How to Tell the Pitch of a Roof

Attach the tape measure's tang to one end of the spirit level. Extend the tape measure across the level. Use a lumber pencil to make a mark 12 inches from the end of the level.

Set a ladder securely against the side of the structure. Climb the ladder and access the roof. Bring both the level and tape measure to the roof. Move approximately 3-feet or more toward the roof's peak.

Turn the level so that its marked side faces downward. Rotate the level so that the end of the level closest to the roof is the side from which you began your previous measurement and mark. Place the end of the level against the roof's surface. If your level is properly positioned, the mark that you made on the level is 12-inches away from the roof's surface.

Adjust the level until its vials indicate a horizontally level position. Place the tape measure's tang against the roof and draw the tape measure from the roof's surface to the pencil mark on the underside of the level. Adjust the tape measure to a straight up-and-down, or "plumb," position. Read the dimension at the point where the tape meets the pencil mark; this dimension represents the roof's rise for every 12-inches. Carefully descend from the roof.

Express the roof's pitch by comparing the rise that you determined to a run of 12-inches. For example, if you measured a rise of 4-inches while on the roof, the roof's pitch is 4-inches in 12-inches or, in building jargon, "4 in 12."