A pergola is a structure that is usually supported on four sides by vertical posts, although some pergola’s that rest against the house have only two vertical posts. The sizes of the overhead joists can vary. Beams attached to posts will support one or more layers of cross rafters. The spacing and size of the rafters will dictate the amount of sunlight that is allowed to penetrate the structure. Sometimes vines are trained to grow through latticework to give it a natural shady canopy.
It is very important to properly attach the posts when installing a pergola over a deck so that it performs well in high wind conditions. Pergolas are top heavy and need lateral bracing so they can withstand the wind swaying and uplift. Attaching sections of rail in between pergola posts will strengthen the pergola. Using 6x6s for support posts will further strengthen the structure.