When designing a pergola, there are several things to take into consideration. First of all, what do you want the overall look to be? Will it be painted or stained? Will it have a rustic "grape arbor" look, or do you visualize something that would be more at home in a classic Greek garden? How big will it be, and how will it tie in with the rest of the house? Do you want a lot of shade or just a little? Will it support vines, lights or anything else?
As your design takes shape in your mind, the next step is to consider how the individual elements of the pergola will come together, starting with the columns or posts. For a simple design, standard 4-by-4 or 6-by-6 lumber can be used. If pressure-treated posts are needed, consider 4-by-4s that can then be wrapped with a finish grade of lumber for a cleaner look. Cedar and redwood are great choices, and if the pocketbook will allow it, select a clear grade for a really stunning finished look.
Another option is to use round, turned columns instead of square posts. For a much more rustic look, you can use natural wood logs. If your tastes run more to the classic, there are turned and fluted columns available in a wide variety of styles. You can get wood columns for painting or staining, or for less maintenance consider prefinished resin or aluminum columns.
With the posts or columns selected, attention now turns to the rafters and beams. Here again, you'll want to select materials that have the right look and feel for your design, while still providing the necessary structural support.
In most designs, structural beams are run between the posts, then the rafters are placed on top of and perpendicular to the beams, forming a symmetrical pattern. The rafters can be installed so that they are parallel to the house or perpendicular to it, depending on what you think will look best and what is the most effective from a structural standpoint.
The rafters themselves are one of the most important aesthetic elements of the entire design. They typically overhang the beams by a foot or more, and the ends are cut at an angle or in some type of simple or intricate curve. Since the rafters don't support anything other than their own weight, you have more flexibility in both material and layout. The rafters may be 4-by-4 lumber spaced 4 feet or more apart, or they may be 2-by-2 lumber set with only 6 inches between them.