To make a roof pitch table, measure the rise and the run of a roof, and express the rise and the run as a ratio, such as rise:run. Divide the rise by the run, find the tangent inverse, express the tangent inverse as degrees, and repeat for any further rise:run measurements.Continue Reading
A standard sloping roof has a single rise:run relationship, or pitch, which is a proportion that indicates how many inches the roof rises vertically across the number of inches it spans horizontally. A roof pitch table merely shows the rise:run of roofs that slope at various angles. Standard pitch tables assume a run of 12 inches.
To find a roof's pitch externally, mark a spirit level at 12 inches. Hold the level horizontally, and measure the distance from the roof's sloping surface to the mark on the level. To find a roof's pitch internally, mark a spirit level at 12 inches. Using a ladder, hold the level horizontally to the bottom side of a rafter. Measure the roof's sloping surface to the mark on the level. This is the roof's pitch, or 6:12, for example
Dividing the rise by run yields the tangent 0.5. For the simplest way to obtain the most accurate result, use a scientific calculator to find the inverse tangent. Type 0.5 into the calculator, push the inverse button, and push the Tan button. The result is the slope angle expressed in degrees.
Follow the same steps for any number of rise:run increments. This constitutes a roof pitch table.Learn more about Carpentry