How to use this calculator
- Choose US customary or metric measurements.
- Enter each roof section’s projected length and width.
- Enter the pitch rise per 12 units of horizontal run for each section.
- Set a waste allowance and calculate surface area and roofing squares.
How the calculation works
Each projected rectangular area is multiplied by a geometric pitch multiplier. The adjusted surface areas are combined and waste is applied once to the total.
Formula
Pitch multiplier = √(1 + (rise ÷ 12)²) Section surface area = projected length × width × pitch multiplier Adjusted roof area = combined surface area × (1 + waste % ÷ 100)
Worked example
A 30 ft × 20 ft projected roof at 6:12 pitch has a 1.118 pitch multiplier and about 670.82 square feet of surface area. With 10% waste, the planning area is about 737.9 square feet, or 7.38 roofing squares.
Project considerations
- Projected dimensions must represent the horizontal plan area, not measurements taken along the slope.
- This simplified geometry does not account for every valley, dormer, curve, or penetration.
- The calculator does not recommend structural dimensions or material systems.
Frequently asked questions
Why does pitch increase roof area?
A sloped surface is longer than its horizontal projection. The multiplier converts projected area into idealized sloped surface area.
Does a gable roof need to be doubled?
Not when the entered projected length and full building width represent the complete horizontal footprint. The multiplier converts that full footprint to the combined sloped area.