About

When estimating roofing materials, a common rookie mistake is to order based on the ground floor footprint. However, a roof is a 3D object; as the pitch increases, the surface area grows significantly compared to the flat run. The "Slope Factor" is a precise multiplier used by estimators to convert flat area into true roof area without needing to climb a ladder with a tape measure.

This tool is designed for contractors and project managers. By inputting the building's flat area (including overhangs) and the pitch, it applies the correct trigonometric multiplier to reveal the actual square footage required for shingles, underlayment, and decking. It also integrates a financial module to provide immediate rough estimates based on the cost per square (100 sq ft).

Formulas

The Slope Factor S is the Secant of the pitch angle, or the ratio of the hypotenuse to the run:

S = √12² + pitch² ÷ 12

The True Roof Area A_roof is derived from the Flat Area A_flat:

A_roof = A_flat × S

Reference Data

Pitch	Slope Factor	Area Increase (%)
1:12	1.0035	0.35%
2:12	1.0138	1.38%
3:12	1.0308	3.08%
4:12	1.0541	5.41%
5:12	1.0833	8.33%
6:12	1.1180	11.80%
8:12	1.2019	20.19%
10:12	1.3017	30.17%
12:12	1.4142	41.42%

Frequently Asked Questions

In the US roofing industry, a "Square" is a unit of area equal to 100 square feet. Contractors quote prices "per square" rather than per square foot. For example, a 2,500 sq ft roof is 25 squares.

Yes. Mathematically, the total surface area of a hip roof is identical to that of a gable roof with the same footprint, pitch, and overhangs. You can use the same slope factor multiplier for both styles.

Measure the length and width of the building at the ground level. Then add the length of the overhang (eaves) to all sides. For example, if a house is 40x24 and has a 2ft overhang on all sides, the calculation dimensions are 44x28.