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How to Build a Canopy or Porch Roof

With over 30 years of experience as a carpenter, I'm an expert at building roofs and housing fixtures.

In this article, we are going to explore how to build a porch roof the easy way. Undertaking this task on your own will save you hundreds of dollars and will be a great learning experience.

What You Need to Know to Build Your Own Porch Roof

  1. The pieces of a porch and gable roof.
  2. How we use run and rise to find the angle of a potential roof.
  3. How to make a simple, but effective, roofing square.
  4. How to work out the lengths of rafters.
Gable Roof

Gable Roof

1. What Is a Gable and Porch Roof?

Gable roof: The most common porch roof is a gable roof. The gable roof is a triangle-shaped roof that projects along a ridge down the center of the enclosure. The sides are sloped rise over run at a particular pitch in order to shed snow and water. This type of roof consists of two roof sections sloping in opposite directions, placed such that the highest, horizontal edges meet to form the roof ridge. This is the most common, classic roof shape in parts of the world with cold or temperate climates.

Parts of a Gable and Porch Roof

  • Ridge
  • Rafter
  • Purlin
  • Ceiling joint
  • Wall plate

Before we can start to build any kind of roof, it helps to know a few basics. Below are illustrations showing the main components of a gable roof followed by a porch roof (as mentioned above).

Porch Roof

Porch Roof

2. How to Use Run and Rise

The next thing we need to know is how to work out the angles. Now, before you say, "I hate trigonometry! I couldn't do that in school!" you may be pleased to know that most of the best traditional roofers couldn't do complex math either, so they devised this method. A roof was worked out by run and rise. To work out the angle of our roof, we need to find the distance it travels (run) and the height (rise).

Run and Rise

Run and Rise

Calculating Run and Rise

A roof is calculated by foot per run. For those who are not familiar with imperial measurements, there are 12 inches in a foot (1 foot) so calculations are worked out by how much the roof rises per foot/every 12 inches.

In the next picture, you will see that for every inch a roof rises there is an equivalent angle. For example, if a roof rises 10 inches after traveling 12 inches (1 foot), the angle of the roof would be 40 degrees. If it rises 12 inches after traveling 12 inches, it would be a 45-degree pitch roof.

Run and Rise

Run and Rise

How Do These Measurements Correlate to My Roof?

The run:

  • When talking about the basic construction of a porch roof, the run would be the distance out from the wall along the ceiling joist.
  • This is less than the thickness of the ridge.
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The rise:

  • The rise would be the height from the top of the ceiling joist to the top of the ridge.
  • The formula to work out the angle is the rise (in inches) divided by the run (in feet).
  • So, if the run was 26 inches (distance out from the wall) minus a 2-inch thick ridge board that would give us 24 inches, or 2 feet.
  • If we had a rise of 20 inches (height from top of ceiling joist to top of the ridge) we would divide 20 by 2, giving us 10. This would be equal to a 40-degree pitch roof.

3. How to Make a Roofing Square

The next thing we need to do is make a roofing square (unless we already have one) and the easiest way to do this is to cut the corner off a sheet of plyboard- making it roughly 24 inches on one side and 16 inches on the other side. Then, starting from the corner, in both directions, mark it out at 1-inch intervals.

Roofing Square

Roofing Square

4. How to Work Out the Lengths of Rafters

Pythagorean theorem: We can use the Pythagorean theorem (run2 + rise2=c2), then find the square root. In our example, our porch roof would be 24 squared (576) plus 20 squared (400), which added together equals 976. Then, taking the square root of 976, we get 31.24. So, your rafter length would be 311/4 inches

Example: 242 + 202 = 976

√976 = 31.24

Using a roofing square:

Let's say our roof is a 10 in 12

  • 10 x 10 = 100, then 12 x 12 = 144 add them together = 244
  • Find square root, which is 15.62
  • This means that for every 12 inches our roof travels, the rafter will be 15.62 inches longer.

But, as not all roof runs work into exact multiples of 12, we need to take it a step further to make it easier.

  • once we have our square root of 15.62, we need to divide it by 12 = 1.30.
  • This means that for every inch our roof travels, our rafter will extend by 1.30 inches.
  • So, if our roof run is 24 inches, then we would multiply 24 by 1.30 = 31.2 inches or 311/4 inches.
  • If we were doing a big gable roof with a span of say 20 feet that would be 240 inches minus a 2-inch ridge board = 238 inches.
  • divided by 2 = 119 inches.
  • Multiply by our 1.30, making our rafter 154 3/4 long.
  • If you drop a tape measure across our square, between the number 12 and number 10, you will see that it measures exactly 15.62.
  • Multiply this by 10 feet and minus the 2-inch ridge. This would get you 154.75.
  • You could also divide 15.62 by 12 and get 1.30.
  • Change our 10 feet into inches = 120 x 1.30 - 2 inch ridge = 154.75
  • In our porch example, we would multiply our 24 inches by 1.30, giving us a rafter length of 31.2 or 311/4.

(See image below) To cut our rafter, we place our square onto our timber. Lining up the marks 10 and 12. We draw a line down the top cut side (no 10 sides) we then measure down the timber 31.2 inches, put our square back on, lining up the 12 and 10 again, but making sure this time that the no 12 is on our bottom mark, then draw a line for our bottom or seat cut.

Rafter lengths

Rafter lengths

If you want to learn more about roof framing, or simply want to improve the skills you have, then I recommend these books from firsthand experience:

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

New Guestbook Comments

Timothy Macg on May 01, 2015:

it is a great article i enjoyed it,my porch is going to build by my self and a help i find on keep up good work

roofingvancouve1 on January 22, 2013:

I like the way you have laid it out in sequence. Nice job

jp_smith86 on November 05, 2012:

Thanks for sharing. High performance reinforced EPDM flat roofing systems from AR Systems. We supply Duoply, new reinforced EPDM flat roofing systems to roofing contractors for residential, commercial and industrial projects.

sandra65 on October 21, 2012:

easy and simple guide lines

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