How to measure your roof with Google Earth
Using your computer to get a ballpark estimate of your roof area is easy and fast. However, using satellite imagery may not be an ideal solution if your structure is in a very remote location that hasn’t been 3D scanned. That’s because we’ll need to grab an accurate representation of your roof’s pitch for measuring purposes. From above with a 2D image, everything looks flat. If you were to just measure your roof area from directly above, you wouldn’t be accounting for its pitch, or slope.
Consider two homes with the same base area square footage, but one of them has a much steeper roof pitch than the other. That structure would need to be taller, and there would be a lot more roofing material to account for. If you’re just measuring from above in 2D, both of these structures might look identical. If you don’t account for roof pitch, you’re going to underestimate your materials requirements, which could be a costly mistake.
Satellite technology has progressed a lot in the past few years. With Google Earth, you can easily find your address and a 3D replica of your structure, including your roof’s pitch. Let’s get to it. Once you load the site, you’ll be presented with our beautiful planet:
Image source: Google Earth
Click the search icon in the upper left corner of the screen and enter your address. You’ll get a birds-eye 3D circling view of your property. You can press the spacebar to stop the rotations.
While you can switch to 2D mode by clicking the button in the lower right, you’ll want to stay in this 3D view so you can get a good view of your roof’s pitch.
Use your mouse to click and drag, and hold down the shift key to easily rotate the view around your property. Once you’ve got a good frame of reference, take a screenshot and print it out, or copy it into a program such as Google Slides or Powerpoint, so you can easily mark up the dimensions of the various surfaces you’re about to measure.
Now’s the time to start measuring. Click the ruler at the bottom of the left sidebar. Google Earth will assume you want to be in 2D mode for this, but we don’t want to be here, since we’ll get an inaccurate representation of your roof area. To fix this, click the small circle in the lower right corner which now says, “3D”. Also, click the “0 m” in the upper right corner of the screen to change the display to “feet”.
Start by focusing on one roof slope or shape at a time. Click one of the edges of one of your roof surfaces, move to another edge, and keep clicking on the perimeter to close the shape. You’ll be presented with a measure for the perimeter and the area. We’re interested in both. Again, you’ll need to click on the area measurement box in the upper right to change the units to “square feet” from “square meters”.
Note the length of each of the sides of every shape on your roof in your printout or Powerpoint. Also, note the area calculations. Add up all of your roof shape areas, and you’ll have your total estimate for square feet of roofing materials needed.
Roofing material is meted out in “squares”. One square is 100 square feet. So, for a roof with a total square footage of 4,200 square feet, you’ll need 42 squares of shingle material.
To be sure these measurements are accurate, you might want to measure the width of your driveway with an actual tape measure and compare it to what Google Earth tells you its distance is.
The purpose of using Google Earth to measure your roof really should just be for a rough estimate. The numbers are not going to be 100% accurate, which is especially true if trees are in the way of your roofline, or Google has had a difficult time determining where your 3D structure ends.
While the display is impressive, many of the structures still have splotchy 3D representations. When you use the measuring tool, you may become frustrated if your roofline is splotchy as Google will include a lot of unnecessary distance in your calculations because of all the noise. If your roof has a few dormers, your frustration could be compounded due to the added complexity.
How to measure your roof area from the ground
A more accurate way to calculate the area of the roof than using Google Earth is to get outside to estimate the roof pitch and the base area of the property. Using these two figures, we can get a good idea of the shingle roof’s square footage and estimate your asphalt shingles’ needs and costs. This DIY technique can be helpful, especially if you are not comfortable getting on your roof, or if you have limited access to it.
Ground measures only work well for a gabled roof, since there are usually just two main rectangular pitches to calculate area for. For more complex roof shapes, like hip roofs, you’ll need to work with a roofer or measure from on top of the roof itself to calculate its total area.
Calculate roof pitch from the ground in three quick steps
Roof pitch is one of those things that middle school math and geometry instructors can easily point to, and say, “See? Learning how to work with ‘rise over run’ is fun and useful, right?” If at any moment you start having PTSD flashbacks to algebra or geometry class, take a deep breath. We’ll walk you through this slowly in three simple steps. All you need is a tape measure, and possibly a helper to hold it.
Step One: Grab that tape measure, notepad, and pencil and head outside. Find the slope of your roof you’d like to measure, and start measuring the distance from the outer edge of the eave to the point at which the plane of the roof slope is barely visible to your eye. Write that number down in both inches. This figure is the “horizontal run”.
Step Two: Stand directly underneath the gutter or edge of the roof plane you want to calculate the slope for. Measure the distance from your eye to the top of the drip edge of your roof overhang. Write that number down in inches, too. This figure is the “roof rise”.
Step Three: Take the “roof rise” figure in Step Two and divide it by the “horizontal run” from Step One. In the example above, the rise is 60 inches and the run is 120 inches. This reduces down to a roof pitch of 6/12. We’ll use this figure in just a bit, so keep it handy.
Measure the base area of your property
Now, use that tape measure again and take the distance from the edge of one of the eaves to the other edge along the same wall. Record that figure in feet. That will be the roof base width.
Then, measure the distance of the other side of the structure from the edge of the eave to the other. That will be the roof base length.
Multiply those figures together to get an estimate of the base area for the roof structure. For example, if your roof base width is 35 feet and your roof base length is 60 feet, the total base area would be 2,100 square feet.
Calculate the square footage of the roof area with the roof pitch and base area
Find your roof pitch from the first step in the following table and multiply the amount you just got by the corresponding roof pitch multiplier. The result is the total roof square footage you’ll need to get roofing material for.
In our example, the roof pitch was 6/12. The pitch multiplier would be 1.118. So, take the 2,100 square feet, multiplied by 1.118 to get 2,347 square feet of roofing material. Since roofing material like shingles come in squares of 100 square feet apiece, we’d need at least 24 of them. If you have a relatively flat roof (< 3 in 12), you can simply avoid using a pitch multiplier and make sure to round up on your equipment estimate.
Roof Slope (Rise-in-Run) |
Roof Slope (In Degrees) |
Roof Pitch Multiplier |
3-in-12 |
14.04 degrees |
1.031 |
3.5-in-12 |
16.26 degrees |
1.042 |
4-in-12 |
18.43 degrees |
1.054 |
4.5-in-12 |
20.56 degrees |
1.068 |
5-in-12 |
22.62 degrees |
1.083 |
5.5-in-12 |
24.62 degrees |
1.100 |
6-in-12 |
26.57 degrees |
1.118 |
6.5-in-12 |
28.44 degrees |
1.137 |
7-in-12 |
30.26 degrees |
1.158 |
7.5-in-12 |
32.01 degrees |
1.179 |
8-in-12 |
33.69 degrees |
1.202 |
8.5-in-12 |
35.31 degrees |
1.225 |
9-in-12 |
36.87 degrees |
1.250 |
9.5-in-12 |
38.37 degrees |
1.275 |
10-in-12 |
39.81 degrees |
1.302 |
10.5-in-12 |
41.19 degrees |
1.329 |
11-in-12 |
42.51 degrees |
1.357 |
11.5-in-12 |
43.78 degrees |
1.385 |
12-in-12 |
45 degrees |
1.414 |
How to measure your roof from the top of the structure
The most accurate way to measure your roof is by being on top of the roof itself. You can make sure each roof section’s slope area is accounted for. You’ll need some graph paper, measuring tape, and a calculator. For our example, we’ll use a hip roof, though you can use this technique to measure any shape roof with varying complexity.
Step one: get up on our roof safely and take initial measurements
Sketch out the shape of your roof on graph paper. In this example, you’d only need to measure the overall width and length, the central ridge, and one of the hips (marked above with blue stars). If you have a more complex roof shape with dormers, take all the measurements you can and prepare to use your eraser to refine the image of your overall roof structure. Make sure you have solid footing and if possible, use fall protection to keep yourself safe. Be mindful of hazards like ridge vents. Note the lengths in feet:
Step two: split tricky shapes into squares and rectangles
Once you have these figures, return to the safety of your home. Notice in the image above that the hip roof is made of two triangles and two trapezoids. Trapezoids are tricky to get the area measurement of, so we’re going to split each of them into two triangles and a rectangle:
We know enough about the overall measurements of the roof to fill in the missing values. Let’s fill the figures in now:
Step three: draw each shape individually, calculate each area, and the total square footage of your roof
Now we have eight shapes to find the area of – six triangles and two rectangles. Instead of breaking out the old textbooks to calculate triangle areas, we like to use a triangle calculator. It makes figuring out the area of triangles a lot simpler.
In the example above, we have two triangles with sides of 30, 20, and 20 feet. Pop those figures in the three-sides calculator and you’ll get an area of 198. Since we have two of those triangles, multiply by 2 for a running total of 396 square feet.
We have four remaining triangles, all with dimensions of 15, 20, and 25 feet. That same calculator reveals a result of 150 square feet. Since there are four of those triangles, multiply by 4 to get 600 square feet.
Lastly, we have two rectangles of 15 by 30 feet. This one is easy, just width times height for a total of 450 square feet for each one. Multiply by 2 and their total square footage is 900.
Adding all the areas up yields a total of 1896 square feet (396 + 600 + 900). Since roofing material comes in 100 square foot “squares”, you’ll need at least 19 of them for your roofing project. It’d be wise to get at least 20 to account for mistakes and waste.
Final thoughts on measuring your roof
If you’re looking for a simple ballpark estimate of your roofing materials needs, Google Earth really does a great job of quickly allowing you to measure the square footage you’ll need to cover. While its technology sure is surreal, it has drawbacks in its ability to precisely measure distances represented in 3D. Therefore, it can be helpful to gauge estimates of your materials costs by measuring from the ground and on top of your structure. As always, if you’re feeling confused, don’t hesitate to get in touch with a local roofing contractor to help you out. Many will provide estimates for free. Simply ask how many squares of material they estimate you’ll need, and compare that to the results you got from using any of the methods we outlined above.
Dan is a home improvement journalist and content advisor with Fixr.com. He also works with solar installers and solar nonprofits. Dan earned his B.S. in Financial Management from the University of Illinois in 2001 and his M.S. in Industrial/Organizational Psychology from Portland State University in 2008.