For a simple gable or shed roof, use horizontal plan dimensions including intended overhangs. Complex roofs require each plane’s true dimensions or a qualified measurement report; this tool does not infer hips, valleys, dormers, or curved surfaces.
01 / Formula
How this is calculated
Sloped area = plan length × plan width × √(run² + rise²) ÷ run + additional measured area02 / Assumptions
What the estimate assumes
- ✓Length and width describe horizontal roof plan, not sloped rafter length.
- ✓One pitch applies to the entered plan area.
- ✓Additional area is already measured on the roof slope.
03 / Worked example
See the math in context
A 40 × 30 ft plan at 6:12 has a 1.118 pitch factor and approximately 1,342 sq ft of sloped area before allowance.
04 / Guidance
Include overhangs correctly
Building footprint is not always roof plan area. Add horizontal eave and rake overhangs to the applicable plan dimensions before applying pitch.
05 / Guidance
Separate mixed pitches
Calculate each pitch group independently. Add actual plane areas rather than applying one average factor to a complex roof.
Questions
Frequently asked
Can I measure or inspect a roof by walking on it?+
Do not access a roof without appropriate training, equipment, conditions, and fall protection. Use plans, ground measurements, aerial reports, or a qualified roofing professional when direct access is unsafe.
Does this calculator approve a roofing system?+
No. Verify deck condition, slope limits, underlayment, ice-barrier, flashing, ventilation, fasteners, wind requirements, fire classification, local code, and the exact manufacturer instructions.
What should be measured separately?+
Break out each roof plane plus valleys, hips, ridges, eaves, rakes, walls, penetrations, dormers, low-slope transitions, and special membranes. Geometry alone does not create a complete roofing order.
Sources