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How to Build a Porch / Canopy Roof

Updated on September 15, 2014

Build your own porch roof the easy way

Here we are going to show you how to build a porch roof the easy way which can save you hundreds by doing it yourself.

First we will go through a few roof basics.

what and how we use run and rise.

how to find the angle of a potential roof,

making a simple but effective rafter square,

how to work out lengths of rafters

and a simple way to mark angle cuts

For more how to do it yourself

click here

Roof Basics

Before we can start to build any kind of roof it helps to know a few basics.

Below the figure shows the main componets of a gable roof followed by a porch roof

Gable Roof

Gable Roof
Gable Roof
Stanley 45-910 24-Inch x 2-Inch Steel Rafter/Roofing Square (English)
Stanley 45-910 24-Inch x 2-Inch Steel Rafter/Roofing Square (English)

frame any roof with this and no need for rafter table book


Porch Roof

Porch Roof
Porch Roof

Run and Rise

run rise
run rise

The next thing we need to know is how to work out the angles, Now before you faint and say i hate trigonometry i couldn't do that in school, you may be pleased to know that most of the best old traditional roofers couldn't do maths either, so they devised this method,a roof was worked out by run and rise

Run and Rise

A roof is calculated by foot per run, for you that are not familiar with imperial measurements there are 12 inches in a foot (1ft) so calculations are worked out by how much the roof rises per foot or every 12 inches.

In the next picture you will see that for every inch a roof rise it equals an angle eg if a roof rises 10 inches after travelling 12 inches (1 foot) the angle of the roof would be 40 degrees,if it rises 12 inches after travelling 12 inches it would be a 45 degree pitch roof.

How do we work out angles - To work out the angle of our we need to find the distance it travels (run) and the height (rise)

If we look at the picture on the left this is the basic construction of our porch roof the run would be the distance out from the wall along the ceiling joist, less the thicknes of the ridge, 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 = 2 ft, if we had an rise of 20 inches (height from top of ceiling joist to top of ridge) we would divide the 20 inches by 2 ft giving 10 this would be equal to a 40 degree pitch roof.

Make a Roofing Square

The next thing we need to do is make a roofing square (unless we have one) and the easiest way to do this is to cut the corner off a sheet of ply board etc.making it roughly 24" one side and 16" the other side then starting from the corner in both directions mark it out at 1 inch intervals

My recommended roofing squares

Checkout my favourite roofing Squares

Swanson Tool S0107 12-Inch Speed Square Layout Tool with Blue Book
Swanson Tool S0107 12-Inch Speed Square Layout Tool with Blue Book

We have the Speed Square (like the one you can make out of plywood) This Square is 12" X 12'' and marked out down both edges in inches, It is marked out in degrees from 0-90 also showing its Equivalent In inches as per foot run eg. 40° = 10"

Stanley 45-910 24-Inch x 2-Inch Steel Rafter/Roofing Square (English)
Stanley 45-910 24-Inch x 2-Inch Steel Rafter/Roofing Square (English)

Here me have the traditional roofing square which is 16" x 24" the blade of the square is mark out with the length of every rafter, Hip rafter, Jack rafter and valley board per foot run for any angle roof, No need for rafter tables book

Johnson Level & Tool 405 Stair Gauge 2-Pack
Johnson Level & Tool 405 Stair Gauge 2-Pack

these stair guages fit on the above square and are to hold the square at your desired angle for ease of multiple marking out

Stanley 46-052 Adjustable Quick Square Layout Tool
Stanley 46-052 Adjustable Quick Square Layout Tool

Similar to the speed square we have the quick Square or Squ angle this is the same as the speed square but has a moveable arm that Locks on the angle making Plumb and Seat cuts quicker


How do we work out rafter lengths

How do we work out the rafter lengths ?

Well we have a couple of ways, we could buy a rafter table book - expensive, can be complicated.

We can use pythagoras theory, which would be the run squared + the rise squared find square root which i would recommend for major roofs , in our example porch roof it would be 24 squared = 576 + 20 squared = 400 add together = 976 square root = 31.24 so your rafter length would be 311/4 inches

The best way is to do it the same way on our square as our roof is a 10 in 12 we do 10 x 10 =100 then 12 x 12 = 144 add them together = 244 find square root 15.62 this means that for every 12 inches our roof travels the rafter will 15.62 inches longer,but as not all roof runs work in to exact mutiples of 12 we need to take it a step further to make it easier, so once we have our square root of 15.62 we need to divide it by 12 = 1.30 this now means that for every inch our roof travels our rafter will extend by 1.30 inches, so if our roof run is 24 inches we would mutiply 24 by 1.30 = 31.2 inches or 311/4 inches. so if we were doing a big gable roof with a span of say 20ft that would be 240 inches - a 2 inch ridge board = 238 inches half it = 119 inches multiply by our 1.30 would make our rafter 1543/4 long,if we look at the figure below of our square if you drop a tape measure across our square between the number 12 and number 10 you will see it measures exactly 15.62 multiply this by 10ft as in our big roof and minus the 2" ridge you would get 154.75 or divide 15.62 by 12 you get 1.30 change our 10ft into inches =120 x 1.30 - 2" ridgde =154.75, so in our porch example we would multiply our 24" by 1.30 would give us a rafter length of 31.2 or 311/4.

Now look at the figure below, to cut our rafter we place our square on to our timber lining up the marks 10 and 12 level to the top we draw a line down the top cut side (no 10 side) 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 is now ready to cut.

Get the new ebook covering basic roofing with practical examples

Chocolate fudge Cake
Chocolate fudge Cake

Once your'e finished your roof

Chocolate fudge Cake Click Here

Stanley Rafter Square Review

In my opinion if you a building any kind of roof this is the best square to use and maybe a cheap calculater for a couple of bucks the stair guages are a useful added extra for speed of marking out but not essential, with this square you can work out anysize or and any angle of roof without the use of fancy roofing calculators or books of rafter tables, just this square and a cheap calculator. This square has stamped on it rafter tables to work out rafters,hips,jacks and valleys so it's not going to get damaged by a bit of rain like your rafter book.check out my lens on how to use a rafter square

Recommended reading

if you want to learn roof framing or improve the skills you have I recommend these from firsthand experience

New Guestbook Comments

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    • profile image

      Timothy Macg 

      3 years ago

      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 profile image


      5 years ago

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

    • profile image


      5 years ago

      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.

    • profile image


      5 years ago

      easy and simple guide lines


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