Squaring the posts for a pergola is essential to getting the project off to a good start; without a squared foundation, the rest of the measurements for the pergola may be off. Once the posts for the foundation have been squared, leveling them will further ensure the project's success.
Squaring the posts
Drive a stake into the ground where the first post of the pergola will be placed. Measure from the stake to where the second post should be placed and drive the second stake into the ground. Repeat for the third and fourth stakes so you have a square or rectangular layout for your pergola.
Tie the string around the first stake, run it around the other three stakes and back to the first, forming a square or rectangle. Tie the string off on the first stake.
Make a mark on the string 3 feet away from the first stake in one direction and 4 feet from the stake in the other direction. The diagonal measurement between the two points should be 5 feet, which creates a right angle. If it is not 5 feet, the corner is not square and you can adjust the positioning of the other stakes to achieve the 5-foot measurement. Mark and measure at the remaining three corners to ensure they also are square.
Dig a hole a foot deep for the pergola posts in place of each stake.
Leveling the posts
Set the posts upright in the holes. Brace the posts with the 2x4s.
Hammer nails through the 2x4s and into the posts for stability while the concrete sets. Place the level on top of each post to ensure they are level. If necessary, adjust the 2x4s until the posts are level.
Mix the concrete according to the package directions and pour it into the post holes to secure the posts permanently. As the concrete settles and begins to harden, place the level on top of each post again to ensure they are still straight. If the posts are not level, gently adjust them until they are. Continue checking the level until the concrete is completely hard.