Roofs are generally described by their “pitch”. Roof pitch does not refer to the sticky substance found on pine trees. Pitch is usually stated as a ratio like 3:12, or 4:12. This is read as “three and twelve”, or “four and twelve”. It is always one number followed by 12. The ratio is easily understood by thinking of the first number as the horizontal distance from a starting point and 12 as the vertical distance. The starting point is connected to the ending point. If drawn out, it is always a right triangle with the horizontal portion as the first number and the vertical portion as 12. The roof pitch is the slanted part of the triangle. 1:12 is quite flat, where a 12:12 is quite angled. It can be measured with any increment (Inches, feet, yards, meters, etc.).
according to an article at angieslist.com, “Roof Pitch”
“One thing to consider when building a home is roof construction. A roof must do more than just protect you from the elements, it must support external weight.
The pitch of a roof is a numerical measure of its slope. Pitch is also called the roof angle or pitch degree. A flat roof has no pitch or
slope. The more slope a roof has, the more it’s able to shed rain and snow. Flat roofs can collapse under the weight of heavy snow, so roofs in areas with heavy snowfall need to have pitch.
Understanding the numbers
Roof pitch is the rise of the roof over the run of the roof. The rise is the height of the roof, from the base to the top. For example, if a roof is 8-feet high and spreads out from the center covering 12 feet in each direction, the rise over the run would be 8/12.
Consider a right triangle. The bottom, horizontal line of the triangle is the base of the roof. The straight, vertical line runs from the base to the top for total height. This distance is the rise. The angled line, from the top to the base, is the slope or run of the roof.” To read the entire article click here.