Okay to imbed deck posts in concrete?

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Old 04-26-12, 12:06 AM
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Okay to imbed deck posts in concrete?

Iíve read about a contractor who always imbeds deck support posts deep into concrete that he pours into holes in the ground. This method has the big advantage of making rock-solid posts that donít need any bracing with the deck joists. Still there are other contractors and carpenters who are concerned about the posts possibly absorbing moisture from the concrete, causing rot. Just to be on the safe side, I took the advice of a local contractor and put my deck posts on top of above-ground concrete foundation blocks, but I am still interested in the imbedding method for future projects if I can possibly use it safely. Iím wondering if this is just a regional climatic issue or what, like maybe it is okay in some places but not in others. Anybody have any personal experience, good or bad, with this method?
 
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Old 04-26-12, 03:57 AM
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For the reasons you mentioned is why deck posts should never be buried. The stability factor is there for a few years, then rot sets in and you'll have an unsafe deck. Using sonotubes poured with concrete and installing post bases above grade will lengthen the life of the posts and still give a very stable deck. If your contractor used dek blocks, I question the load that will be placed on them and their ability to hold it without sinking. Of course, in Hawaii, you don't have a frost line so that's good. In the northern US, some footing depths go to 3' or better to prevent freeze heaving.
 
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Old 04-26-12, 09:49 AM
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Don't do it - Chandler steered you right.
 
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Old 05-09-12, 02:44 AM
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Thanks for your input, and thanks for suggesting the sonotubes. They are a great product, and they are used here in Hawaii in some applications. Iím sure they would have made my deck super solid, like you said, but since this particular deck is short and narrow and will be getting pretty light loading, I think that it will probably be okay without them. Iíll definitely keep them in mind for possible use in future projects, so thanks for suggesting them.

Here are a few more details about my deck post installation: The foundation blocks are 12Ē high and about 11Ē square at the base, resting on top of heavily-tamped and compacted gravel, which in turn is on top of heavily-tamped soil. So fortunately, settling is probably not a problem.

The contractor who imbeds posts in concrete below grade lives and works in North Carolina, where the frost line is 20Ē down. He claims that most building codes allow this method. He also says heís been doing it for years with no problems. I donít know how reliable his information is, and Iíd like to get your take on what he said.

He uses treated lumber, and that may make all the difference. Do you think that could be so? Based on what you said, I wouldnít try it myself. But have you seen anyone ever do this successfully in any situation? I obviously cannot do a long-term test myself, so Iím still hoping to hear if anyone has done this successfully, especially in a place like where I am, where it rarely gets below 65 degrees and the rain is light. If I donít hear, Iím giving it up and sticking with the sure thing.

Thanks for your time and help.
 
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Old 05-09-12, 04:15 AM
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Your NC friend has probably been out of touch with best practices for a while. We never embed deck posts in the ground. NOW, with that said, you live in an area where there is little chance of frost , so your rules will differ a little. If the deck is low and the ground as you say, you may be able to get by using a product called dek blocks. They are pyramidal shaped concrete post supports that sit on the surface of the ground. This may be what you referred to in post #1.
We embed pressure treated fence posts, but you aren't likely to have a party on top of a fence post. Rotted deck supports over time will cause problems with stability.
 
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Old 05-09-12, 05:37 AM
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I don't know about today but before I retired it was common practice here to embed the posts in concrete but that doesn't make it a good idea!! I agree with Larry that the bottom of the posts should be above ground.... and we always use PT wood for the deck posts and framing.

Doesn't Hawaii have a bigger termite problem that would dictate no lumber touching the ground?
 
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Old 05-15-12, 01:32 PM
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Thanks for the good perspectives. The blocks I used fit chandlerís description of dek blocks, so they must be the same thing. I capped them with galvanized sheet metal plates that we call termite shields here, because, yes, termites are a major problem here in Hawaii. The ground termites will make mud tubes up concrete surfaces to get to the wood, so they have to be thwarted with the shields. I used treated lumber, too. After all that everyone has said, Iím glad I built the deck the way I did, since youíve convinced me that the imbedding-in-concrete option is really not an option. Thanks again for your advice.
 
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Old 05-15-12, 04:33 PM
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Looks like some of the locals are still embedding posts in concrete. About a mile from me they're putting a good size addition on a log cabin including a wrap around porch.... and they've embedded all the 6x6s in concrete
 
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Old 05-15-12, 11:50 PM
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Don't be surprised--on a new $500,000 "lake cottage" a few doors down from our last place in Colorado, the high-end "builder" constructed the big, fancy deck using elephants' paws inside of 10" Sonotubes without even cutting off the reducers! In effect, reducing the 10" diameter to less than 4". So much for quality construction.

Let the buyer beware.
 
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Old 05-16-12, 03:51 AM
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Makes you wonder! Here, if we are within so many feet of a dropoff (mountains) we can't even use sonotubes for deck posts. Continuous footings 12" wide 24" deep (12" deeper than the frost line).
 
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