01
Formula and assumptions
Surface area = plan area × √(1 + (rise ÷ run)²). A uniform shed or gable roof is assumed, with plan length × width already covering the full footprint. Bundle coverage must come from the product label.
Roofing calculator
Plan a uniform shed or gable roof with five transparent calculators for geometry, ordering, and material cost.
Convert plan dimensions and rise/run pitch into sloped roof surface area.
Result
Enter values and calculate.
Convert rise and run into pitch ratio, angle, slope, and area factor.
Result
Enter values and calculate.
Estimate geometric slope length from horizontal run and overhang.
Result
Enter values and calculate.
Add waste, apply the manufacturer coverage, and round up whole bundles.
Result
Enter values and calculate.
Total bundles, underlayment, accessories, and sales tax; labour is excluded.
Result
Enter values and calculate.
Sources
Method, example, and limits
02
Worked example
A 12 m × 8 m footprint at 6:12 pitch has factor 1.1180 and surface area 107.33 m². With 10% waste and 3.096768 m² per bundle, order 118.06 m² or 39 whole bundles.
03
Limits and safe use
The rafter result is geometry only: it does not size lumber or account for loads, spans, birdsmouth cuts, ridge details, valleys, dormers, tear-off, labour, delivery, or local code. Measure each roof plane separately when geometry is irregular.
Frequently asked questions
Why are bundles rounded up?
Roofing is sold in whole bundles. The calculator applies waste first, divides by the coverage you enter, then rounds upward so the order is not short.
Does this calculate structural rafter size?
No. It estimates slope length only. A qualified designer or local code official must determine member size, spacing, connections, and loads.