Beadboard ceiling planks are as boards, flat roof regular, except they have parallel along its length lines to give the impression that the individual panels are only about 2 inches wide. In fact, usually they are 8 to 12 inches wide. If you have beadboard 12 feet and your room is more than 12 feet, you can stagger the ends of boards at different beams. This will give the roof a random, natural appearance.
Check each beam in the beadboard ceiling planks, using electronic stud finder to locate and a level and a pencil to draw the lines. Measure along the roof edge of the wall where you want to start. It has to be a wall that is perpendicular to the joists. Select a piece of beadboard in that measurement. Subtract 3/8 inch from the measurement. Cut with a miter saw, 90 degrees straight.
Hold the beadboard ceiling planks on the roof, from one end wall to the beam as you measured. Place the groove facing the wall. May shoot finish nails through the board, up, two nails per joist. Measure the space left at the end of the meeting. Measure and cut the first plate to the second course so that it lands on a beam other than the one that led to the first time. Continue through the roof in the courses, staggering at the end of each board to a different beam board next to it.