Two things lose money on a roofing estimate: area worked out from the wrong plane, and flashings left out.
Roof area is not floor area. Price the slope area, not the projection, and add laps and offcuts — typically 5–10% for profiled sheet, more for tiles on a complex roof.
Sections
Strip-out and disposal (asbestos cement sheeting is its own line with its own rules); structure — checking rafters, replacing damaged members, treatment; membrane, counter battens and battens; insulation where applicable; the covering itself; flashings — ridge, eaves, verges, abutments to chimneys and walls, snow guards, all in linear metres; rainwater goods with gutters and downpipes priced separately; roof windows, vents, ladders and safety hooks; and access — scaffolding or a lift.
What else to allow for
Weather delays, temporary covering while the roof is open, and a 5–10% contingency for rotten battens and rafter ends found after strip-out.
For quantities and prices see our calculators; for the full form, the estimate template.