The illustration below depicts the pitch of a 7-12 roof slope, which means that for every 12" of horizontal measurement (roof run), the vertical measurement (roof rise) equals 7. This measurement is best performed on a bare roof since curled roofing shingles will interfere with your measurement. A roof that is not fully constructed cannot be measured accurately, so have a contractor estimate the size of your roof before it is built.
Pitch is usually expressed in degrees. One degree is equal to 57.5 degrees. So, a pitch of 5 degrees is equivalent to 35.7 degrees. Pitch is used to describe the angle at which the sloping surfaces of a roof meet. The higher the pitch, the steeper the roof. Low-pitched roofs are generally less expensive than high-pitched ones of a similar design and construction.
The term "pitch" comes from the building trades when describing the angle of a roof's slope. The higher the number that follows "pitch", the steeper the roof. A "1-4" roof has slopes that are almost perpendicular, while a "5-12" roof has slopes that are nearly parallel. There are also semi-sloped roofs that fall between these two extremes. Slope measurements should always be taken by qualified personnel using accurate tools. Failure to do so could result in injuries from falling objects.
Roofs are classified by type and material.
Roofs require slope for proper drainage, and even flat roofs must have some inclination. A 4/12 roof pitch means that the roof rises 4 inches for every 12 inches measured horizontally from the edge of the roof to the centerline. This is a moderate slope, which is generally acceptable for most climates.
A 4-pitch roof has two rows of shingles, each with four sections. The vertical distance between these rows is called the layering. The thickness of the shingle material is the depth of the layer. For example, a 3/8-inch-thick piece of metal folded over twice will make one layer thick. A 4-pitch roof has layers of metal about 1/4 inch thick. A common type of material used for shingles is asphalt, but other materials can be used as well. Wood shakes and tiles are also commonly used instead. The number indicating the pitch of the roof is usually written on an arrow pointing up from the left side of the house near where the roof meets the wall. If there is no such mark, then assume a 3/8-inch-thick sheet of metal is used for the layer, which makes a 4-pitch roof.
The purpose of adding more than one layer of shingles is so that if some of them get damaged or worn out, it won't affect the overall look of the roof.
A roof pitch of 10:12, as shown below, signifies that the roof rises 10 inches for every 12 horizontal inches of roof. This is a roof with a steep slope. A shallow slope is one where the rise is less than 10 inches for every 12 horizontal inches. A flat or level roof has no rise in excess of 2 inches per 12 inches.
Heights and depths of buildings are usually expressed in terms of feet, meters, yards, or miles. The height of a building is the distance from its lowest point to its highest. The depth of a building is the distance from its front edge to its back edge. Heading (or run) is the angle between a vertical plane and the ground. If a building's axis is aligned with the direction it is facing, the heading is zero degrees. If the building is turned toward the east, its heading is positive; if west, negative.
The rise of a roof is how high it goes above its foundation. The fall of a roof is how far it extends down from its peak. The slope of a roof is the ratio of its rise to its fall. So, a roof with a rise of 10 feet and a fall of 12 feet has a slope of 1 to 0.5 = 20/12, or 1/6.
Roof pitch (or slope) indicates how much the roof rises for every 12 inches of depth. A "6/12 pitch" roof, for example, signifies that the roof rises 6" for every 12" inward towards the apex (or ridge). This means that the roof would be 3 feet high at its highest point and 1 foot low at its lowest point.
The advantage of a 6/12 pitch roof is that it can support a large amount of snow load without leaking. The disadvantage is that it tends to look flat, not like a real roof.
Real roofs have various shapes, but they all have one thing in common: they are very steep. A real roof with a 30 degree angle might look like this:
As you can see, there's no way something as flat as paper could stand under its own weight without tearing or breaking. But the shape of a real roof is necessary to keep it waterproof even when it does get wet.
Modern houses usually have sloped roofs too. They usually use a 6/12 pitch because it's easy to build. But any number of slopes between these two extremes is possible. For example, a house might have a starting pitch of 5/12 then go down 2 inches for every foot off the peak. If it was raining really hard, this would allow water to run off before it reached the edge of the roof.
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