How it’s calculated
For a simple roof of uniform pitch, sloping area = (length + 2 × overhang) × (width + 2 × overhang) ÷ cos(pitch). Pitch is entered in degrees. Apply waste, divide by effective coverage per pack or sheet and round up. Use the product’s cover width after side laps and usable length after end laps, not raw dimensions. Consult the NZ Metal Roofing Manufacturers Code of Practice for profile, pitch and installation requirements; the geometric area calculation is not a roof design approval.
Worked example
A 12 × 8 m footprint with 0.45 m overhangs is 114.81 m² in plan. At 30° the roof surface is about 132.57 m². Adding 10% waste gives about 145.83 m²: 49 packs with 3 m² effective coverage, costing NZ$4,410 at NZ$90 per pack.
Before you use the result
Measure the full horizontal footprint, not one roof plane; a standard gable is already included and must not be doubled again. Uniform-pitch hip roofs have the same area relationship, but cuts increase waste. Mixed pitches, dormers, curves and intersecting roofs need separate measured sections. Pack-equivalent counts do not create a long-run metal sheet cutting schedule. Excludes underlay, flashings, ridge caps, fasteners, gutters, structure, labour, access and tax unless included in prices. Do not use this for drainage sizing or structural design.
Frequently asked questions
Do I double the result for a gable roof?
No. The full building footprint already accounts for both slopes. Doubling would over-order.
How do I allow for laps?
Enter effective installed coverage after side and end laps. Do not use the sheet’s nominal physical area.
Can I use it for a flat roof?
Zero degrees gives plan area, but manufacturer minimum pitches and drainage still apply.