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I was planning a relatively simple, 6x8ft "2-post" pavilion, just a small structure to provide protection to a small bench.

This design would require to bury the wood into the concrete several feet. Is it OK to bury wood in concrete this way? How to avoid getting water in it so it does not rot? Or is there an alternative that is considered better?

Alessio Sangalli
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"Pressure treated wood" isn't one thing. It's many things. Some is rated for ground contact or below grade use. Most isn't. You'd need lumber that is.

You won't keep it from getting wet underground, but you can keep it from being constantly saturated by giving the concrete sleeve a drain at the bottom. One strategy is to set the post on gravel and just pour the concrete around the post, not under it. This only works if the surrounding soil drains well also, though.

isherwood
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    I like the gravel at the bottom and regularly use that method it also keeps the concrete from collecting a bunch of dirt when poured into the hole. – Ed Beal Jul 06 '20 at 22:57
  • Yes of course I've heard of "Ground Contact" treatment but that does not (at least in my mind, hence the question) necessarily mean that it's OK to bury three feet in concrete, so I infer there is an ever higher grade called "below grade"? I'll look it up. – Alessio Sangalli Jul 07 '20 at 07:36
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    "Ground contact" implies acceptability for use below grade. The wood doesn't care which side the ground contacts. :) If you read my post again you'll see that I used the terms synonymously. – isherwood Jul 07 '20 at 13:19
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In my last house, I built a very large deck and all the uprights were installed in concrete that were buried 3' minimum into the soil below. The deck was very solid and done to my perfection. 20 years later when I was selling that home, the treated 4X4's that were the uprights were soft and seemed rotted about 1/2" into the wood. I was told by a home builder that was because the wood posts were in contact with the concrete surrounding those posts. I was told that burying the treated wood in the concrete made the deck extremely strong but that the softening of the wood, just above the concrete was the draw back. He also said that the posts would have to be replaced when the softening of the wood was compromised enough to weaken the posts. He did not specify when that would happen but said that it would probably happen in 50 or 60 years. (just a guess)

d.george
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  • A 4x4 has 12.25" of cross-sectional area. Reducing it to 2.5" square brings it down to 6.25" of area. That's a reduction of nearly half. The posts should've been replaced then, not in another couple decades. – isherwood Jul 07 '20 at 13:16
  • Also, this doesn't really answer the question unless you can specify to what degree the posts were treated. As I mentioned, there are various grades. Perhaps yours weren't rated for ground contact. (Most 4x4s aren't.) – isherwood Jul 07 '20 at 13:18
  • @isherwood; I built this deck 42+ years ago when they still produced a good treated wood. And @ Anthony Stevens; the holes were 3" deep X 24" in diameter with the concrete being reinforced with rebar. ( I was much younger then and work was easier). – d.george Jul 07 '20 at 15:01
  • Your post says 20 years. I'm confused. I did assume CCA treatment, which also has always come in various concentrations. There's no "good" and "not good". :) – isherwood Jul 07 '20 at 15:37
  • The load bearing of a vertical post is surprisingly high, so maybe even with half of the cross sectional area removed it's still way above the working limits...??? However, why did the wood became soft, that means it stayed wet pretty much all the time? – Alessio Sangalli Jul 07 '20 at 20:10
  • @ isherwood, the house that had the deck was built around 1978 and was sold in 1998. I knew the people that bought that house and I asked about the deck. They said that the deck is still in reasonable shape and that the uprights are still in descent shape. @Alessio Sangalli, I asked at the factory where the wood was "Wolmanized" and they said that posts should not be in contact with concrete and if they are the result would be deterioration or softening of the wood at the contact point. – d.george Jul 08 '20 at 15:59
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The biggest problem with that approach is that the slightest lateral movement of the post is going to crack the concrete. It’s just too much leverage against a brittle substance.

Since you are bothering with a hole and concrete, I would suggest using anchors. Pretty easy, just fill the hole and insert the anchor. Let it fully set and build. This keeps the wood out of the wet.

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Anthony Stevens
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  • I've not had good experience with anchors on anything which can have a high side-loading from the wind. I had a whole new fence snap it's anchors over a winter. Of course once one goes, it increases the stress on its neighbours. – SiHa Jul 07 '20 at 14:52
  • Anchors like that aren't intended to hold anything up. They just maintain position. – isherwood Jul 07 '20 at 15:37
  • Of course I'd like to use anchors but with a 2-post pavillion, the lateral forces are substantial, and at least the anchors in the picture do not seem good enough, but I'd be very happy to discover they are adequate. I am not sure about "since you are bothering with a hole and concrete" - do you have any other solution in mind for a two post pavilion? – Alessio Sangalli Jul 07 '20 at 20:07
  • Ah, I misunderstood the problem. I just dislike the idea of burying wood. Sooner or later, that is going to weaken and lean. Any chance of upgrading to something like aluminum? If this was going up a commercial property, the whole thing would be aluminum. Now I know you don’t want to do that, but if you set a 2 foot aluminum tube in the concrete, leaving 6 inches out of the concrete, you could insert the wood post into that. You could even undersize the metal and carve down the end of the post. – Anthony Stevens Jul 07 '20 at 21:17
  • https://www.onlinemetals.com/en/buy/aluminum-rectangle-tube – Anthony Stevens Jul 07 '20 at 21:19
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    @Anthony Stevens Aluminium and concrete don't play nicely together (unless the aluminium is fully protected by some coating ) - it would be worse than using wood. – NMF Jul 07 '20 at 21:33
  • Ah, did not know that. Well, better leave this one to the pros. – Anthony Stevens Jul 07 '20 at 22:31
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There are stronger anchors that involve embedding large and long treaded bolts into the concrete that has a bottom piece that attaches to the concrete while the upper piece attached the wood to the bottom piece.

Also, could put the wood into the poured concrete but the wood will need to have heavy plastic bags that cover the entire wood that is in the concrete and should show above the soil level. A barrier is needed so that wet concrete doesn't contact the pressure treated wood. I used this method for a wooden swing set using 6inch by 4inch by 12 feet A frames on each end of swing set where the ends went into the ground. The holes were 3 feet down (below frost line) added rocks to fill bottom of holes and then positioned the covered wood (about 2 feet down) and then pour the concrete which needs to fill in holes between the rocks (which means rocks needs to be large enough). This would be overkill for a ladder. That was in 1994 and is still as strong as it was back then. Grandchildren are using it now.

Hank Schrandt
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