Use a framing square to transfer the edge of the lentil to the top of the rafter (Image 1). Mark the line from the location of the first purlin. To keep everything even, measure over every 20-and-5/8" and mark for the rest of the purlins. Stretch a chalk line (Image 2) and mark the guides for the purlins.
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.
Once the purlins are installed across the rafters, there's only one small detail left, and that is to decorate the top of the columns by installing capitals. To install the capitals, first you need to measure each side at the top of each column.
Make 45-degree cuts on the miter saw for the four-sided capital. Attach each side of the capital one at a time to the top of each column with a nomadic brad nailer. If you want capitals at the bottom of each column, repeat the previous steps. Fill all the nail holes and cracks with an exterior caulk. Now you're ready for the final coat of paint.