Roofing Material Calculator

How to Estimate the Size of a Roof

If you’re replacing a roof, one of the first considerations you make needs to be the roof’s size so you can accurately estimate project materials.

In the US, roofs are measured in square footage, and roofing contractors typically quote projects based on the size of the roof in squares, which are equal to 100 sq. ft. If your roof measures 2,300 sq. ft, then you would need 23 squares of material.

Find the Area of the Footprint of the Roof

To calculate a roof’s dimensions, you need to measure the roof as if it were flat, then account for roof pitch, calculate square footage, and finally determine how many squares of material are required for the roof.

Keep in mind that you will need to do this for each section of the roof. While the illustration above shows a simple gable roof, a cross gable roof will require you to take the length and width of each individual section, and complex roofs like gambrels and mansards will have two sections of roofing to each side.

The calculator above can handle all of this; simply enter the length, width, and pitch, and it will determine the size of the roof in squares.

Finding the Total Roof Area

Once the area of the roof’s footprint is known, the overall roof area can be found by accounting for the roof’s pitch. The pitch of the roof is the rise over a 12-inch run.

This means that for every 12 inches horizontally, your roof will rise a specific number of inches. Most roofs will fall between a rise of 4/12 and 8/12, but a gambrel roof will likely have a section that is 20/12 and a section that is 7/12. This can mean that you need to calculate for each area separately.

It’s also important to keep in mind that each section of roofing may have its own pitch. This is true even of gable roofs, which may be what is known as a dual-pitch gable – one side of the roof will have a different pitch than the other side. You will need to calculate each side separately in this case.

Use our roof pitch calculator to find the pitch of your roof.

Next, multiply the footprint of the roof by the multiplier below for your roof pitch to find the overall roof area.

Snow load is the additional weight on a roof structure added by snow and ice buildup on the roof. Calculating the snow load is crucial to determining if the structure can handle the snow’s additional weight.

The ATC has a snow load hazard tool that can help you identify when a snow load presents a hazard to the structure.[1] You can calculate snow load in a few easy steps.

Step One: Calculate the Volume of the Snow on the Roof

The volume of snow on the roof directly relates to how much it will weigh. To find the snow volume, start by measuring the roof’s footprint.

Measure the Roof

We strongly recommend against walking on the roof to take these measurements for obvious safety reasons. Instead, measure the length and width roof from the ground to find the footprint; we’ll account for the pitch later. Keep all measurements in feet to simplify the formulas.

When you have the roof’s length and width as measured from the ground, multiply them together. This will give you the area of the footprint. Alternatively, you can also use a volume calculator to find this measurement.

Account for Roof Pitch

If you measured from the ground and did not measure the roof’s actual dimensions, then it’s time to account for the roof pitch. If you don’t know your roof pitch, then try our roof pitch calculator to find it.

To account for the pitch, you’ll need to multiply the area by the multiplier for the given roof pitch. You can find a list of multipliers on our roofing calculator. The calculator above applies this formula. If the roof is a flat roof, then this step is not needed..

Before moving on, multiply the area by the snow depth in feet to find the volume of snow on the roof.

Step Two: Find the Snow Density

Snow varies in weight depending on the density. Fresh powder weighs much less than wind-packed drifts. Check out our snow weight calculator to find the density of various types of snow.

Step Three: Calculate Snow Load

The final step in calculating the snow load is to multiply the volume of snow on the roof by its density. If you have a density range, then multiply the volume by each part of the range separately to find the minimum and maximum snow load.