Slightly shorter than the bench, the back fits snugly between the two back posts. The actual size is determined after the bench seat is in place. The back is held together by screws running through a 12-inch 2-by-2 (ripped from a scrap of 2-by-4) at each end of its three boards.
1. After checking for squareness of the assembled sides and bench sections, measure the distance between the two rear posts. Cut two 2-by-4s and one 2-by-6 to this size.
2. Space the boards 1/8 inch apart, with ends flush. Overlay and attach the 2-by-2s, flush to the ends, with 21/2-inch screws. (Predrilling holes will avoid splitting the wood.) Round outside corners.
3. Stand the bench upright and recheck for squareness. Slip the back in place so the bottom board is 6 inches above the seat and the 2-by-2 backer boards are centered on the posts. Tack loosely in place with one screw through each 2-by-2 backer. Angle the back so it feels comfortable to lean against, then add another screw through each side. Set the screws firmly.
Adding the trellis
The top of the bench has two pairs of 2-by-6 beams connecting the posts and attached with countersunk carriage bolts. Seven 2-by-3s are spaced across the top.
1. Cut four 7-foot-long 2-by-6s. Using the compass, draw identical curving cutouts at the ends of each beam. Make cutouts with saber saw.
2. Lay the arbor bench on its back. Clamp a pair of the beams to the front and back of the front posts so they're flush with the tops and extend equally on each side. Find center line of posts, transfer to top 2-by-6, and mark points 1 inch from top and bottom edges.
3. Drilling the holes through the beams and posts is the trickiest part of the whole project. When doing this step, it's important to keep the drill perpendicular to the work surface. Use a square as a guide. Start drilling with the 15/16-inch paddle bit and make a 3/4-inch-deep hole at each of the four marked points. Switch to the long 1/2-inch bit and continue drilling through the top 2-by-6, the post, and almost through the bottom 2-by-6. Stop when bit begins to break through. Remove clamps and bottom 2-by-6. Switch back to a 15/16-inch bit and drill back into board (using break-through hole as a guide), making a 3/4-inch-deep hole.
4. Reposition and clamp boards, aligning them over the holes. Slip the carriage bolts through all the boards, add washers and double nuts, then tighten firmly with socket wrench.
5. Flip arbor bench on front face; repeat steps 2 through 4 for rear set of posts.
6. Cut seven 48-inch-long 2-by-3s.
7. While bench is still on its front, find center of beams and mark points, evenly spaced (approximately 9 inches apart), for the 2-by-3s.
8. Stand arbor upright. Place 2-by-3s at points marked so they extend equally to front and back. Secure with 31/2-inch deck screws.
Sand all the wood surfaces except lattice. Transport structure to outside location (two strong people can do this, but three are better). Using a small pump sprayer - a life-saver with all that lattice - apply clear sealer to all surfaces. Wipe off excess sealer with cotton rags. When dry, apply second coat, following directions. The last step is to insert the copper-pipe end caps in the holes for the countersunk bolts. Depending on the fit, you may have to hammer or glue them in position.