The slope of a roof (also know as the "pitch") is based on the vertical rise in inches for every horizontal twelve inch (12") length (this is called the "run"). Slope is expressed with the rise shown first and the run shown second. For example, if your roof has a three inch (3") rise for every horizontal foot (12"), then the slope of the roof is a "3 in 12" slope (or pitch). This is written as 3:12, 3/12, or 3":12".

You can easily determine the slope of a roof using a 12" level. Set one end of the level on the roof surface and make it level. Then take a ruler or a tape measure and measure from the other end of the level down to the roof surface. This gives you the slope of the roof.

The slope of the roof in the picture below is expressed as "4 in 12" or "4 on 12". It is written as 4/12, 4:12, or 4":12".

How to Determine the Slope of Your Roof