you got to take your total run of the building which is half the width . I dont know what pitch you want the roof , buts lets take for an example you want a 1/4 pitch which is 6x12 roof cut . Use 6 on the small part of the square which is the tongue and 12 on the blade of the square (holding the framing square in front of you the tongue of the square is in your left hand and the blade of the square is in your right hand . To mark the plumb cut you mark on the tongue . To find the lengght of a rafter lets say your building width is 14 feet (span) half of that is 7 feet (run). So if its a 6x12 roof cut or some say pitch you square those two numbers together and take the square root you should get 13.42 (this is called the bridge distance ) the unit rise of this roof is 6 inches and the unit run is always 12inches . Its a right angled triangle 6 inches is the vertical measurement 12 inches the level measurement and 13.42 is the hypotenuse. So you take 13.42 multiply by 7 (7 units of run ) which equals 93.94 inches divide that by 12 =7.828 or 7 foot nine and fifteen sixtennths or 7 foot ten inches . The total rise of this example would be 7' (total run) x 6" (unit rise ) = 42" . And you would take half the thickness of the ridgeboard of the rafter at right angles to the plumb line .So if you were using a 2x6 ridgeboard youd take 3/4 off the rafter lenght . The lenght of the rafter is from the ridgeboard to the wall plate . Im not sure if your gonna cut a birdsmouth (seat cut where the rafter rest on the plate or not but if your not sure how to do that just ask . Id let the rafter be long enough so once you have them all on you can come back after and snap a line on the ends of them once you know what projection you want on the over hang