The formula L=(R+H) is used to determine rafter length (L) I have a roof, where R is the run an H is the overhang. The quantity U represent you in length. When the run and the overhang are in feet, the rafter length will will be in inches. If a Rafter 248 inches long is used on a roof with a run of 16.5 feet and U equals 12.0, how long will the overhang be?