Simple pitched-roof area calculator

Convert horizontal plan area to sloped surface area with an explicit rise-to-run ratio and waste allowance.

Direct answer

Multiply horizontal roof plan area by sqrt(1 + (rise/run)^2), then apply explicit waste. Calculate complex roof planes separately.

Interactive tool

Enter your measurements

Enter valid measurements and select Calculate.

Transfer these inputs to a new local editor project

Formula

Sloped area = plan length x plan width x sqrt(1 + (rise / run)^2) x (1 + waste).

Worked example

A 40 ft by 30 ft plan at 6:12 has a 1.118 slope factor: 1,342 sq ft before waste and about 1,476 sq ft with 10%.

Assumptions

The entire input rectangle shares one slope magnitude.
Plan dimensions include the intended overhang.
Waste is added after geometric area.

Caveats

Hips, valleys, dormers, openings, low-slope areas, and overlaps need separate geometry.
Underlayment, starter, ridge, flashing, ventilation, and drainage are separate.
The result does not design roof structure, fastening, wind, or snow resistance.

Primary references

Home Builder's Guide to Coastal ConstructionFederal Emergency Management Agency; accessed 2026-08-02.
2024 International Residential Code: Building PlanningInternational Code Council; accessed 2026-08-02.

Understand and continue

Roof geometry, area, and coordination

Model slope and plan area for early quantities while separating geometry from structural and weather design.

Read supporting guide

Two-bedroom ranch concept

A one-level rectangle with bedrooms grouped away from an open shared living zone.

Open starting concept

Planning quantity, not construction directionThis calculator does not verify codes, specifications, structure, utilities, product suitability, purchasing, pricing, installation, or safety. Verify the result and all project decisions with qualified local professionals.