Repairing or replacing your roof can be expensive, especially if you don’t know how to calculate its square footage. A wrong estimate can cost you thousands of dollars and might even delay your roof repair, leaving your home exposed to the elements. The good news is you can easily estimate the square footage of your roof with a few tools and basic geometry.
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. 
