Free roof area calculator. Calculate true roof area from plan footprint and pitch for materials and labour estimates.
This calculator turns a roof's plan footprint into its actual sloped area, then into roofing and underlay quantities, ridge length and a sheet and fixing count. A roof is always larger than the rectangle it sits on, and this is the factor that closes that gap.
Sloped area is the plan footprint multiplied by a slope factor, which is one divided by the cosine of the pitch. The page offers four bands: flat, low, standard and steep.
Each band's factor is taken at the TOP of that band rather than the middle, to two decimal places, so the figure over-states area everywhere inside the band rather than under-stating it. The steep band, for example, uses 1.22, which is the factor at 35 degrees.
These are area factors and nothing else. Never apply one to a linear measurement such as a ridge or a barge.
Rafters cross the shorter plan dimension and repeat along the longer one, so the ridge runs the length of the longer dimension. That is the same convention CostMate's estimating engine uses, so the free page and a paid estimate agree.
That is the gable rule. A hip is the longer dimension less the shorter one, because the hips eat into the ridge from both ends, and a skillion has no ridge at all. Either way the answer does not depend on which order you type the two dimensions in.
CostMate's allowance adds 10% to roofing and 5% to underlay, because underlay comes in wide rolls with far less cutting loss than profiled sheet.
Sheets and fixings come from the sheet cover width you enter, the purlin spacing, and the fixings per sheet per purlin crossing. Those are your inputs: cover width varies by profile and manufacturer, and fixing patterns vary with exposure.
Pitch, member sizes and weathertightness detailing are design decisions. Check them against your plans and NZ Building Code B1 (Structure), and measure the real pitch rather than picking a band by eye, before you order.