01
Separate the footprint from the roof
A floor plan describes horizontal space. A pitched roof has a longer sloping surface, and overhangs extend beyond the walls. Multiple stories add floor area without necessarily adding roof area. Dormers, porches, and intersecting slopes make a simple footprint comparison less reliable. Ask whether an estimate comes from measured roof planes, drawings, or a preliminary aerial model.
02
Read the measurement basis
Request an area schedule showing each roof zone and identifying which surfaces receive slate. Keep field area separate from cutting allowance and special pieces. If two bids show different quantities, compare their measurement assumptions before comparing their rates. A useful estimate allows you to trace the total back to recognizable roof areas.