How to Estimate Roof Size

When replacing your roof, the first thing to consider is the size of the roof so you can accurately estimate the materials needed for the project.

Measuring in Square Feet

In the United States, roofs are measured in square feet, and roofing contractors typically quote prices based on the square footage of the roof. If your roof area is 2,300 square feet, you will need materials for 23 squares (each square is equivalent to 100 square feet).

Collecting Square Footage and Square Numbers

There is no direct correlation between the size or square footage of a house and the size or square footage of its roof. A 2,000-square-foot house may have a simple gable roof, a complex cross-gable roof, or a hip roof, and each will have a different size.

Finding the Roof Footprint

To calculate the roof size, you need to measure the flat roof area, consider the roof pitch, calculate the square footage, and finally determine how many squares of material are needed for the roof.

Remember, you need to do this for each section of the roof. While the illustration above shows a simple gable roof, a cross-gable roof requires you to measure each section's length and width, and complex roofs (like mansard and gambrel roofs) have two sections each.

Using a Calculator

The calculator above can handle all of this; just input the length, width, and pitch, and it will determine the square footage of the roof.

Measuring the Roof's Length and Width

Stand on the ground and measure the length and width of the roof (in feet). Remember, the length and width are for the roof, not the house, so consider any overhang.

Calculating the Roof Area (Assuming Flat)

Multiply the length by the width (in feet) to get the area measurement.

Finding the Total Roof Area

Once you know the roof's footprint area, calculate the total roof area by accounting for the roof pitch. The roof pitch is the amount of rise over 12 inches horizontally.

Most roofs have a pitch between 4/12 and 8/12, but complex roofs may have one section at 20/12 and another at 7/12. This means you need to calculate each area separately.

Also, remember that each section of the roof may have its own pitch. Even a gable roof can be a so-called dual-pitch gable where one side of the roof has a different pitch from the other. In this case, calculate each side's pitch separately.

Using the Roof Pitch Multiplier

Multiply the roof footprint area by the roof pitch multiplier below to find the total roof area.

For example, a 4/12 pitch roof area of 100 square feet:

100 × 1.054 = 105.4 square feet

What is the most commonly used type of roofing material?

Asphalt shingles are commonly used as roofing materials due to their low cost, ease of installation, and ability to withstand various weather types.

What is the most affordable roofing option?

The cheapest roofing material is roll roofing, with an average cost of $1.50 to $2.00 per square foot.

Which type of roof is the easiest to install?

Metal roofing is not only the easiest to install but also the fastest to install.