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Wood Post Rot

 
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Hi Everyone,

I'm getting ready to start a small 1000 sq. ft. building and I tried Googling this but to no avail.

Instead of using large timbers spaced 8-10 feet apart, I plan on using 4x4's on 4 feet centers. Basically the construction will be similar to a pole barn. According to Cornell University...https://courses.cit.cornell.edu/arch264/calculators/example7.1/index.html...the 4x4's should be more than sufficient given the weight bearing capacity of the soil, the number being used, and the axial, uplift, and lateral loads being applied.

The question I have is this:

Has anybody ever put the bottom 3-4 feet of a treated 4x4 inside a piece of 6" schedule 40 PVC pipe with a solvent welded end cap? Either fill the internal and external voids with cement or some "Secure Set" post foam? Since the posts will be behind fiber cement siding and protected from the weather, moisture intrusion and UV exposure should be almost zero. Heck, you could even fill the post/PVC void with octoborate and the system would have multiple levels of protection, especially if you used spray foam on the top to seal out moisture. When you do the math for the compressive strength of PVC it far outstrips the bearing capacity of the soil, even with just end-loading. The lateral strength is also well within specs when filled. Also, the frictional coefficient of the PVC is far in excess of any uplift issues. Despite what most people think, PVC is not very slick from a physics standpoint. The cost is minimal if using standard 6" pipe and end caps. The cost per post would come in at roughly $15. Things like permacolumn or any kind of concrete piling with a post base end up being way too expensive and have a problem with lateral loading at the wood base. I plan on furring out the front of the 4x4's to give a flat treated wood surface to nail on the fiber cement siding.  

From what I've personally witnessed, over 100's of times in the Houston, TX area, are ground contact rated 0.6pcf CCA treated 4x4 posts rotting out in under 20 years. I can't imagine the newer wood preservative formulations would fare any better. This was not an isolated case of a post or two rotting out but an absolute certainty 100% of the time. It never mattered if you put gravel at the bottom, used gravel instead of concrete everywhere, sloped the concrete, blah, blah, blah....the damn things rotted out 6" above and 6" below grade. Houston has got several feet of clay and moisture will stay in the hole for decades no matter what you do. I have zero faith in the studies done by placing treated wood in the ground and looking for failure. My guess is these studies are sponsored by the Wood Preservative Association or something to that effect.

Does anybody have any practical knowledge of why this won't work? Please shoot as many holes into the idea as you can. It's much better to look like an idiot here than have a decrepit building a few years down the road.

Thanks Y'all
 
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Hi JP;  
Very interesting idea. I have not heard of, nor thought of doing such a thing !

Trying to shoot holes...  
A standard cement tapered block would distribute the weight better than a 6" pvc would.  
With a clay base, that tapered cement block should stay stable, I could see 6" pvc sliding under a load.  In Texas the only load I could imagine would be wind. Or a herd of long horns scratching on it.
Keeping everything square while erecting the building could be more work with extra bracing.

Seem's I'm running out of bullets here JP.   We both might be missing something but I don't know what.
I'll be watching your post to hear what others are thinking.

 
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I would be concerned that most of the things you might fill the void with would trap/ attract moisture to the wood posts.
The addition of the octaborate might make this irrelevant.
 
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