To ensure that a pergola can withstand strong winds, it is important to consider the depth requirement for its footings. Wind load calculation plays an essential role in determining the appropriate depth of footings needed for your pergola. The general rule of thumb is that the footing should be at least one-third of the height of the post above ground level. Therefore, if you are building a 9-foot-tall pergola, your posts will need to be set at least three feet deep into the ground.