It can happen that your plans say one measurement and that the actual measurement is slightly different. This is not unusual. Contractors sometimes run into items out in the “field” that the architect doesn’t know about that cause things to come out a little different than what the architect specified on the plans. Because of this, we always recommend measuring the roof yourself to make sure you are furnishing us with accurate measurements.

There are many ways in which to measure a roof. This section will show one method of how to properly do it.

Before you get started, you need to know some simple geometry.

The base length (b) times the height (h) of any triangle is twice its area (A). So if you divide the product of the base and height by two, then you have the area of a triangle.
(b×h) divided by 2 = Area

Therefore, the following two triangles, R and H, though different in looks and shape, have the exact same area.

(20' × 20') divided by 2 = 200 square feet

Now that you know this simple mathematical fact, you will easily understand how to measure the following roof.

The picture shown is an aerial view of a roof with both a hip end and gable ends. It is strongly recommended that you make a rough sketch of your roof. In order to make it easier for measuring, the roof will be broken up into sections A through F.

Section A

This section is a simple triangle. Simply measure the length of the eaves and the perpendicular line from the eaves to the peak. Multiply these numbers and divide the answer by two.

(30' × 15') divided by 2 = 225 square feet

So Section A has 225 square feet in it.

Section B

The easiest way to measure this section is to divide it up into three different sections: x, y, and z. It’s pretty obvious that sections x and z are the same size, even without being marked. But because it’s much easier to work in theory and numbers on pieces of paper (like architects do) than actually performing the construction (like contractors do), it’s always a good idea to go ahead and measure both triangles.

x = (15' × 15') divided by 2 = 112.5 square feet
y = 55' × 15' = 825 square feet

Because we know that z is equal to x after measuring, we’ll simply add another 112.5 square feet to our current list of numbers.

So Section B has a total of 112.5 + 112.5 + 825 = 1,050 square feet.

Section C

Again, with this section it’s pretty obvious that we have symmetrical sides. Let’s go ahead and divide it up into sections, measure one side and get the answers that we want and then we’ll check the other side.

w = 30' × 15' = 450 square feet
y = (15' × 15') divided by 2 = 112.5 square feet

A quick measurement verifies that x and z are the same as w and y so we’ll multiply the sum of w and y by 2 for the final Area.

Section C = (450 + 112.5) × 2 = 1,125 square feet.

Section D

x = (15' × 15') divided by 2 = 112.5 square feet
y = 10' × 15' = 150 square feet


Section D = 112.5 + 150 = 262.5 square feet

Section E

This section has several different subsections so we’ll have to be careful and make sure we do it right. If your roof has a section similar in shape, double check your sketch to make sure that every piece is either rectangular or triangular.

s = (15' × 15') divided by 2 = 112.5 square feet
t = 40' × 15' = 600 square feet

Remember to check both small triangles in the center part to make sure that they are the same size.

x = (10.5' × 10.5') divided by 2 = 55 square feet
y = 21' × 4.5' = 94.5 square feet

The actual answer of x is 55-1/8 square feet, but we rounded off for ease of measuring.

z = 34' × 15' = 510 square feet

Be careful adding all of these up. Remember that there are two different x sections so we’ll need to add it twice. Section E = 112.5 + 600 + 55 + 55 + 94.5 + 510 = 1,427 square feet.

Section F

This section is simply a smaller version of Section C so we’ll do it the same way.

w = (10.5' × 10.5') divided by 2 = 55 square feet
y = 20' × 10.5' = 210 square feet

Don’t forget to make quick measurements to ascertain that x and z are equal to w and y.

Section F = 55 + 55 + 210 + 210 = 530 square feet.

Sum It Up

Now take all sections and add them up.

The total square footage of the above example is 4,619.5 square feet. When you’ve figured out the square footage of your roof, note it down on a piece of paper because you’ll need that to figure to obtain an Instant Estimate.