Dig holes 18-inches wide and 3-feet deep for the footings. The holes should be placed at the corners and midpoints of each side, for a total of eight holes. Place the bottom end of a 12-1/2 foot by 6-inch wooden post into each hole. Plumb the post to make sure it is perfectly vertical.
Nail temporary supports to the wooden post to ensure it stays plumb while the concrete sets, which usually takes about seven days. Pour concrete around the post base until it is level with the surrounding ground.
Attach 10-foot by 2-inch by 12-inch lumber between each of the wooden posts. The boards should be attached using four lag bolts per board. The boards should be attached at the top of the posts so that the top edge of the post is flush with the top edge of the boards. For a more elegant look, attach a series of boards on the outer surface of the posts as well as on the inside.
Attach 24-foot 2-inch by 12-inch lumber on top of the double beams, using wood screws. These pieces should be installed at 12-inch intervals to adequately block the sun. Use four wood screws at each end of these pieces to ensure a strong connection. The 24-foot length includes the 20-foot span as well as 2 feet of overhang at either end to extend the area of sun protection, as well as the additional style and custom look.
- Lag bolts are similar to wood screws, only more durable.
- If the pergola is being built in a high-wind area, metal brackets can be used to connect the top members to the double beams.
- For a more distinctive custom look, cut decorative shapes on the ends of the top members where they extend past the double beams.