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How do You Calculate the Size of Your Roof?

How do You Calculate the Size of Your Roof?

If you’re trying to determine how much a roof replacement will cost you, you’ll need to know the size of your roof, as it’s one of the main things that contractors will look at to estimate your costs.

The size of your roof can be pretty difficult to determine when you don’t know the right way to do it. This is because it involves a combination of your home’s square feet, the pitch or slope of your roof, and how far it hangs over the edge of your house’s footprint.

So to help you do just that, we’ll teach you how to find your roof’s pitch, and how to use that to calculate the area of your roof!

Note: Estimating the area of the roof without getting on the roof is only accurate for simpler, rectangular roofs. For more complex roofs, you’ll need to get on the roof and measure each part to accurately determine the size of the roof. Roofing companies also use aerial imaging tools to correctly measure the size for a roof replacement, so they will be able to give you a good measurement in the cost estimate for your project.

With that being said, let’s get started!

4 Steps to Calculate the Area of Your Roof
1. Find the Length and the Width of Your Home’s Exterior Walls

Rotated Floorplan_WebP

The first step to find the area of your roof from the ground is by measuring the length and width of your home’s exterior walls. Once you’ve done this, you’ll also need to account for and estimate the length of any overhangs on each side of the house. Add those extra feet to the measurements you made for the length of your walls, and multiply the length and width to find the “footprint” of the home. For example, if we had a home that was 50 ft long and 60 ft wide, our footprint would be 3000 sq ft.

2. Calculate Your Roof’s Pitch

measuring roof pitch


The next step is to calculate your roofs pitch. For this, you’ll need to get up on a ladder that can reach the edge of your roof, and to bring a level and a ruler up top with you.

The pitch of your roof, also known as the slope, is a measurement that indicates how steep the roof is. The way to find this measurement is by placing the level a couple of feet in from the edge of the roof, holding it so that it is level, and measuring from the edge of the level, in towards the roof.

The number of inches you measure there goes into the “__-in-12" format that we use to discuss pitch, using a colon to to represent the “in.” So for instance, if you measure 6 inches, you would write the pitch as 6:12.

3. Use the Pitch Multiplier Table

In order to properly use the pitch of your roof to determine the area, you need to use a tool called the “pitch multiplier table.” This table, shown below, lists the different factors that you multiply by the footprint area to find your roof’s surface area.


Find your roof’s pitch on this table, and take note of the multiplier so you can use it to find your roof’s surface area in the next step. Ex: Our pitch of 6:12 that we got from earlier would give us a multiplier of 1.118.

4. Use the Pitch Factor to Find the Surface Area of Your Roof

This last step is pretty self-explanatory, as you just need to multiply the footprint area, by the pitch multiplier, and you have your result! In our example, that would be 3000 x 1.118 = 3354 sq ft. And there you should have your roof’s estimated surface area!

Bonus: Use the Surface Area to Find Your Roof “Squares”

Roofing contractors often don’t use square footage when measuring roofs, and instead use “squares.” Don’t worry though, this isn’t a hard measurement to get. Just divide your surface area by 100, and boom! You’ve got your roof measured out in squares! Ex: 3354 sq ft / 100 = 33.54 sq.

Need a Roofing Contractor to Find Your Roof's Size?

If your roof is a little more complex, or if you want to get a more exact measurement, reach out to a roofing company!

Reputable roofing companies can give you an accurate measurement either by coming to inspect your roof, or by using that aforementioned aerial imaging tool to measure it.

On Tops Roofing has been inspecting, estimating, repairing, and replacing roofs in the Triangle for over 30 years. If you aren’t sure where to look get in touch with us using the Contact Us button below! Whatever you need, we’re happy to help!

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