by Gene Denardo

Created on: May 02, 2009

The pitch of a roof is a ratio that measures the steepness of the roof. The base number used in the measurement is "12". The pitch is the number of units that the roof rises over the base number. Because the base is 12, you can think of the pitch as the number of inches the roof rises for every foot or twelve inches the roof extends horizontally into a theoretical plane, or the plane of the joist or floor system below. This change in height of the roof is also called the "rise" and the change in the horizontal plane is also called the "run".

With this in mind, a perfectly flat roof would have a "pitch" of 0 in 12; for every 12 inches the roof extends in the horizontal plane the roof rises zero inches. This type of roof is really simply a level floor or joist system and of course, would not shed water! They are uncommon, although "ponding" roofs are used in some hot areas to collect a certain amount of water in order to cool the house. They are very effective but need to be able to withstand the considerable weight of the water.

As the pitch rises the roof becomes better at shedding water. What is referred to as a "flat" roof, actually has a pitch of 1 to 3 in 12; it rises a couple of inches for every 12 inches horizontal. This is common on commercial buildings and large "big box" buildings and require what are known as "hot" tar or rubber roofs to seal out the water that can pool during heavy rains or snow.

Many houses have a roof pitch in the range of 5 to 8 in 12. This is a good pitch for shedding water but not prohibitively steep for access and work, although it takes some experience to feel comfortable on any roof! This is probably what the average person perceives as the "normal" roof, one not too flat and not too steep, and is a favorite of "ranch" style architecture.

A roof that has a 12 in 12 pitch, has a rise equal to its horizontal projection. It is helpful to picture the roof pitch as a triangle, with the base of the triangle as the horizontal projection and pitch as its vertical side and the long side of the triangle connecting these two sides as the roof surface. When a roof reaches 12 in 12 pitch, the two angles of the triangle that the roof surface side of the triangle intersect with are both 45%. This is the only time this occurs. Roofs with this pitch and steeper are utilized on traditional architecture such as English or Tudor.

Theoretically, a roof can never reach the complete vertical, unless you consider a vertical line a roof! The roof must

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