I recently added an option to my storage shed plans for increasing the roof pitch from 3/12 to a steeper 4/12 and 5/12 pitch.
Even though I work with this all the time, when I did these calculations I had to stop and think because the reason behind the numbers isn’t intuitively obvious. So I decided this would be a good opportunity to explain what those roof pitch numbers mean.
Roof pitch (or slope) is typically expressed in terms like 3/12, 4/12 and 5/12.
This is a measurement of how much the surface of the roof rises vertically over a horizontal distance.
It’s expressed as a ratio of the vertical rise over the horizontal run, or rise over run, and is always stated in terms of a 12 foot run for standardization..
For example…
For example a roof that increases a height of 3 feet over a 12 foot distance has a pitch of 3/12.
A roof that increases 4 feet over a 12 foot distance has a pitch of 4/12.
And a roof that increases 5 feet over a 12 foot span has a pitch of 5/12.
All together
This graphic shows the 3 pitches overlaid together to give you an idea of their relationship to each other. 
Gable roof example
A gable roof consists of 2 symmetrical slopes together. The run is the measurement to the center of the roof, not the total width.
This shows that the slope is the same even though the run isn’t.
- 8ft wide gable roof shed has a run of 4ft and a rise of 1ft.
- 10 wide gable roof shed has a run of 5ft and a rise of 1 1/4ft.
- 12 wide gable roof shed has a run of 6ft and a rise of 1 1/2ft.
- 24 wide gable roof has a run of 12ft and a rise of 3ft.
My shed plans offer all 3 of these roof pitches. Which one you choose to use will depend on the weather in your area and what you want the final appearance of your shed to be.
The 3/12 pitch roof is the easiest to work on and is the most economical in terms of materials usage. However it might not be suitable in a larger shed if you have a lot of rain or snow load.