How to calculate roof area

Installing a new roof is a capital intensive affair that requires you to establish your budget in advance. Thankfully, it is easy to estimate the costs by calculating the roof area and then using the sum to determine the cost of shingles per square foot.

Here’s how to calculate roof area of your home:

Requirements: A strong ladder, large level, tape measure, marker, paper, and pencil.

1.) Measure the pitch

This is the first thing to do when you set out to calculate the area of your roof. Use your measuring tape to measure 12 inches on your large level and put a mark at the 12-inch line.

Place the ladder beside your building at the gable end. Ascend to the top of your roof. Thereafter, place the level against the gable trim and flat against the side of the house.

READ: How to estimate building costs

Use your measuring device to measure from the 12-inch mark on your level all the way to the bottom of the edge of your gable trim. This figure is your rise measurement.

Each rise dimension is inscribed as “# in 12” and this is the pitch of your roof. For example, if your rise measurement is 4 inches, your roof’s pitch is 4 in 12.

2.) Calculate the estimated roof area

When pressed for time, you can easily use your pitch calculation to get an estimated area (in square footage) of your roof.  This is how to do it:

• Take your pitch number and divide it by 12. For example, if your pitch is 4 in 12, you need to divide 4 by 12. The answer will be 1/3.
• Next, you need to square your outcome.  In our example, 1/3 squared will yield 1/9.
• Thereafter, add 1 to your result: 1/9+1=10/9
• Calculate the square root of your new number: The square root of 10/9 is 1.05.
• Next, determine the square footage of one floor of your house. For example, if you have a two storey house that is 2,600 sq. feet, a single floor would be 1,300 sq. feet.
• Multiply this figure by your most recent sum to get the estimated roof area: In our case, the estimated roof area will be 1,300 x 1.05 = 1,365 square feet.

3.) Calculate the exact roof area

Follow these steps to figure out the exact area of your roof:

• Take your pitch number and divide it by 12. For example, if your pitch is 4 in 12, you need to divide 4 by 12. The answer will be 1/3.
• Next, you need to square your outcome. In our example, 1/3 squared will yield 1/9.
• Thereafter, add 1 to your result: 1/9+1=10/9
• Calculate the square root of your new number: The square root of 10/9 is 1.05.
• Measure the length of your house – including the overhangs.
• Measure the width of your house – including the overhangs.
• Multiply the length of your house by its width to get the area. For example, 40 feet x 20 feet = 1,200 square feet.
• Thereafter, multiply the area by the pitch to get the true roof area: 1,200 x 1.05 = 1,260 square feet. 

In order to allow for ridges, hips, and waste, it is advisable to add 10% of your final roof area for a gable roof or 17% in the case of a cottage roof.

As a result, you should be working with 1,386 sq. feet or 1,474 sq. feet for a gable roof or a cottage roof respectively.

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