Flat pergola requires careful planning
Q: 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. Can you offer any ideas? — Joel Z., Sylvania, Ohio
A: Your primary concern should be horizontal wind load. The weight of the materials used to build a typical pergola can be in the hundreds of pounds, especially if someone decides to grow vines on it.
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 weight of wet, heavy snow.
All the fasteners need to be premium hotdipped, galvanized or stainless steel. If you use modern treated lumber, the fasteners and all metal framing connectors must be approved for the lumber’s higher copper content. Failure to do this will cause advanced and rapid corrosion of the metal.
You can’t bolt your posts to piers, as the pergola would tip over with little effort. You need to bury your posts much like utility poles. 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 in the spring.
Let’s talk about the looks of the pergola. I’m a big fan of multi-colored and multi-textured looks. You can achieve this by using different species of wood or different colored exterior semitransparent stains. Mixing rough-sawn Western red cedar with redwood can produce a stunning look.
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. Your top beams that will be on either side of the posts need to be at least 2-by-10s so they don’t look undersized. I’m a big fan of cutting quarter circles out of the ends of all the crisscrossed lumber that creates the shade aspect of the pergola.
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. But 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.
Good luck and let me know how your wife and friends feel about it.