
For products with laps or exposure requirements, use effective installed coverage rather than gross package dimensions. Manufacturer instructions should control the final conversion from square feet of roof to bundles, panels, rolls, or sheets.
Waste depends on layout and roof complexity
Waste can increase with valleys, hips, diagonal cuts, short runs, pattern constraints, damaged decking, and complicated intersections. A simple gable roof and a roof with many valleys should not automatically use the same allowance.
Keep geometric roof area, product coverage, and waste as separate parts of the estimate. That makes it possible to change one assumption without losing track of the underlying roof measurement.