Roofing Calculator
Calculate the sloped area of a roof based on its flat footprint dimensions and pitch (slope). Computes roofing squares, shingles bundles, and felt underlayment rolls.
Calculation Results
The horizontal flat footprint area of the roof.
The multiplication factor representing the angle of the roof.
Total actual surface area of the sloped roof, including waste.
Total roofing squares (1 square = 100 square feet).
Number of bundles of standard 3-tab shingles (3 bundles cover 1 square).
Number of standard 15lb felt underlayment rolls (approx 400 sq ft of coverage per roll).
How This Calculator Works
To calculate roofing materials:
- Enter the horizontal footprint dimensions of the roof. Be sure to include the overhang (eaves/rakes) in your length and width measurements!
- Select the roof pitch. Pitch represents how many vertical inches the roof rises for every 12 horizontal inches (run). Standard homes are typically 4/12 or 6/12 pitch.
- Include a waste margin. A simple gable roof needs 10% waste, while a complex hip roof with valleys and ridges should use 15-20% waste.
Formula & Calculations
The footprint area is calculated by multiplying footprint length by width:
Footprint Area = Length × Width
Since the roof is sloped, its actual surface area is larger than its footprint. We calculate a slope factor using the Pythagorean theorem based on the rise over 12 run ratio:
Slope Multiplier = √(1 + (Pitch / 12)²)
The actual sloped area is then computed by multiplying the footprint area by the slope multiplier and adding the waste factor:
Sloped Area = Footprint Area × Slope Multiplier × (1 + Waste / 100)
In roofing, materials are estimated in "Squares", where 1 square = 100 square feet. Standard architectural shingles require 3 bundles per square.
Step-by-Step Calculation Examples
Standard Gable Roof (30ft x 24ft footprint, 4/12 pitch)
Estimating shingle bundles and felt rolls for a standard roof with a 10% waste factor.
- length: 30
- width: 24
- pitch: 4
- waste: 10
- flat_area: 720.0 sq ft
- slope_factor: 1.0541
- sloped_area: 834.8 sq ft
- squares: 8.35
- bundles: 26
Frequently Asked Questions
What is a roofing square?
A "square" is a trade unit of measurement in roofing equal to 100 square feet of roof surface. For example, a 2,000 square foot roof measures exactly 20 squares.
How many bundles of shingles do I need per square?
For standard 3-tab or architectural asphalt shingles, it takes exactly 3 bundles to cover 1 square (100 square feet) of roof surface.