How to measure square feet of a roof

Figuring Out the Square Footage of Your Roof

While it may seem difficult at first, you can divide your roof into 10’ x 10’ roofing squares, a standard unit of measurement used by roofers.

But first, some terminology you'll need to know:

• Ridge - Where two sloped roof areas meet (usually the higest point)
• Valley - Where two sloped roof areas meet (usually the lowest point), often with a metal channel running along the fold
• Hip Roof - Type of roof where all sides slope downwards to the walls 
• Hip End - The triangle shaped end of a hip roof

Related: Roofing 101: What Are the Different Parts of the Roof Called?

Get Measurements

You’ll need accurate measurements of your roof before making any calculations. Make a bird’s-eye view sketch of your home and measure the length of its sides. For example, a home with a hip roof is 40 feet long and 30 feet wide, and includes an additional lean-to section that is 7 feet long and 5 feet wide. There are no overhangs, but if there were, you would simply add it their length.

You should outline every plane of your roof and include measurements for any ridges, valleys, or hips. We'll create an imaginary home as an example with 20-foot long ridge along with four 10-foot long hips.

Squares, Rectangles and Triangles

Nearly every roof, no matter how simple or complicated, can be broken down into squares, rectangles or triangles. You can draw lines on your diagram from the ridge to the eaves to make triangles out of the hips. This will leave you with four triangles for the hips, two rectangles for the rest of the hip roof, and another rectangle for the lean-to roof.

Related: 7 Warning Signs You Need a New Roof ASAP

Determine Roof Pitch and Slope Factor

The pitch of your roof can dramatically increase its surface area, so it’s important to take it into account in your estimation. Simply use a 12-inch level to measure how many inches it rises for per foot it runs. This can help you determine your roof’s slope factor. You can use pre-made tables online, or you can calculate it yourself. Simply square the rise and run of your roof’s slope, add them together, then find their square root and divide it by 12.

In this case, let’s assume your roof has a pitch of 3/12, which means for every twelve inches of length, it will rise by two. Your roof’s slope factor would then be equal to (sqrt (9 in + 144 in)) / 12 in, or 1.03078. Most tables will round this to 1.03.

Calculate the Roof Area

Now that you have all of your roof measurements, you can calculate its surface area. Simply multiply the length and width of each rectangular plane, and then add the combined area of the hips. Then multiply this number by the roof’s slope factor, and then multiply this final result by 1.10 to allow for error.

In this example, our imaginary home's hip roof would cover a surface area of (20 ft x 30 ft) + (10 ft x 30 ft), while the lean-to roof would be equal to (7 ft x 5 ft). This gives you a total surface area of 935 square feet. Multiplying this by the slope factor of 1.03 equals 963.05 square feet. Multiply this by 1.10 to obtain 1059.4, which is a good estimate of your roof’s surface area.

Related: As a Homeowner, Can I Deduct My Roofing Repair Costs?

With that, you can estimate the total cost of your roofing supplies or double check against your contractor's estimates. 

The following tools estimate the area of a roof, as well as the amount of materials necessary to construct a roof of a given area. The "House Base Area" is the area of land that the house covers, and for more complex shapes, it can be estimated using the Area Calculator.

Roofing Area Calculator

Roofing Material Calculator

The calculated area is only an estimation. In cases where a roof has a complex shape, such as in the image to the right, measuring the dimensions and areas of each part of the roof to calculate the total area will result in a more accurate measurement of area. The calculator cannot account for complex shapes based on a measurement of square footage alone. The Area Calculator can be used to calculate the area of a variety of simple shapes that together can comprise the area of the roof. Using the aggregate area of these simple shapes can yield a more accurate roof area to be used with the Roofing Material Calculator.

The most common roofing materials used in the United States include shingles, membrane roofing, and ceramic tile, all of which have different life spans. Shingle roofs typically have a life span of 15-30 years, while membrane roofs usually last 5-15 years. Although ceramic tile roofs are expensive, they can have a life span of over 100 years.

Roof Pitch

Roof pitch is the measurement of a roof's vertical rise divided by its horizontal run. It is often compared to slope, but is not exactly the same. In the United States, a run of 12 inches (1 foot) is used, and pitch is measured as the rise of the roof over 12 inches. For instance, a 7/12 roof pitch means that the roof rises 7 inches for every 12 horizontal inches. Outside of the U.S., a degree angle is typically used.

Roof pitch is a determining factor for the cost of the roof, as well as the roof area, and the type of materials used. It affects walkability as well as drainage, and roofs in areas of high rain or snowfall tend to have steeper pitches.

Roof pitch affects the actual area of the roof. Depending on whether the roof area is measured horizontally (possibly from a drawing or photograph), a correction factor is necessary to determine the actual area of the roof. Given pitch and a horizontal area measurement, multiply the horizontal area by a correction factor corresponding to pitch, provided in the table below, to determine the actual area of the roof to be used in the Roofing Material Calculator. While it is possible to estimate the amount of necessary materials using only the total roof area measurement, as can be seen from the table, depending on how large the pitch of the roof, the actual area of the roof can differ by up to 2.236 from the measured total area at a pitch of 24/12. As such, while it can be cumbersome, measuring the area and pitch of each part of the roof and multiplying by the corresponding correction factor will result in the most accurate estimate of necessary roofing materials.

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