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When replacing your roof you may be tempted to choose the material based on the one that lasts the longest. Depending on your roof, this could be a mistake.

The slope of your roof will dictate what material you can use. As you are getting ready to replace your roof, our roofing calculator can help you determine the materials you'll need, even if you aren't sure of your roof area.

For those just learning, we are going to show you how to calculate your roof area in this handy guide.


How can a roofing calculator help me?

If you don't want to do the math, a roofing calculator can help you estimate the materials needed and the area to cover. The base area of your home is a good indicator of the area of your roof.

You need to know the roof area to know the number of materials needed. Roofing contractors charge by the square foot for materials and knowing the area will let you compare the prices of the materials appropriate for your roof.

Getting Started

It is important to know the pitch of your roof so you pick the right material. If you choose the wrong material moisture will not flow off of your roof and cause rot and mold to grow.

This can become a much bigger problem than a roof issue.

What is roof pitch?

The angle or slant of your roof is the slope.

The slope ratio (or roof pitch) is the amount of vertical rise relative to the horizontal distance covered by your roof.

How It's Measured

There is an easy way to determine the slope of your roof from inside your attic. You'll need to count how many inches the roof goes vertical for every 12 inches it goes horizontal.

We'll cover this more fully in the next section.


A pitch of 5:12 means that the roof goes up 5 inches for every 12 that it goes horizontal.

Don't get fooled into thinking you need a slope calculator. This figure only requires two measurements that you can easily find in your home to determine your ratio.

What's the easiest way to measure roof pitch and slope?

You can measure your roof slope from inside your attic. You need an 18 or 24-inch level, a pencil, and a tape measure.

Step One

Start by measuring 12 inches from the bottom of a roof rafter out. Be sure to use a level and not a ruler for this.

Step Two

Then measure vertically from your 12-inch mark to the roof rafter.

The number is how much your roof rises every 12 inches, or the roof pitch.

Complex Roofs

For complex roofs, you may need to measure your roof in sections. Your contractor will have conversion charts for the right amount of materials needed.

How is roof area calculated?

The easiest way to calculate your roof area is to use a roofing calculator.

Knowing how to calculate the roof area isn't too difficult if you know your square footage and pitch. Simply plug these values into the calculator or the formula below.

Roof area = ((roof pitch)2 + 1)1/2) x square ft of base area

Here's how it works:

To figure our your roof area, start with your roof pitch over 12.

Take this number and square it.

Add one, and then get the square root.

Multiply this final number by the square footage of the base of your home.

The number you end up with is the square footage of roof you will need to cover.


Let's illustrate how this works using a two-story home with a roof pitch of 5:12 and a square footage of 2,000 with 1,000 sq ft of base area.

Roof area = ((roof pitch)2 + 1)1/2) x square ft of base area

=((5/12)2 + 1)1/2) x 1000

=(0.4162 + 1)1/2 x 1000

=(0.173 + 1)1/2 x 1000

= 1.1731/2 x 1000

= 1.08 x 1000

= 1080 sqft

Complicated Roof?

If you have a complicated roof you need to follow these steps in sections of your home. Figure out the area for each section. Then add the square footage for each section together for the total area coverage.

Are there different types of roof materials?

Yes, and you can only use certain materials depending on how steep your roof's slope is. The key is to choose the roofing material that will direct water away from your home.

Flat Roofs

You have a flat roof if your slope is under 2:12.

Acceptable Materials

Most modern flat roofs use a large sheet of EPDM synthetic rubber or PVC.

BUR or built-up roofs use a hot tar and gravel layer technique. This is a traditional method for a flat roof.

Low Slope Roofs

Your roof has a low slope if it is 4:12 to 2:12. Slate shingles are not the best choice here.

The low angle of the roof means the water will drain off slow. This is not good and so roofing materials that depend on mechanical drainage are not used.

Acceptable Materials

Similar to flat roofs, you'll want to go with rubber or PVC. If the slope is 3:12 you may be able to use ashphalt shingles, provided a double layer of felt is used.

Steep Roofs

Your roof is steep if the slope is between 4:12 and 21:12.

Acceptable Materials

Asphalt shingles are the most common material used on these roofs. You can also use slate, clay, or concrete shingles.

Slate and clay shingles are more expensive, but will likely be the last roof you buy. They last a life time, literally. The tradeoff is that they both need a lot of added support structure for their weight.

Pitch Tax

Working on a steep roof is more dangerous than their flat alternatives. Expect a roofer to add a "pitch tax" of up to 20% for this added danger.

Final thoughts?

When its time to replace your roof the first thing you need to figure out is the pitch and slope. This will help you figure out the area and what materials to use.

If you have a flat roof look for large solid materials like metal or PVC. By contrast, a roof with a steep slope can use shingles made of asphalt, slate, or clay.

Once you know your pitch, you can use this to find out the area of your roof. Knowing your roof area helps you shop around for your next roof and gives you an idea of what to expect before hiring a contractor.

Figuring out the square footage of your roof has never been easier when using our roofing calculator. For help with other home renovation projects, take a look at our Tile Calculator.

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