New Zealand · metal roofing
Roof area calculator
Sloped area is plan area including overhangs, divided by cos(pitch). That is the whole formula. The rest is not forgetting the eaves.
A 12 m × 8 m gable at 15° is not 96 m² of roof. Add 450 mm eaves on both long sides and 50 mm verges, then divide by cos(15°). The metal has to cover the overhangs; the underlay usually does too. This calculator shows plan area and sloped area side by side so you can see the pitch factor instead of hiding it in a “12% extra” fudge.
Hip roofs at a single pitch have the same area relationship — plan rectangle including eaves, times 1/cos(pitch). What changes is cutting pattern and flashing length, which is why waste is higher even when area looks tidy.
Multi-section NZ houses (house + garage + porch) should be entered as separate planes. Adding footprints and applying one pitch is how people miss a 5° lean-to.
FAQ
Is roof area the same as squares?
A US roofing square is 100 square feet. NZ merchants work in m² and lineal metres of sheet. This tool stays metric.
Does roof style change area?
Not for a simple rectangle at one pitch. Gable vs hip changes flashings, waste and sheet cuts, not the raw sloped area of that rectangle.