These plans will build a 16 x 20-foot pergola. Use rot-resistant lumber, either pressure-treated or the heartwood of cedar or redwood.
1. Cut, soak, and position the 4 corner posts. Trim 4 of the 4x4 posts to 10 feet. Soak the bottom 18 inches of each post in wood preservative. Set the corner posts in position and temporarily brace them. On paved areas, anchor the braces with concrete blocks; otherwise, stake them in place. Check that each post is plumb. [see diagram, next page]
2. Construct the overhead framing and set it in place. With the help of a friend or two, construct the top frame in place with the 20-foot-, 12-foot-, and 8-foot-long 2x6s. Use 3-inch galvanized screws. Fasten the frame to the corner posts with angled 3-inch deck screws or 3-sided corner brackets. [see diagram, next page]
3. Cut and position the remaining 4 posts. Mark the position of the remaining 4 posts. Drill holes in the concrete for the shields and securely attach U-brackets with lag screw [see diagram, next page]. Trim each of the remaining 4x4 posts to size. (Take into account how high the anchors will hold them off the slab. Also, the posts may differ in length due to variations in the slab.) Drill a hole in the bottom of each post to fit over the lag screw head. Set each in place, check for plumb, and attach to the frame with angle-driven screws or corner brackets. [see diagram, next page]
4. Cut and attach braces. On the overhead 2x6 frame, mark the halfway point between each pair of posts along the sides (not the front) of the structure. Measure and mark the same distance down from the top of each post. Measure between the two marks to determine the length of the long side of each 2x4 angled brace. Cut both ends of each brace at a 45-degree angle and attach with 3-inch deck screws [see diagram, next page].
5. Cut, position, and attach rafters. Cut one 2x6 and use it as a template for the others. Evenly spaced, 31 rafters will be 8 inches apart. You can experiment with different spacings: Closer together, the rafters create more shade. Attach with hurricane ties and 1-inch screws [see diagram, next page].
6. Build a planter box around each corner post. Attach the 1x4s to 2x2 framing with 1-5/8-inch screws. Butt-join the corners, line with 4 mm plastic sheeting held in place with staples, and cover with 1x4 trim pieces. Top it off with a 1x4 ledge and fill with soil [see diagram, next page].
7. Add plants and shade as desired. Plant climbing plants and creepers to increase the shade your pergola provides, or top the rafters with a large sheet of fabric to ensure a shady spot.