How to Measure a Roof

• Measure the length, width, and pitch from the ground with a tape measure. Use a pitch multiplier and calculator to figure out square footage.
• Get up on the roof to take and record precise measurements.
• Draw the diagram using the precise measurements and break the diagram into smaller sections.

Taking Measurements from the Ground

Find the length and width of the building’s exterior walls. Stretch your tape measure along the walls from end to end in both directions. Record these measurements in your notebook. Along with the roof’s pitch, you’ll use these 2 dimensions to calculate the roof’s overall square footage. Once you've measured the building’s walls, estimate the length of the overhangs on either side (if your roof has them). The easiest way to do this is to simply eyeball them and approximate how long each one is in feet. Add these figures to your recorded measurements to account for the additional area. Estimating the area from ground level is less precise than measuring each side individually, but it can make a quick, easy, and safe alternative when redoing a square or rectangular roof. To measure an irregularly-shaped roof, you'll need to get on top.

Calculate the pitch of the roof. The pitch is the steepness of the roof. To find the pitch of your roof, you’ll first need to set up a ladder tall enough to reach it. Place a level against the rooftop a few feet in from the edge and angle it so that the floating bubble is centered, then measure the distance from the end of the level to the surface of the roof. Pitch is expressed as “X-in-12 (inches)”. If you’re using a 1 ft (0.30 m) level, the pitch is the distance between the edge of the level and the roof’s surface. A distance of 7 inches (18 cm), for example, would mean your roof has a pitch of 7-in-12. If you’re using a 2 ft (0.61 m) level, divide the distance between the level and the roof by 2 to get the true pitch. A 14 in (36 cm) difference, for example, indicates a pitch of 7 in (18 cm). The terms “pitch” and “slope” are sometimes used interchangeably when describing roofing projects.

Use a pitch multiplier table to determine what figure to use. Pitch multiplier tables make it easy to use a particular pitch to estimate a roof’s square footage. Run a search for "pitch multiplier table" and look for the measurement of your roof’s pitch in inches on the left side of the table. Write down the corresponding pitch multiplier for use in your following calculations. For example, a roof with a rise of 3-in-12 will have a pitch multiplier of 1.031, while one with a rise of 8 in 12 will use a multiplier of 1.202. You should be able to pull up a number of reliable pitch multiplier tables with a quick Internet search.

Multiply the area by the pitch multiplier to get the roof's square footage. First, multiply the length by the width. Then, take the product of these two dimensions and multiply it by your pitch multiplier. The number you get will be an accurate estimate of how much area you have to cover for your roofing project. If your roof measures 48 feet (15 m) x 24 feet (7.3 m), multiplying its length by its width would give you a square footage of 1,152 square feet (107.0 m). If its pitch is 6-in-12 (a pitch multiplier of 1.12), multiplying 1,152 sq ft (107.0 m) by 1.12 would give you an overall square footage of 1,290 square feet (120 m).

Divide your estimated square footage by 100 to estimate your materials. Roofing materials are bundled in "squares" that contain enough shingles to cover 100 square feet (9.3 m). Dividing 1,290 sq ft (120 m) by 100, for example, will tell you that you need to order at least 13 squares to get the job done. Always order 10% more materials than you think you need to account for waste, and round up to make sure you don't come up short. In the above example, you'd need 15 squares.

Diagramming Your Roof

Find a safe way onto the roof. In order to be able to take precise measurements of your roof, you’ll need to actually be on it. If you have an interior window that provides roof access, use it to step out onto a secure section. Otherwise, it will be necessary to set up an extension ladder and climb onto the roof carefully. Bring a tape measure, pen or pencil, and notebook or pad of paper with you. You’ll need these items to record your roof’s measurements. Make sure your ladder is resting on a flat, stable patch of ground. If possible, have a helper hold it for you to stabilize it as you climb.

Measure each side of the main section of the roof. Extend your tape measure along the outer edges of the roof to find its length and width in feet. For square or rectangular roofs, you’ll only need to record the length and width. For sloped sectional roofs, write down the dimensions of each individual plane. Complete one side at a time before moving on to other areas. You’re just looking for the outer perimeter at this stage. In this context, a “plane” is defined as each flat, continuous section of the roof.

Measure any additional structures separately. If your roof features any architectural elements like hip ridges, valleys, or dormers, don’t forget to find the length and width of these areas, as well. These measurements will factor into your calculations and, therefore, the amount of materials you’ll need to buy. Ridges and valleys are the top and bottom contours where smaller hip sections join the main part of the roof. Dormers are separate protruding structures that house windows in the upper floors of the house. They're ordinarily roofed separately. There’s no need to account for chimneys, pipes, or other irregularities. You or your roofing contractor will work around these areas once the project is actually underway.

Record your measurements precisely. As you take your measurements, jot them down in your notebook or on a separate sheet of paper. You’ll be using these numbers to calculate the total square footage of the roof later on. Write out each measurement exactly as it appeared on your tape measure to the nearest ⁄2 inch (1.3 cm). Don’t round your roof measurements up or down. This will be done once it’s time to determine the number of shingles you need. Rounding before you begin multiplying and adding may throw off the accuracy of your final estimate.

Draw a diagram of your roof on a large piece of paper. Make a rough top-down sketch of your roof. Pencil in the outlying shape, then flesh out your diagram with details like ridge lines, hips, valleys, and dormers until you have a rough representation of your roof’s basic layout. Your diagram doesn’t need to be perfect. All that matters is that you have a visual aid to keep up with the various dimensions you’ll be using to make your final calculations. Use a pencil rather than a pen. That way, you’ll be able to make corrections and revisions as needed while you work.

Use the measurements you took to label your diagram. Write down the length and width of each corresponding section of the roof, starting with the outer edges where the eaves are. Then, put down the length of the ridge lines on each peak, hip, and valley. The idea is to reduce your roof to a series of lines in order to simplify your subsequent calculations.

Divide the roof up into simple geometrical sections. At this point, you may notice that many of your roof’s planes are irregular. The easiest way to get around this problem is to draw a few extra lines to turn complex shapes into simple ones. For example, drawing a line across the tapered end of a gabled roof will create a long rectangle connected to a small triangle. Keep lining off the different parts of your roof until each section is a square, rectangle, or triangle. Breaking your diagram up into smaller sections may seem unnecessarily complicated, but it will actually make your final calculations much faster and easier (unless you want to figure out how to find the area of a trapezoid).

Multiply the length by the width to find the area of rectangular sections. Determining the square footage of these sections is easy. Just multiply the length and width dimensions. Make a note of the square footage of each square or rectangular section on your diagram. If a section is 120 feet (37 m) x 100 feet (30 m), its total square footage would be 12,000 square feet (1,100 m). Be sure to find the area of any dormers on your roof, as well.

Work out the area of the remaining triangular sections. This isn’t as difficult as you might think. First, use a ruler to draw a line from the center of the longest side of the triangle (the eave) to the point (the roof’s peak). Then, multiply the length of the longest side by the length of this central line. Divide this number by 2 to get the square footage of the triangle. The basic formula for finding the area of a triangle is ½ the base times the height (in this case, the distance between the base and the peak). If you have a roof section that's 30 feet (9.1 m) long and 12 feet (3.7 m) high, multiplying those dimensions would give you an area of 180 square feet (17 m). Measure and mark the square footage of each triangular section carefully, as these tend to differ in size.

Add up the areas of each section to get your total square footage. Punch each of the area measurements you deduced into a calculator one by one. The sum of these numbers is the overall square footage of your roof, which you can then use to work out how much material you’ll need for your project. Together, two 750 sq ft (70 m) rectangular planes and four 135 sq ft (12.5 m) triangular planes would give you a total area of 2,040 square feet (190 m). Add up the numbers a few times to ensure that the final figure you get is correct. Take your time and work carefully to prevent mistakes. Even a small miscalculation could end up costing you time or money.

Divide your square footage by 100 to estimate your materials. Roofing materials are typically packaged in “squares,” each of which is the equivalent of 100 square feet (9.3 m) of roof space. Dividing the total area of your roof by 100 will therefore help you figure out how many squares worth of shingles to order. For a 12,000 sq ft (1,100 m) roof, you’ll need a minimum of 120 squares. It’s a good idea to add an extra 10% to your estimated material requirements to account for waste and ensure that you have enough shingles to cover your roof when it’s all said and done. Using the above example, that would be 132 squares.