Roof Size from House Square Footage Calculator
Last reviewed How we calculate
Most people know how big their house is, not how big the roof is. This turns the square footage on a listing or a tax record into an estimate of roof area and roofing squares, so you can sense-check a quote or a materials list before you have a tape in your hand.
Roof area from the size of the house
Estimate your roof area and roofing squares from the square footage of the house, the number of stories and the pitch.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Estimated roof area Footprint with overhang, times the pitch multiplier, times the roof shape allowance.
- Roofing squares
- Likely low end About 10% under the estimate.
- Likely high end About 10% over the estimate.
- Roofed footprint, walls only Living area divided by the stories, plus the garage or porch.
- Footprint including overhang
- Pitch multiplier
An estimate, good to about 10 percent for an ordinary house. It assumes a roughly rectangular footprint about one and a half times as long as it is wide, because the house size alone does not say what shape the walls make. Measure the roof itself, or use the roof area calculator with the real footprint, before you order materials.
How the estimate works
The roof covers the ground floor, not the whole floor area. A 2,400 sq ft house on two stories has a footprint of about 1,200 sq ft, and a one-story house of the same size has twice that. So the calculator divides the living area by the number of stories, then adds any garage, porch or addition that sits under the same roof.
It then adds the overhang all the way round, applies the pitch multiplier because a sloped roof is larger than the ground beneath it, and adds a small allowance if the roof has dormers, valleys or several levels.
A worked example
A one-story house of 2,000 sq ft with a 400 sq ft attached garage has a roofed footprint of 2,400 sq ft. With a 12 in overhang that grows to about 2,600 sq ft, and at a 6 in 12 pitch the roof comes to roughly 2,910 sq ft, or about 29 roofing squares. A two-story house of 2,400 sq ft at 8 in 12 with a few dormers lands near 1,780 sq ft, about 18 squares. The same floor area gives a much smaller roof once it is stacked.
Why it is only an estimate
House size does not say what shape the walls make. The calculator assumes a rectangle one and a half times as long as it is wide, which is close for many houses and wrong for a long ranch or a deep, narrow rowhouse. Cut-up roofs with many planes carry more area than their footprint suggests, and the shape setting only approximates that. Expect the answer to be good to about 10 percent for an ordinary house, which is why the result shows a low and a high figure.
Before you order anything
Use this figure to compare quotes and plan a budget, not to place a materials order. For that, measure the footprint and pitch and use the roof area calculator, or work out each part of a complicated roof with the multi-section calculator. Add a waste allowance on top: about 10 percent on a plain gable, and more where there are hips and valleys.
Frequently asked questions
Why is the roof area bigger than my square footage?
The overhang adds area on every side, and a sloped roof is longer than the ground it covers. Together they usually make the roof 10 to 30 percent bigger than the footprint, even before a garage is counted.
Does a two-story house have twice the roof?
No. The roof sits over the ground floor, so a two-story house has about half the roof area of a one-story house with the same total floor area. The second floor sits under the first floor's roof line.
How accurate is a roof size estimate from square footage?
Within about 10 percent for an ordinary house. It is least reliable for long, narrow houses and for roofs with many dormers and valleys. Measure the roof, or use satellite imagery or an inspection, before you order materials.