lay out common rafters with combination square
As  Aloysius Defenestrate commented I realized it can be done with any piece of flat wood that has a right angle and straight sides. 

Pitch is a ratio between rise and run, it doesn't even matter in wich units you mark it. For example: 6:12 ( or 1:2 ). Mark 6 cm (or inches) on one side of the right angle, mark 12 on the other side. Now use that as a framing square, marking the plumb lines and bidsmouth the same way as you would with a real square.

Thanks