To determine the roof pitch, place a level on the roof's high side, level the level, and measure the distance between the level's 12-inch mark and the roof's surface using a measuring tape. Use this distance or roof rise to express the roof pitch. A formula commonly used to express the roof pitch is roof rise in inches/12-inch run.
Begin determining the roof pitch by attaching the tang of the measuring tape to a spirit level's end. The level must have a minimum length of 12 inches. Pull the tape over the level, and mark the 12-inch point from the attached end on it using a lumber pencil.
Now, place a ladder against the building's side, climb it, take the level and the tape with you, and move towards the peak of the roof by approximately 3 feet. Then, position the level on the roof's high side so that it rests lengthwise with the marked side facing in a downward direction.
Shift the level's lower side upwards so it becomes fully horizontal and its bubble becomes centered. Next, position the measuring tape at a 90-degree angle to the roof, and use it to measure the distance between the roof's surface and the 12-inch mark on the level.
To express the pitch of the roof, compare this distance to a 12-inch run. For example, if the distance calculated is 4 inches, the roof pitch is 4/12.