Build your own Picnic table kit
The next step is to assemble the picnic table structure. First, position the tabletop assembly flush with the top of the legs and clamp it into place. At each end of the top, drive 3-inch deck screws through the inside of the frame and into the legs. Here's where a stubby impact driver really comes in handy, to fit between crossmembers.
Later, install carriage bolts to bolster the joints. Fasten the seat frames by driving deck screws through the outside of the seat support into the frame. Be sure the frame is flush with the seat support to ensure that seat planks, well, seat evenly.
Step 6: Reinforce the Base
To reinforce the table base and ensure that it doesn't tilt out of square, install two diagonal braces. Use a power miter saw or a circular saw to cut the angled ends on these pieces. Slip the braces between the seat support and top-frame cross support and secure them with 3-inch deck screws. Now the heavy work is done.
Step 7: Attach the Legs
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Use a drill and a 13/32-inch-diameter bit to bore carriage-bolt holes through the legs and tabletop frame. To ensure that any splintering occurs on the inside face of the legs, run the bit from the outside to the inside. Use a hammer to tap the carriage bolts through the holes, slide on galvanized washers and nuts and tighten with a socket wrench. If you use longer bolts than those I've specified, be sure to cut off any excess bolt shank and file the cut surface smooth. This will prevent the possibility of injury from a protruding shank. You'll probably have to tighten the parts slightly in a few months after the lumber dries out.
Step 8: Crosscut the Planks
Crosscut the composite decking for the top and seats. To increase accuracy, gang-cut several planks at once. Lay the decking planks across the frame with their woodgrain texture facing up. The planks should have a 5-inch overhang on each end, and the end planks should project 1 inch. Bore 1/8-inch pilot holes through the plank and frame. Use a square to align the bit with the frame crossrails. Fasten the planks with 2½-inch-long trim-head deck screws.
Step 9: Round the Ends
To maintain a uniform ¼-inch gap between the planks, use plastic spacers designed for composite decking. Fit these between each plank.
Finish the project by rounding off the square ends of each plank with a router and a 3/8-inch-radius rounding-over bit. Finally, fire up the grill and sit down to your first meal at your new table.