I marked it in red to make it easier to spot.
Roof rafter layout framing square.
Square guides are available at most hardware and home improvement centers.
The standard two foot framing square also referred to as a carpenter s square and historically as a steel square is an l shaped tool that is used to mark angles for cuts used in building framing particularly roof rafters stair stringers and many other cuts or angles other than 90.
18 12 roof slope rafter lengths let s figure the rafter length for an 18 in 12 roof.