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Measuring Your Roof

There are many ways in which to measure a roof. This section will show one method of how to properly do it. We’ll express our final answer in "squares". In all roofing except polyurethane foam, roofers express Area in "squares".

1 square = 100 square feet

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.

(bh)/2 = Area

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

(20' 20')/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')/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')/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')/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')/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')/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')/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')/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.

Section A = 225
Section B = 1,050
Section C = 1,125
Section D = 262.5
Section E = 1427
Section F = 530

Out total square footage is equal to 4,619.5 square feet. Or, roughly 46 squares. Remember that 1 square = 100 square feet.


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