Now measure for the length of the purlins and be sure to add 2" for the overhang. The purlins for this project are 2 x 4s that are ripped (sawed) down the center. Don't forget to paint all the lumber for this project while it's still on the ground. Set the purlins on the chalk line, pre-drill the holes and secure with galvanized wood screws. As the pergola takes shape, you'll notice that the roof looks familiar. It has all the elements of a normal pitched house roof -- joists, rafters and purlins. The only difference is that the pergola roof is flat and has no shingles. Continue to attach and secure the purlins, repeating the previous steps.