Keep a hip roof’s plan dimensions separate from sloped plane dimensions.
Why is rectangular plan area smaller than a supplied four-plane roof surface?
Use the matching calculator
Use this decision sequence
- Use the rectangular plan boundary, already including the intended eave projection.
- Use the supplied rise rather than selecting pitch.
- For the symmetric equal-pitch model, calculate trapezoidal sides and triangular ends separately.
- Move their measured areas into material coverage tools without counting the plan area again.
Keep the quantities distinct
| Quantity or assumption | How to use it |
|---|---|
| Plan footprint | A horizontal projection. |
| Roof surface | Sum of sloped planes. |
| Unequal pitches | Outside the symmetric equal-pitch model. |
Worked comparison
An illustrative 6 m by 4 m plan with 1.5 m rise has half-width run 2 m and sloping distance 2.5 m. Its ridge is 6−4 = 2 m. Each side trapezoid is (6+2)×2.5/2 = 10 m², and each triangular end is 2×2.5 = 5 m². All four planes total 30 m², compared with the 24 m² plan.
Check before using the estimate
No pitch, roof structure, weather detailing or drainage capacity is chosen by this geometry model.
All dimensions, product properties, prices and specifications in this example are illustrative arithmetic inputs. Use the values from your measured plan, selected product data sheet and supplier quote. This guide does not choose construction specifications or certify safety.
Calculation and scope checked 2026-10-04. Methods and scope.