The most common type of rafter is those with roof framing options; especially gable roofs that are built with common rafters which are mostly the shed roofs. The other type of rafter is the hip roof which depends on the length of the building and has four rafters. Cutting a rafter is not difficult provided you have found the right length. Measuring the length and width of the building most importantly can get you the idea of the span.
In case the width of the entire building is about 30 feet with the roof pitch around 6/12 you can work out the length of the rafter accordingly. Rafter table books and construction calculators are used to find the rafter length. Both these methods determine the number precisely used to cut the rafter into. Once you have your length decided cutting common rafters become a really easy task then and you don’t need to worry much about the exact width of it anymore.