aerial roof measurement

Roofing Calculator – How to Measure a Roof

How to Calculate Total Square Footage of Your Roof. To find your roof’s total square footage: Measure the length and width of each plane on the roof (including dormers) then multiply length times width. Add the square footage of each of the planes together.

What is a roofing square?

A lot of different factors determine the cost of your new roof. One of the biggest factors is the size of your roof that’s calculated by a measurement called a “roofing square”.

A roofing square is a 10×10 area that’s equal to 100 square feet of roof surface. The amount of roof squares determines how much roofing materials and labor it takes to replace your roof. 

The more squares your roof has, the more your roof replacement costs. Usually, it takes about 3 bundles of roofing materials (asphalt shingles) to fill a roofing square. 

But it ultimately depends on what type of roofing material you choose for your roof replacement. Some homeowners get confused by the term “roof square” and like to see the cost broken down by square footage. 

If this sounds like you, all you have to do is take the cost per square and divide it by 100. For example, if the cost of your roof is $600 per square, that comes out to $6.00 per square foot. 

If there’s any confusion about the terms you see on your roof estimate, don’t hesitate to ask the roofing contractor to explain them until you fully understand. 

Estimating Roof Area Using Pitch and Square Footage

If you are mathematically inclined or have a roofing calculator, you can estimate the roof area if you know the pitch of your roof and the square footage of your home. To illustrate, if your pitch is 4/12 and your two-story home has 1,400 square feet on each floor, these are the calculations.

• Divide the pitch. For a 4/12 pitch, the answer is 1/3.
• Square the above answer, which in this example would yield 1/9.
• Add 1, expressed as a fraction. That is, you would add 9/9 + 1/9 for a total of 10/9.
• Take the square root of 10/9, which is approximately 1.054.
• Multiply 1.054 by 1,400, the square footage of the home used in this example. You should get 1,475.6.
• Divide 1,475.6 by 100 to determine the number of squares required. You will need at least 14.756 squares of roofing. However, you should add about 10 percent for starter strips, valleys, waste and hip and ridge.

Estimating Materials Based on Actual Measurements

Measuring each roof plane requires you to actually go onto your roof. If your roof has a number of planes and numerous hips, ridges and valleys, this is the most accurate way to calculate the materials needed. Take a metal tape measure and a pen and paper. Measure each plane’s length and width. Record your measurements. Once you are safely on the ground, use your measurements to determine the square footage.

• For rectangular areas, multiply the length by the width. For example, if a plane measures 30 feet by 100 feet, this would be 3,000 square feet.
• For triangular areas, you may need to dust off your geometry book. If the area forms a right triangle, you can make use of the Pythagorean theorem. Otherwise, you might want to look up how to calculate the semiperimeter and then use Heron’s formula to find the area.
• Be sure to add extra for valleys, hips and ridges.

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