Rafter² = rise² + run² (this formula is from the Pythagorean theorem) Here are the equations you may use to solve for the length of the slope and the rafter: Use the following formula which also describes a right triangle. You can also calculate roof pitch even without using a roof slope calculator. They have a pitch ranging from 4:12 to 9:12.

Generally, such roofs have a pitch ranging from 1/2:12 to 2:12. Flat roofs which aren’t perfectly flat require a small slope to allow for water runoff.You can divide roofs into these categories: Often, you express roof pitch as the ratio between the rise and the run in the form of x:12. You can assess this in two ways, either as the roof pitch angles which the rafters make with the horizontal or the proportion between the run and the rise of the roof. Roof pitch refers to the slope which the rafter creates.