Getting Started the Basics

Build a 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

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

Porch Roof

 

Run and 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

 

A roof is calculated by foot per run, for you that are not familar 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 picture above 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 above 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

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 recomend 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 above, 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.

 



Putting it all together and a NEW ebook on Basic roof construction click here with an introduction to the carpenters framing square and the speed square.