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I have a large arbor attached to my house, and one of the two 6x6 supports has rotted through at ground level. The posts are 12ft above ground level and the original invoice from the build says they used a 16 ft beam, which matches theory of 3:1 ratio between above- and below-ground wood.

Theory, again, says the replacement post should be in a 4ft deep, 18-inch diameter, sonotube form, which will take 16 bags of concrete to fill.

So, I spent Friday shoring up the arbor and removing the above-ground portion of the post. The weekend was reserved for dreaded task digging out the (presumed) 4ft x 18inch diameter plug of concrete below-ground (I'm in my 60s and doing it myself).

I found was that they had apparently just dug a hole, 3 foot deep and maybe 12 inches wide (obviously they'd only buried 3ft of post), then made a concrete and dirt sandwich, from the bottom up.

  • About 12 inches of packed heavy dirt
  • about 6 inches of concrete
  • More packed dirt
  • Another 6 inch layer of concrete to the surface.

Now, this is clearly way less than most theory calls for, and my first instinct was that they'd cut corners to save money. But, thinking about it, this has some practical advantages:

  • I'd probably still be there next year working my way through 4ft of concrete with a small breaker in order to dig out the post. This approach let me do it in about a day and a half (and not full days as it was in the 90s).
  • It has worked. The post lasted 20 years and rotted before it shifted in any way.
  • Clearly it's less work and cheaper.
  • Replacing it in the future will be simpler

The arbor itself is lightweight, so there's little need for the post to support huge structures. So "theory" is probably a case of over-engineering.

I'm unlikely to be the person replacing this again in another 20 years, but would hate to hit them with the need to pull an unnecessarily large 21 cu ft (3ft deep x 18 inches) plug of concrete (which I estimate would weigh a ton and a half).

On the other hand, I may have just got lucky with this post.

So .. should I replicate the original "dirt sandwich"? Or should I fill the hole with concrete?

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  • For clarity - I'm in CA so frost level is probably somewhere above ground :)
    – kdopen
    Aug 15, 2022 at 17:10
  • If using concrete, I'd set a steel post and bolt the wood to it above soil level, avoiding rotten wood problems. With a wooden post, depending on loading, no concrete at all (or a pad it sets on to increase bearing area at the bottom, but not embeds in) might be perfectly suitable (there's a whole religious war on this point around fence posts...and an arbor is not far off fence posts.)
    – Ecnerwal
    Aug 15, 2022 at 17:52

2 Answers 2

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Not sure how you came up with 21 cu ft.(I think you used diameter squared instead of radius squared) I came up with 5.3 cu ft minus the pole, .75 cu ft would be 4.55 cu ft of concrete. We try not to encourage shortcuts so my advice would be to fill with concrete. If the post did rot in 20 years, the rotted wood could be removed from the concrete and a new post inserted into the hole.

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    Thanks - yo confirmed my own instincts not to cut corners. And, yeah, my life would have been easier if I could have just pulled the post from the existing hole. As it is, I've left the bottom six inches of the hole (in the dirt) undisturbed so I can (a) drop the post into it for alignment and (b) avoid the backbreaking work of digging out another six inches of soil. I may dig that out
    – kdopen
    Aug 15, 2022 at 20:38
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In practice I would pour only concrete in the hole and not a layered mix of dirt and concrete.

That being said, I would never pour an arbor post into a hole- for the very reason you are asking this question- it will rot eventually.

A hole is fine for the concrete pour and then hardware is wet set into this concrete which is designed for holding the post. Attached is a drawing for a Simpson hardware piece (no affiliation to them- I just use their products). This protects the wood from in ground contact with moisture, bugs, etc.

enter image description here

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  • Yep - they used this on the other post for the arbor (no idea how much concrete is below it). Problem is, I'm sure I can get the new post into the hole on my own and set it plum etc. Not so sure I can get that bracket set in the correct place and fix the post on my own.
    – kdopen
    Aug 15, 2022 at 20:35

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