Measure the footprint of your house, then account for roof pitch
The square footage of a roof is not the same as the square footage of your house. A roof that slopes upward covers more area than the flat ground beneath it. To find the actual roof area, you need two numbers: the footprint of your house (length times width) and the pitch of your roof (how steeply it slopes). Once you have those, a straightforward calculation gives you the real square footage a roofer will charge you for.
Most homeowners need this number when getting repair or replacement quotes, ordering materials, or understanding what contractors are charging. The method works the same whether your roof is a straightforward gable (two slopes) or more complex, as long as you break it into sections and measure each one.
Key Takeaways
- Roof square footage is always larger than your house's footprint because sloped roofs cover more area than flat ground.
- You need the length and width of your house and the pitch of your roof (expressed as rise over run, like 6:12) to do the calculation.
- Multiply your house footprint by a pitch factor: a 4:12 pitch uses 1.054, a 6:12 pitch uses 1.118, and a 12:12 pitch uses 1.414.
- For complex roofs with multiple angles, measure each section separately and add them together.
- If you cannot safely access your roof, a contractor can measure it for you, usually at no charge if you get a quote from them.
Gather the measurements you need
Start by finding the length and width of your house at ground level. Measure the outside walls if you can, or use your property survey or home inspection report — both usually list the house dimensions. Write down the length and width in feet.
Next, you need the pitch of your roof. Pitch is written as a ratio like 4:12 or 6:12, which means "4 inches of rise for every 12 inches of horizontal run" (or 6 inches, depending on your roof). You can find this on your home inspection report, your original building plans, or by measuring it yourself. To measure pitch yourself, go into your attic with a level and a tape measure. Place the level horizontally on a rafter, then measure straight up from the end of the level to the rafter above it. That vertical distance, compared to the 12-inch length of the level, is your pitch. A roof that rises 6 inches over 12 inches is a 6:12 pitch.
If you cannot access your attic safely, you can estimate pitch from the outside by looking at the roof angle, but a measurement is more accurate. Many contractors will measure the pitch for you when they come to give a quote.
Calculate the pitch factor for your roof
Once you know your pitch, convert it to a decimal number called the pitch factor. This number tells you how much larger your roof is than your house footprint. Common pitch factors are listed in the table below.
| Roof Pitch | Pitch Factor |
|---|---|
| 3:12 | 1.031 |
| 4:12 | 1.054 |
| 5:12 | 1.083 |
| 6:12 | 1.118 |
| 7:12 | 1.158 |
| 8:12 | 1.202 |
| 9:12 | 1.250 |
| 10:12 | 1.302 |
| 12:12 | 1.414 |
If your roof pitch falls between two of these, round to the nearest one. A 5:12 pitch is close enough to 6:12 for most purposes, and the difference in your final number will be small.
Multiply to find your total roof area
Now do the math. Take your house footprint (length × width) and multiply it by the pitch factor.
Example: Your house is 40 feet long and 30 feet wide, so the footprint is 1,200 square feet. Your roof pitch is 6:12, so the pitch factor is 1.118. Multiply 1,200 × 1.118 = 1,341.6 square feet of roof.
This number is what a roofer will use to estimate the cost of materials and labor. Roofing is usually quoted per "square," which is 100 square feet. In the example above, you would have about 13.4 squares of roof.
Handle roofs with multiple slopes or sections
If your roof has different pitches on different sides, or if it has valleys, dormers, or other complex shapes, measure each section separately. Break the roof into rectangles or triangles, find the footprint of each section, multiply by its pitch factor, then add all the sections together.
For a straightforward gable roof (two equal slopes), you can measure the whole footprint and multiply once. For an L-shaped house, measure the long section and the short section separately, explore the pitch factor to each, and add them. A roof with a steep section and a shallow section requires the same approach: calculate each pitch separately.
If your roof is very complex, a contractor's measurement is worth the time saved. Most will measure for free when they visit to give you a quote.
When to ask a contractor to measure instead
You do not need to measure your own roof. If you are uncomfortable on a ladder, cannot access your attic, or have a complex roof shape, call a roofing contractor and ask for a measurement. Most will do this at no charge as part of the quote process. They have tools to measure pitch accurately and can account for valleys, dormers, and other features you might miss.
If you are getting multiple quotes, each contractor should measure independently. If one quote is much larger or smaller than the others, ask that contractor to walk you through how they arrived at the number. A difference of 5 to 10 percent is normal; a difference of 20 percent or more suggests one contractor may have made an error.
Frequently Asked Questions
Does roof square footage include the overhang?
Yes. The overhang is the part of the roof that extends beyond the walls of your house. It is part of the roof area and should be included in your measurement. If your house is 40 feet long but the roof overhangs by 2 feet on each end, use 44 feet as your length for the calculation.
What if I have a flat roof?
A flat roof has a pitch of 0:12, so the pitch factor is 1.0. Your roof square footage equals your house footprint exactly. Measure the length and width of the roof edges, multiply them, and that is your answer.
Can I use Google Earth or satellite images to measure my house?
Satellite images can give you a rough footprint, but they are not precise enough for a roofing quote. Use them as a starting point, then verify with a tape measure or your property survey. Pitch cannot be determined from a satellite image at all, so you will still need to measure or ask a contractor.
Why do roofers charge by the square instead of by square footage?
A "square" of roofing is 100 square feet. Roofers use this unit because it matches the way roofing materials are bundled and sold. It also makes pricing easier to compare across different jobs. If your roof is 1,341 square feet, that is 13.41 squares, which a roofer might round to 13.5 or 14 depending on their pricing method.
What if my measurements do not match what a contractor says?
Small differences (5 to 10 percent) are normal and come from how you round pitch factors or measure the edges. Larger differences usually mean one of you measured the pitch wrong or did not account for overhangs or dormers the same way. Ask the contractor to show you their measurement and explain any features you missed.