A flat 10% is how hip roofs run short and simple gables get over-ordered. On this site a clean gable, sheets ridge-to-eave, no valleys, is about 5% area allowance on top of per-plane sheet round-up. Hips start at 12% of sloped area. Each valley adds 3%. Pitches of 35° and over add 2%. Override to 0% if you are only checking geometry — do not order a hip from 0%.
The waste calculator applies that. Flashings are not inside the percentage.
Count sheets first
Waste is extra after a real layout. Sheet count from cover width per plane (sheet calculator). A 7.85 m face on 762 mm cover is 10.30 → 11 sheets. That remainder is already layout waste on a gable.
Hip ends cannot be counted as full cover-width sheets; the takeoff switches toward area ÷ (cover × common-rafter length) plus the higher percentage. That is an order estimate, not a cutting list.
Where the offcuts actually go
Hips: repeated diagonal cuts, rarely a restockable full sheet. Valleys: angled cuts both sides plus the valley flashing itself. Multi-section roofs: short leftovers from a porch that will not cover the garage (multi-section takeoff). Penetrations: not modelled — add a valley count as a blunt instrument or a separate section.
Do not apply one percentage to the whole footprint of an L-shape. Split faces, then add.
Checks that are not waste
Cladding pitch and run length still fail the order even if waste looks generous. 6° corrugate is a profile change, not 15% extra (min pitch). Long dark Stratco CGI may need expansion joints around 16–24 m — that is a length split, not waste.
Open the waste calculator with hip + valley count, screenshot the percentage and the flashing lines, and put both on the merchant pack with the NZMRM CoP.