Roof Pitch
In building construction, Roof Pitch is a numerical measure of the steepness of a roof. Roofs may have functionally "Flat" Slopes, "Low" Slopes, or "Pitched" Slopes.
The pitch of a roof is its vertical rise divided by its horizontal span, or "run." It is what's termed a "slope" in geometry and stair construction terminology. In the USA, the run is denominated by the number 12, giving a ratio of how many inches of rise or fall there are to each 12 inches (one foot) of run.
For example, 3/12 = 3 feet of drop in a 12 foot span, , 4 feet in 12 = 4/12, and so on. The pitch matters for a variety of reasons including determining the type of roofing material suitable for use per code and aesthetics, walkability, and the proportions to the building as a whole.
Pitch is sometimes a critical factor in some architectural styles such as a steep pitch in Gothic architecture and a low pitch in Classical architecture, while combinations of pitches form distinctive roof shapes such as a gambrel or mansard roof. The basic ranges of pitch range from:
Flat Roofs, which are rarely perfectly flat but sloped to drain water up to ½ /:12 to 2:12 ( 1 in 24 to 1 in 6).
Low-slope Roofing requires special materials and techniques to avoid leaks and ranges from 1/12 to 4/12.
Conventional Pitch is considered to be from 4/12 to 9/12 (though extra costs for steep installations can start at 7/12 pitch).
Steep Slope Roofing is above 9 / 12 and may require extra fasteners or other installation considerations, such as hand sealing, in the case of asphalt shingles on mansards.
US convention is to use whole numbers when even (e.g. "three in twelve") or the nearest single or two-digit fraction when not (e.g. either "five and a quarter in twelve" or "five point two-five in twelve", each expressed numerically as 5.25 / 12). The exact roof slope in degrees is given by the arctangent. For example: arctan(3 / 12) = 14.0°
The primary purpose of pitching a roof is to redirect water and snow. Thus, pitch is typically greater in areas of high rain or snowfall. The steep thatch roof of the tropical Papua New Guinea Longhouse, for example, sweeps almost to the ground. The high, steeply-pitched gabled roofs of northern Europe are typical in regions of heavy snowfall. In some areas of the U.S. building codes require a minimum slope; for example, areas hit by extraordinary Lake Effect snows like Buffalo, New York and Montreal, Canada, specify a 6 / 12, a rather steep pitch of approximately 26.6 degrees.
Rafters are built, or framed, on an angle to achieve the designed "pitch" of a roof. Gable and other roofs of various pitches allow for lower primary structures with a corresponding reduction in framing and sheathing materials.
