Rather than spending thousands of dollars to tear out a concrete slab and refill it with topsoil, consider building a trellislike pergola. This airy, yet sturdy, structure can be built in two or three weekends. Ideal for climbing plants, it creates an area of dappled sunlight that's delightful for entertaining. You can make it as shady as you like by adding climbing plants and creepers, or by topping off the structure with a sheet of fabric. The four corners of the pergola are anchored with planter boxes.
Use rot-resistant lumber, either pressure-treated or the heartwood of cedar or redwood. No special tools are required, but a hammer drill for boring into concrete and a power miter saw will make the job easier. Have two 12-foot stepladders on hand. While you are building, the structure will be unsteady, with nothing solid on which to lean an extension ladder. You'll need assistance in raising some of the members into place. Line up helpers in advance.
The materials list below is for a structure that is 16x20-feet. The eight 4x4 posts outline an area 12 feet by 20 feet. The frame that supports the rafters is the same dimensions, and rests on top of the posts. The rafters are 16 feet long, and run parallel to the short side, leaving a 2 foot overhang on each long side.
1. Set first posts. The pergola is constructed by first cutting and positioning the corner posts, then setting the overhead framing in place. Begin by trimming the corner posts to 10 feet. Soak the bottom 18 inches of each post in wood preservative. Set the 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. The four corner posts form a 12x20-foot rectangle.
2. Construct the top frame. With the help of a friend or two, construct the top frame in place with 2x6s and 3-inch galvanized screws. (This will be easier if you drill the pilot holes while the framing members are on the ground.) Fasten the frame to the corner posts with angled 3-inch deck screws or 3-sided corner brackets.
3. Cut remaining posts. Mark the position of the remaining posts. Drill holes in the concrete for the shields and securely attach U-brackets with lag screws.
4. Set posts. Trim each of the remaining 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.
5. Add braces. On the overhead 2x6, 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.
6. Add rafters. Cut 2x6 and use it as a template for the others. Experiment with different spacings: Closer together, they create more shade. Attach with hurricane ties and 1-inch screws.
7. Build the planters. Build the four planter boxes around the posts, attaching 1x4s to 2x2 framing with 1-5/8-inch screws. Butt-join the corners, line with 4 mil plastic sheeting held in place with staples, and cover with 1x4 trim pieces. Top it off with a 1x4 ledge and fill with soil.