Building a pergola that is safe and beautiful
Question: I need to build a pergola to separate a small patio from my driveway. I’m trying to create a privacy screen and as much shade as possible. The issue is I have only a tiny strip of ground to work with and the pergola needs to be more like a fence than a table with four legs, if that makes sense.
Answer: Let’s first talk about the structural challenges all pergolas face. Yours is of particular concern because it’s going to be a tall fence as you already understand. Your primary concern should be horizontal wind load. The total weight of all the materials used to build a typical pergola can be in the hundreds of pounds. You don’t want it falling on someone at a later date.
While you may not intend to have vines on the pergola, some future homeowner may decide to grow flowers and fruit on the structure. Not only does this vegetation add hundreds of pounds, but the dense growth also increases the wind load. Your safety and that of all who sit by or under the pergola are subject to how well you obey the laws of physics.
All lumber used needs to be strong enough to resist cracking or snapping under the force of the wind or snow loads. You live where wet heavy snow can coat the pergola, so plan accordingly. All the fasteners need to be premium hot-dipped, galvanized or stainless steel.
If you decide to use modern treated lumber, the fasteners and all metal framing connectors must be approved for the newer treated lumber that has a higher copper content. Failure to do this will cause advanced and rapid corrosion of the metal from galvanic reaction when the pergola gets wet.
You’ve got to be very concerned about strong winds that can blow your pergola over. A traditional per- gola that might have as few as four posts can blow over, but it’s harder to do if the four posts are bolted securely to concrete piers.
You need to bury your posts much like utility poles are installed. My guess is your pergola is going to be about 10 feet high once it’s all done, so I’d want to see the posts buried at least 4 feet into the ground.
I’d coat the wood that’s going to be in the ground with a readily available copper naphthenate solution and surround the posts with angular crushed gravel that’s the size of large grapes. This type of gravel interlocks and acts much like concrete, but it provides for great drainage once the water leaves the soil in the spring.
It’s all about scale at the end of the day so you need to use big posts. You may get by with 4-by-6 posts, but, trust me, 6-by-6 posts would be better.
Think about cantilevering the top of the pergola a little bit. The horizontal rafters don’t have to be equal on each side of the main beams or the posts. I’d not exceed a 3:1 ratio because it will add a rotational force that could cause the pergola to tilt from the offset weight of the overhang.