How deep should I dig the post holes for a 2 post pergola in SoCal to avoid water damage, and is using tar on the in-ground portion a good idea?
Asked by Earl Harrison 
4 months ago
Last Updated: July 26, 2024
I’m thinking about constructing a 2 post pergola (check out the pic). I was considering placing the posts in the ground and coating the lower part with tar to prevent excess water absorption (I live in SoCal, so we don’t get a ton of rain usually). My posts are 6x6x8, so how deep should I bury them, or would it be okay to use sonotubes with anchors instead? If so, what size tube should I use and how deep should it be? Just looking to create a cozy spot to relax and enjoy watching the kids play outside.
Constructed one just last year.
Hey , did you place it straight into the ground? If yes, how far down did you go?
I dug down 30 inches for the pre-stained 6×6 posts.
Hey , how high did you build your setup? Is it 6 feet tall or 8 feet tall?
Place directly in the center, positioned approximately 7 feet above ground level.
By the way, I ended up using a single 4 x 8 beam instead of two 2 x 8 beams.
I’m thinking of using 2×8 on each side instead of just one beam.
You can definitely hang those chairs but make sure they are centered. Adding blocking to hang them from will do the trick.
Hey , I do plan on adding some blocking to keep it centered.
I suggest going with 3 1/2 feet in the ground instead of using a sono tube. Anchors placed low won’t effectively prevent racking for long. Ideally, opt for pressure treated posts rated for ‘ground contact,’ or consider coating the bottoms with wood preservative. If you’re in NV or OR, it’s better to purchase there due to the subpar options we’re limited to in CA. With or without rain, posts without preservative will have a much shorter lifespan.
For the post, a 6x6x12 with 4 feet in the ground would give you the look you want from the picture.
Yeah, I believe it needs to be at least 3-4 feet deep in the ground. I still need to decide if I want it 6ft tall or 8ft tall. I’ll go measure soon to brainstorm some ideas.
Determined that 8 feet is ideal. 6 feet is definitely too short. I don’t know why I even thought 6 feet was enough height.
Show us the final product, please.
Will definitely do it once it’s completed.
I would simply use pressure treated posts and install them at a depth of around 3 feet. I prefer backfilling with rock/gravel and compacting it rather than using concrete, the traditional method. If done properly, you shouldn’t encounter any issues. We used to set up highway signposts in a similar manner. In Southern California, you should be all set. That’s just my opinion.
Without braces, it’s a recipe for failure.
It will sway as you move in it and might bend and fall down.
Oh, it’s actually not a swing. Just a hanging basket. But I really want to make sure it’s safe
Hey, I thought it was meant for sitting on
Yeah, definitely sit in the basket. Just don’t swing.
Hey, but naturally, people will sway back and forth a bit. If they swing out so their center of gravity is in front of the frame, it will put pressure on it. Similarly, if their center is behind the frame, I would prefer more stability. If you have two people and one is slightly forward while the other is a bit backward, it will create a twisting force. It should be fine, but it might reduce the lifespan or suddenly fail in the future.
You can use pressure treated lumber, no need to apply any coating.
My husband did this a few years back.
I wouldn’t seal the part in the ground, but I would seal the top to prevent water from soaking into the wood. The goal is to avoid creating a bowl that holds water, so it’s important for the post to be in direct contact with the ground. Put about 6″ of dirt in the hole before adding concrete to help with drainage.
Make sure to dig below the frost line, typically around 12 inches to 48 inches deep, pour a concrete footing, and securely attach your posts with the right hardware. Remember, posts in the ground can rot, especially if you experience extreme temperature changes, heavy rain or snow, and have people sitting on the swings. It’s better to be safe than sorry!
Keep it simple. Put pressure treated posts as deep as you can with your post hole diggers. Fill the hole with concrete or just pack the dirt back in. Use the money saved from not buying sonotubes to get more beer.
There is no support in either the forward or backward direction.
If I were you, I’d consider renting a 12-15″ auger and dig down 3-4ft. After that, pour a concrete foundation and place the posts in the concrete. It’s important to have out-of-plane bracing for stability. Also, using a 6×6 member for the top horizontal part would be a good idea. You can save the 2x8s for accent pieces if you prefer. It’s not ideal to have them as the main load-carrying member in this type of structure.
You mentioned having kids, I’m pretty sure they will love the swings LOL
Hahaha. No way. I like to keep things organized. Just kidding. I’ll make sure to tell them not to use the swings, friends only.
Let’s go with steel 4×4 wrapped in wood for an above-ground setup. The footings should be 16 inches round and 32 inches deep, with posts embedded securely. This method should last for 40 years, unlike wood in the ground which lasts about 20 years at best. If you go with wood in the ground, make sure the footing is 3 times the size at 32 inches deep.