The maths, explained
Use the horizontal plan dimensions, including any measured overhangs, and enter a uniform roof pitch in degrees. Dividing the plan area by the cosine of the pitch estimates the sloped area. The waste-adjusted area is then priced per square metre or square foot. This is a simplified geometry model for a rectangular footprint with consistent pitch, not a structural or drainage design.
Sloped area = plan length × plan width ÷ cos(pitch angle); order area = sloped area × (1 + waste / 100)
A worked example
The worked example below uses the calculator’s initial inputs and the same calculation function as the live interface. Replace example amounts with your own records; changing currency does not convert the numbers. Physical reference dimensions are stored in metric units, and the interface can display their imperial equivalents. Rounded display values may differ slightly from the full precision retained in the calculation.
| Your inputs | Editable example — not a market quote |
|---|---|
| Plan length | 10 m |
| Plan width | 8 m |
| Pitch angle (degrees) | 30 |
| Order quantity with 10% example waste | 101.61 m² |
Assumptions used
The result assumes simple geometry and the dimensions actually entered. Waste, unit price and allowances are editable assumptions, not verified regional market rates. An allowance is not a guarantee: compare the answer with the chosen product, cutting or laying plan, and delivery constraints. Tax is applied to the complete subtotal including contingency in this model; adjust the budget if your applicable tax base differs.
On your side of the Atlantic
Canadian projects often mix metric drawings with imperial products. Confirm the supplier’s units and the applicable GST, HST or PST treatment for your province.
Common mistakes
Do not enter a rise-in-12 ratio into a degree field. Break complex roofs into sections and account separately for valleys, dormers, flashing and product-specific overlaps. Roofing squares are a purchasing convention, not the displayed price basis.
Before ordering or tendering, compare at least two scenarios: your base takeoff and a realistic alternative. Keep assumptions with the result so differences are explainable. Save an estimate only on a suitable device. A shared link contains the entered values, so avoid including confidential information. An exported estimate is a snapshot and does not update when a supplier changes a price.
Questions from the jobsite
What does this calculator measure?
What is the first mistake to avoid?
Are the example prices supplier quotes?
Which tax assumptions apply in Canada?
How should I handle waste and contingency?
Can I use the result in a tender or estimate?
Planning estimate only. Not a substitute for a professional quote or local building-code compliance. Verify quantities, tax treatment and contract requirements.
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