slope of your roof is determined by the vertical rise in inches for every
horizontal twelve inch (12") length (called the "run"). It is expressed with
the rise mentioned first and the run mentioned second. For instance, if your
roof has a four inch (4") rise for every horizontal foot, then it has a "4
in 12" slope (or pitch).
A fairly easy way
to determine the slope is to take a 12" level and set one end on the roof
surface and make it level. Then take a tape measure or ruler and measure
from the other end down to the roof surface. This will give you the slope of
the roof. The slope of the roof in the following picture would be expressed
as "4 in 12" or "4 on 12" and written as 4/12, 4:12, or 4":12".