What Determines the Pitch of a Roof?

Measurement

• In the United States, the roof pitch is defined as the amount a roof line rises over a horizontal distance of 12 inches. For example, a roof may rise or incline 6 inches every 12 inches of roof length. A flatter-looking roof may have a rise of only 1 inch every 12 inches, while a steep-pitched roof may rise 20 inches for every 12 inches of roof length. The pitch may also be expressed as an angle, such as saying that a roof has a 30-degree pitch looking at the roof's side profile as if it were a triangle sitting atop a box.

Terminology

• Although roofs may be more easily visualized or discussed by mentioning the roof's angle, the roof pitch is written in text in numerous ways. A roof with a 37-degree angle results when the roof rises 9 inches for every 12 inches of roof length. It may be expressed with many different syntax, including 9/12, 9-12, 9 to 12, 9 and 12 or 9 on 12. Building contractors may not readily know the physical angle of the roof they're building but more quickly refer to the pitch being created, such as "we're starting construction on the 9-12 roof section today."

Climatic Considerations

• A structure's roof serves a primary purpose of preventing precipitation from getting inside the building. Gravity and wind work against a roof line, the vectors that help water move downward and breach the building's interior. Flat or no-pitch roofs do not shed water or buildup of snow as efficiently as a steep-pitched roof. In regions with little rainfall and snow, a flat roof makes sense in construction. In a rainy, snowy climate, a sloped roof is advantageous. Where heavy snow accumulates each winter, a steep roof sheds snow and ices quickly, diminishing any weight load on the roof's support walls and trusses.

Building Material Insight

• When a building's roof is designed, factors such as pitch, weight load, cost and stability are all considered. Flat or low-taper roof lines are easier to install and maintain, and they better distribute the shingles' or metal sheeting's weight. Heavy materials on a steeply pitched roof are problematic, as they are difficult to hoist and attach and gravity exerts more force to pull the materials off the roof. Installation specifications and recommendations by roofing material manufacturers usually provide safe and durable guidelines on labels or safety sheets. Certain materials are appropriate for use on flat or low-tapering roofs but are too unstable or difficult to install on steeper roofs. The manufacturer provides guidelines for what range of roof pitches the roofing material is best suited.

Building Code

• Contact your state, county or city office for current lawful building codes for your area. For safety, laws change frequently and dictate which building styles, roof pitches or materials are acceptable to use in construction. These codes ensure construction safety as well as mitigate property losses caused by roof pitch due to earthquakes, hurricane winds, annual rainfall or weight-load distribution of snowfall.