Burying pressure treated posts in ground - waterproofing to last longer

Started by cbc58, August 24, 2017, 08:37:46 AM

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cbc58

Been reading up on burying pressure treated posts in ground - either casing them in cement or in gravel/dirt - and the overwhelming consensus seems to be that they will last a given time depending on how moist the soil conditions are.  Some say a few years, some say you could get 20 years out of them.  All articles I've read said not to cut the bottom end off since it is impregnated with preservative and if you cut it off that accelerates decay.  I remember reading someone building on here that dug holes, dropped an unopened bag of cement in, and then put the posts on top and then backfilled them in.

My question is on how to extend the life of the wood underground:  what about wrapping the part that goes into the ground in ice/water shield material - or covering it with the black stuff they coat the exterior of foundations with.  Is that a good or bad idea?  It would keep moisture out but possibly also moisture in...

Redoverfarm

Pro & cons on any approach.  They do make a black plastic sleeve.  Down side is that you have to seal the top portion if it is going to be exposed to the elements.  It is probably not a bad idea to place a base in the bottom.  I would use 1/2 of a solid block.


ChugiakTinkerer

I'm in the process of rebuilding a cedar fence that was put in about 25 years ago.  The posts are pressure treated 4x4 that were anchored with concrete.  All the posts are rotting away at the top of the concrete plug.  Below ground level the wood looks like it was put in last year.  I'm a believer in the argument that you want to use packed crushed gravel around the posts, not concrete.  It allows it to drain, whereas with concrete when the wood shrinks it forms a small gap that fills with water.  Rot soon follows.

If the ground conditions allow drainage then I think it's better to just use gravel and the treated wood.  If wetter conditions I'd look for treatment like the utility poles have.
My cabin build thread: Alaskan remote 16x28 1.5 story

Don_P

Decay fungi need several things to grow; moisture, temperature, food (the wood), and oxygen. The bottom of a post is out of the oxygen zone, the post doesn't rot more than a few inches to a foot underground. Moisture you can try to exclude with the post wraps. Temperature you have no control of but decay halts in the winter. Food you can poison with treatment chemicals. They are not all the same. Usually the treated at the big box is treated to UC1, Use category 1, above ground, incidental wetting. Foundation grade is UC4. No heartwood, which doesn't accept treatment chemicals and a ground contact quantity of treating chemical. You'll find those classifications somewhere on the tag. That means more than how you field treat or modify the conditions around the post. If the post is load bearing it needs to be supported on a footing large enough to distribute the weight from the post to the ground without sinking. Concrete around a post as often as not traps moisture and admits oxygen to that ground level area.