Why 4-Post Pergolas Are a Design (and Structural) Mistake
Two rules worth knowing before designing a pergola: spans over 12 feet get expensive fast, so use them sparingly, and a pergola should generally have more than 4 posts whenever the layout allows it. A 4-post pergola almost always ends up with what we’d call the “amateur look” — and looks are actually the smaller problem.
We regularly get asked for a design built on just 4 posts spanning something like 16 by 18 feet, and it’s worth explaining why that’s a genuine structural risk, not just a style preference. Running a large structure that close to its structural limits leaves very little margin — the first strong wind, the first sign of rot at a post base, or the first ground tremor can be enough to bring a 4-post pergola down, if it wasn’t engineered with real care for that exact scenario.
The instinct behind 4-post designs is usually one of a few things: an assumption that a post set in the ground simply won’t move, an assumption that a post on a sonotube footing has effectively infinite lateral strength, or sometimes just a desire to avoid extra footings because they’re the most labor-intensive part of the job. Sometimes it’s about keeping a view completely unobstructed — but a pergola is meant to frame a view, not disappear from it, so an overly minimal post count in pursuit of “openness” often works against the very reason someone wanted a pergola in the first place.
The practical guidance: if a pergola design is being pushed toward the fewest possible posts, that’s worth a second look from a structural standpoint before it’s built, not after.
