Cut two pieces of 1×4, each 10 ft. long, and two pieces 9 ft. 5 in. long. Butt the shorter pieces between the 10-footers and drive 1-1/4 in. screws through corner blocks to connect the sides. Tack one side of the frame to the deck and then adjust the frame until the two diagonals are the same length. Fasten a 1×4 angled brace to hold the frame rigid and square.
Assemble a template to position the corners
Build a 10 x 10-ft. template out of 1x4s to help position the corner posts accurately. Screw this frame together so you can disassemble it later, and use the 1x4s to temporarily brace the posts. Shift the template around on your deck to help visualize the best location for the pergola.
Removing deck boards at the corner post positions can be difficult if they’re nailed rather than screwed to the joists. If possible, get underneath the deck and knock the deck boards up with a heavy hammer. If you’re lucky, the boards will pull the nails up with them.Most likely, however, the nails will pull through or split the boards, so you may have to replace several boards.
Build the pockets for the four corner posts
Fasten blocking between the deck joists to support each corner post. Use the template to accurately locate the edges of the corner posts (Figure B).Mark the position of the 2×4 center that extends down between the deck joists (Figure B). Make sure the 2×4 doesn’t land on top of a joist. The one we show sits against the side of a joist.