Roofing Calculator

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This roofing calculator can help you estimate the area of your roof and find out how much materials you would need to construct a roof of your chosen parameters. The calculator also allows you to calculate its price based on the square footage.

Inputs
ft²
$
per ft²
Outputs
Roof Area: ~6,184.66 ft2Materials Needed:
188 bundles of shingles
58 rolls of roll roofing
15 rolls of #15 felt
29 rolls of #30 felt

Understanding the Calculator Inputs

House Base Area - This is the surface area of the house, the parallel plan with the ground. You can simply calculate the base area of your house if it has a rectangular shape. However, for more complex shapes, you would either have to calculate each part of the shape individually or know what the area is priorly. If your house has a complex shape, the calculator would give more accurate results if you separate it into parts and calculate each part on its own.

Roof Pitch - The roof pitch is measured as a ratio of the vertical rise of the house over its horizontal run. In the U.S., this ratio is shown as the number of inches of vertical house for every 12 inches of horizontal run. For example, if the roof pitch is 5/12, this means that the roof rises 5 inches for every 12 horizontal inches.

Price - The calculator allows you to input the estimated price per square foot of constructing the roof in order to find out how much would be needed in total to cover the whole roof’s surface area.

How to estimate the size of a roof?

In order to understand how the calculator works, we will go over a step-by-step example on how to estimate the size of your roof.

Step 1 - Find the area of the base of the roof

The first thing you will want to do is calculate the area of the roof as if it was flat and parallel to the ground. This means that you will need the length and width of the roof. As mentioned earlier, depending on the shape of your house, this can be a simple multiplication, length times width, or a more complex one.

Imagine that the length of the roof is 15 meters and the width is 12 meters. The area of the base of the roof would in turn be 15 multiplied by 12, or 180 square meters.

Step 2 - Find the total roof area by using roof pitch

Imagine that the roof has a pitch 8/12. As explained earlier, this means that for every 12 horizontal inches, the rise is 7 inches vertically. It also shows the slope of the roof measured in degrees. To make it simpler for us to find the area by using roof pitch, we will use the multipliers for the specific roof pitch. A table of these multipliers and angles is given below:

PitchAngleMultiplier
1/124.8°1.003
2/129.5°1.014
3/1214.0°1.031
4/1218.4°1.054
5/1222.6°1.083
6/1226.6°1.118
7/1230.3°1.158
8/1233.7°1.202
9/1236.9°1.250
10/1239.8°1.302
11/1242.5°1.357
12/1245.0°1.414
13/1247.3°1.474
14/1249.4°1.537
15/1251.3°1.601
16/1253.1°1.667
17/1254.8°1.734
18/1256.3°1.803
19/1257.7°1.873
20/1259.0°1.944
21/1260.3°2.016
22/1261.4°2.088
23/1262.4°2.162
24/1263.4°2.236

The corresponding multiplier of the ratio will be multiplied by the base area of the roof. In our example, the multiplier for a roof pitch 8/12 is 1.202. That means that the roof surface area is 1.202 * 180 = 216.36 square meters.

How to estimate the roofing materials needed?

To estimate the roofing materials needed, we will use the U.S. standards, which can be summarized as follows:

  • One bundle of shingles covers ~ 33 ft2
  • Rolls of roof roofing are 36 inches wide and 36 feet long ~ 108 ft2
  • Rolls of #15 felt are 36 inches wide and 144 feet long ~ 432 ft2
  • Rolls of #30 felt are 36 inches wide and 72 feet long ~ 216 ft2

For example, for a roof of 230 square footage, one would need:

  • 7 bundles of shingles
  • 3 rolls of roll roofing
  • 1 roll of #15 felt
  • 2 rolls of #30 felt
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