How To Measure A Roof For Shingles

Whether taking on a roofing job on your own or hiring someone else to do it, knowing how many shingles you’ll need for the job is key. To figure that out, one needs to measure the dimensions of their roof. While this may be a simple task for a small shed, determining the square footage of a roof that’s 20 feet off the ground can be daunting.

While climbing onto the roof with a tape measure is the most straightforward way to calculate a roof area, doing so isn’t always feasible or safe. Fortunately, there are many options for measuring the square footage of a roof. Many of these methods can be conducted with your feet firmly on the ground or even from a computer with the help of online resources.

In this article, we’ll discuss how to measure a roof for shingles from both the roof or the ground. We’ll also take you through how to use these measurements to determine how many shingles you need to buy for your roofing project.

How To Measure A Roof For Shingles

How to Measure a Roof for Shingles

There are numerous ways to measure roof area. While physically measuring the square footage by getting on the roof is the most accurate way to take measurements, sometimes it simply isn’t possible to walk on a roof.

Most roofers consider a roof with an 8/12 pitch or greater to be too steep to walk on. One needs specialty tools such as roof jacks and scaffolding to safely work on a steep roof. Fortunately, there are other options that don’t require you to get on the roof.

Method 1:

Using rope, a good throwing arm, and a little ingenuity, it’s possible to take fairly accurate roof measurements without pulling out the extension ladder. Begin by measuring the length of the exterior walls of the home parallel to the ridge. Make sure to include the roof eaves in your measurement.

Next, throw one end of your rope over the ridge, pull it tight and mark where the rope meets each ridge with a market or tape. Remove the rope, spread it out on the driveway, and measure the distance between each mark to get the total width of the two roof planes.

Multiply that number by the length to get the total square footage of the roof. For example, if the length of the roof is 40 feet and the length between the two marks on the rope is 80 feet, then the roof is 1,200 square feet (80×40).

Method 2:

Johnson Level & Tool 750 Pitch and Slope LocatorThe second method involves more complicated math than the rope method. You’ll also need roof pitch measurement tools, an affordable tool available at most home improvement stores. Use the roof pitch tool to calculate the roof slope. Numerous apps allow you to use your phone to find roof pitch, some of which are free.

As in method one, measure the exterior wall plus the length of the eaves to get the measurement for the length of the roof. Next, measure the width of the roof, remembering to include any overhangs.

Multiply length times width to get the total plane area of the roof. For a home that’s 60 feet long and 40 feet wide, you would have an area of 1,200 square feet (40×60).

You still need to account for the roof’s slope, which adds additional surface area. Use the chart below with the reading you took with the roof gauge to determine the roof’s slope factor.

Multiply the slope factor times the plane area to get the total roof area. For example, a 40×60 roof with a 4/12 pitch would have an area of 1,260 square feet (40x60x1.05).

Method 3:

If you’re installing a new roof or replacing the shingles and underlayment on an old roof, you can use the sheet count method, which is fast, accurate, and doesn’t require you to climb on the roof.

Roof sheathing is made up of 4-foot by 8-foot pieces of plywood. This means each full sheet is exactly 32 square feet. To determine the total square footage of the roof, simply count how many sheets are on the roof and multiply that number by 32. A roof with 50 sheets would have an area of 1,600 square feet (32×50).

Estimate the square footage for sheets cut down to size to fit edges or around dormers. For example, a half-sheet would be about 16 square feet, while a quarter sheet would be 8 square feet.

Method 4: Roof Measuring App

These days it seems like there is a smartphone app for everything, so it should come as little surprise that there are apps for measuring the square footage of a roof. While these apps are very powerful, most are also really expensive, limiting their use mainly to professionals. However, if your project involves a more complex roof, it could be worth it.

Apps such as RoofSnap and iRoofing use drone imagery to provide measurements of roofs from a bird’s eye view. This is especially helpful for larger structures with complicated rooflines that would be difficult to measure accurately using one of the above methods.

Subscriptions to RoofSnap start at $99 per month, while iRoofing memberships start at $129 per month. Given the total cost of a roof, that price tag might be worth it for those looking to purchase a single-month membership for estimating the number of shingles needed for a large, complicated roof.

The money you save in supplies with an accurate measurement might actually pay for the software.

How to Measure a Roof Using Google Earth

How to Measure a Roof Using Google Earth

If you’re looking for a free way to measure your roof using imagery you can find on a smartphone or computer, consider logging into Google Earth and searching for your home. If you live in a large enough metropolitan area, you’ll find both the 2D and 3D imagery needed to take roof measurements online.

Begin by entering your address into Google Earth’s search function. You’ll get a birds-eye 3D view of the property, which will allow you to get a good view of the roof pitch. Position the view, so you’re looking down on the roof.

Use Google Earth’s area measurement tool to outline the section of roof you want to measure, making sure to set the display to feet. Once you create a box, Google will tell you the length of each side and the total square footage.

You can use that number with the slope factor of your roof (discussed below) to come up with a fairly accurate estimate.

This method isn’t as precise as the expensive apps, but it will give you a good enough estimate of the square footage of your roof to purchase shingles.

How to Measure a Roof for Shingles From the Ground

Remember when your high school algebra teacher said you needed to know how to calculate slope. He may have been thinking about measuring the square footage of a roof.

This method allows you to calculate roof slope and determine the roof area while keeping both feet firmly on the ground.

Begin by measuring the distance from the edge of the eave to the point at which you can barely see the slope of the roof. This works best if you can attach the tape measure to the eve, then walk to the appropriate place and take the measurement at eye level.

This measurement is the horizontal run of the roof. Now, stand under the gutter of the roof you are measuring the slope for and measure the distance from your eye to the roof overhang. This is the roof rise.

Divide the roof rise by the horizontal run to get your roof pitch. For example, if the rise is 60 inches and the run is 120 inches, you’ll have a roof pitch of 6/12.

As in the method described above, measure the length of the roof from the edge of one eave to the other and the width from one eave to the other. For example, a roof that measures 60 feet by 40 feet would be 2,400 square feet.

Next, use the table below to determine the slope factor. In our example, the 6/12 roof would have a slope factor of 1.118. In our above example, you would multiply 2,400 square feet times the slope factor to get the total square footage of 2,683 square feet.

How to Calculate Square Footage of Roof for Shingles

The most precise way to calculate the square footage of the roof for shingles is by taking exact measurements. This is also one of the riskier methods, as it involves you climbing up on the roof with a tape measure and physically measuring each plane of the roof. If the roof is 8/12 or steeper, do not attempt this method!

Once on the roof, use a tape measure to take measurements of the length and width of each plane of the roof. Multiply the length and width of the roof to get a measurement of each plane’s total area. For example, a roof with a single plane that is 50 feet long by 30 feet wide has a total square footage of 1,500 square feet.

For roofs with two roof planes, such as a gable roof, add up the areas of each plane to get the total square footage of the roof. For example, a roof with two sides that are each 30 by 50 square feet would have the total square footage of 3,000 square feet.

How Do I Measure a Steep Roof?

Most roofers won’t go on a roof with an 8/12 roof pitch or greater with special equipment such as scaffolding and roof jacks and neither should you. Instead, use one of the four methods described above that allow you to measure the roof while standing safely on the ground or from the comfort of your computer.

While these methods may not be quite as accurate as taking actual measurements from the rooftop, they’re close enough for calculating how many shingles you’ll need to purchase for a new roof.

How Do I Calculate Roof Pitch?

While using a roof gauge or the sight method described above are two ways of measuring roof pitch, if you have an attic, there’s an even easier method. Head to the attic with a level and tape measure and choose a rafter.

Mark a length of 1 foot on the level. Hold one end of the level against a rafter so that it is sticking straight out and perfectly level. Measure upward from the level to the rafter. That measurement over 12 is the pitch of your roof.

For example, if the vertical measurement from the level to the rafter is 6 inches, the roof has a 6/12 pitch.

If you have a smartphone, you can also download one of several free roof pitch apps, such as Pitch Gauge. These apps allow you to determine the roof pitch simply by holding the phone against the roof or the rafter.

What Is Slope Factor?

Taking a 2D measurement of a roof either online via Google Earth or by measuring the exterior walls of the home doesn’t account for the elevation of the roof, which adds square footage. Slope factor is a convenient way of using the pitch of your roof to convert a 2D measurement to an accurate measurement of the roof’s square footage.

By multiplying those 2D measurements by the slope factor, you can figure out the actual square footage of the roof. The chart below shows slope factors for common roof pitches.

Roof Slope Slope Factor
2-12 1.01
3-12 1.03
4-12 1.05
5-15 1.08
6-12 1.12
8-12 1.20
10-12 1.30
12-12 1.41
14-12 1.54
16-12 1.67
18-12 1.80

How Do I Measure Odd Roof Shapes?

Measuring a gable roof with its square planes may be easy enough, whether on the roof or standing on the ground, but what about roofs that don’t have square planes? What do you do for roofs with triangle and irregular polygon shapes?

Hip Roof

Measuring roofs for shinglesA hip roof is notable for its four roof planes that come together at the peak. Hip roofs have two shapes. Some have a pyramid shape, in which the roof planes come together at a point. Other hip roofs have a ridge with two triangular-shaped roof planes and an irregular polygon shape.

To measure a pyramid-shaped roof, measure the distance from the center of the eve of one side to the tip of the point. Next, measure the width of one side of the roof (the base of the triangle) from end to end.

Multiply the values together and divide by two to get the square footage of one roof plane. Multiply that number by four to get the square footage of the entire roof. For example, a roof that measures 30 feet from point to eave with a 40-foot-wide base will have the square footage of 600 square feet (30×40/2).

For the second type of hip roof, first measure the triangular-shaped roof planes using the method above. Next, measure the irregularly shaped roof planes.

First, measure the length of an eve that runs parallel to the roof ridge. Next, measure the length of the ridge. Finally, measure the distance from the ridge to the eve.

Add the length of the ridge to the length of the eave, then multiply by the distance from the ridge to the eave and divide by two.

So, a roof plane with a ridge that’s 30 feet long, an eave that’s 40 feet long, and a ridge to eave distance of 50 feet will have the total square footage of 1,750 square feet (30+40) x 50/2.

Mansard Roof

How to figure shinglesCalculating the square footage of a mansard roof can be a challenge as it involves five roof planes. Start with a flat rooftop by measuring the length and width and multiplying the two values together to get the square footage.

Then work on the four irregularly-shaped sides. Measure the length of the eave and the ridge of one side. Then measure the distance between the eave and the ridge. Add the ridge length to the eave length. Multiply that value by the distance between the ridge and eave and divide by two to get the square footage.

Repeat that process for the remaining sides if they aren’t identical. Add all five values together to get the total square footage.

Dormer Roof

How to calculate shinglesDormers can make measuring a roof more complicated. Begin by measuring the length and width of the roof plane that has the dormers. Multiply the two values together to get a square footage total for the roof plane.

Next, measure the length and width of the areas of the roof from which the dormers protrude. Calculate the area of each, then subtract that from the total area of the roof.

For example, if a 30- by 40-foot roof has an area of 1200 feet and each dormer takes up 40 square feet, the total would be 1,120 square feet.

Now, measure the two planes that make up the roofs of each dormer. For example, if a dormer has a roof with two planes that measure 5 feet by 3 feet each, it would have a roof size of 30 square feet (5x3x2).

Two identical dormers on the same roof plane would double that to 60 square feet. Add that number to 1,120 square feet for a total of 1,180 square feet for that roof plane.

Complex Roofs

If nothing else, those many hours spent learning algebra and geometry in high school have prepared you for measuring the roof of your home.

By understanding how to calculate the area of a square, triangle, rectangle, and irregularly shaped polygons, described above, you can estimate the square footage of virtually any roof by breaking it up into these shapes.

When working with complex roofs with complicated shapes with dormers and hip roofs, print or draw a diagram of the roof. You can use that diagram to break up the roof into measurable shapes and label the measurements.

How Do I Calculate How Many Shingles I Need?

How to figure roof shingles

Once you know the square footage of your roof, you can figure out how many shingles you need to buy. First, it’s important to understand that roof shingles are sold in bundles. To determine how many bundles you need, you must convert that square footage total you so carefully calculated into a roofing square.

A roofing square is equal to 100 square feet of roof. So, a 2,700-square-foot roof is 27 roofing squares (2700/100=27). Next, you must determine how many bundles of shingles you need to cover the roof.

There are three bundles in each square of roofing, so a 2,700 square-foot roof would require 9 bundles (27/3=9). So, for a 2,700 square-foot roof, you should purchase 9 bundles, right? Not so fast.

How Much Waste Should I Account For?

As with many construction projects, you need to account for material that is wasted. For roofs, this means shingles that must be cut to account for corners, edges, and walls. You’ll also want some leftover shingles in reserve for repairs further down the line.

With that in mind, add 10 to 15 percent to your shingle total to account for waste and future repairs. For complicated roofs, go with 15 percent as they will have more waste. So, for a 2,700-square-foot roof, it’s wise to purchase 10 bundles.

Conclusion

Figuring out the square footage of your home’s roof for shingles can at first seem like a challenge. Fortunately, there are numerous methods for making this calculation, ranging from climbing on the roof with a tape measure to taking measurements of your home via satellite imagery on Google Earth.

Once you know the square footage of your home’s roof, calculating how many bundles of shingles you need is easy.

When deciding on a method for measuring your roof, always put safety first. If your roof is 8/12 or above, do not attempt to climb on the roof to take your measurements. If the pitch of your roof is low enough to allow you to safely walk on it, remember to take proper safety precautions by wearing the proper footwear and using a stable ladder.

Leave a Comment