When you are adding onto an existing structure and you find the pattern, never assume that the pattern will continue throughout. In the case of this video, the rafters are 24 inches apart. APPROXIMATELY. Some are 23.5 inches apart and others are 24.25 apart. Check the distance Every. Single. Time. In this case, if I had rounded to 24 inches, it would have meant that some of my boards would not be vertical and that would increase the likelihood that I might miss a stud when I attach the drywall.