Construction of any structure requires stabilization from the ground up, starting with the footing. Outdoor pergolas encounter winds, rains and storms that can sway or topple them if they are not structurally sound. Using large posts in larger postholes provides area for concrete in the footing to hold the posts in place. Additional lengths of vertical posts in the ground also add sturdiness to a pergola.
Determine the placement of the pergola. Stretch a string line to mark the exterior outline on the ground.
Dig holes with posthole diggers to one-third the depth of the vertical support posts. For example, for a 9-foot-tall pergola, use 12-foot posts with 3 feet underground for ample support. Dig a hole for each corner post to the correct depth and 12 inches in diameter.
Pour 3 inches of gravel into the bottom of each posthole. Tamp it down with a shovel or hoe handle.
Place a 4-inch-by-4-inch post in the center of a post hole. Level the pole horizontally and vertically, checking it with your level. Lean a 2-inch-by-4-inch board up against the post at a 45-degree angle with the other end on the ground. Nail the board to the post to support the post in its level position.
Combine ready-mix concrete and water according to the package directions. Place the dry powder in a wheelbarrow, add water and mix with a shovel until the concrete is consistent.
Shovel the concrete into the posthole around the post until it is at ground level.
Repeat this process for each vertical pergola post.
Let the posts set for 24 hours. Remove the two-by-four braces and continue the building process.