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I'm geting ready to install a couple new wooden privacy fence gates on the sides of our house. One of my co-workers suggested to wrap the 4"x4" with plastic stretch wrap around the 2' part that will set in the concrete. He had heard that the plastic wrap will keep the lime in the concrete from eating into the wood over time.

Has anyone heard of this or what do you think?

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I've never heard of that and I think your co-worker is a little crazy. –  DA01 Apr 27 '12 at 20:10
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Anything that will seal in water will accelerate the rot. It's fungal and microbial action on wood lignins and cellulose that is what destroys fence posts. Capping the top of the post so water doesn't get into the end grain, providing slope in the concrete away from the post to prevent water pooling and providing gravel in the bottom that the post sets on for drainage will do more for your concerns. As Ryan says below. –  Fiasco Labs Jun 28 at 17:11
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Advice I've heard is "Pack the end-grain of the end that goes into the ground with epoxy. This will keep it from wicking up moisture from the ground, while still leaving enough exposure that it'll reach equilibrium." Gods only know whether that's good advice, which is why this is a comment. –  keshlam Sep 12 at 21:36

10 Answers 10

Most concrete and cement these days do not contain lime. When it contains lime its called Limecrete and generally it is used as a type of mortar for older buildings. Anyway If you wanted to protect the posts you are best soaking the post in creosote it's a type of oil that is used specifically to treat wood for this very reason its also used in tar distillation and is commonly available. You can also soak the wood in general motor oil for a week as it absorbs it well and repels water. The plastic would in fact trap moisture in the wood which in effect would increase the rot. Ideally you want to dig the hold deeper and put gravel under the post to allow water to drain easier and then fill the rest of the hole with concrete

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I'm definitly putting gravel in the bottom first. –  TornadoKnight Apr 27 '12 at 21:49
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Also remember to pack and compact it. because the wiggle room in the bottom is amplified the further towards the top of the post you get. water expands the post and then it contracts in the dry. Water is public enemy number one. –  Ryan Walkowski Apr 27 '12 at 21:52
    
Thanks. Good to know. –  TornadoKnight Apr 27 '12 at 21:56
    
Both Kreosote and Tar are both toxic materials, though. Common, but just something to consider before sticking into the ground. –  DA01 Apr 27 '12 at 22:27
    
A valid point however the toxic variety of creosote was coal tar creosote but it has been superseded with wood creosote which is very much less toxic and you now get a water/gas formulations which are meant to be non toxic. –  Ryan Walkowski Apr 27 '12 at 22:39

If what I am thinking your plastic wrap is, I think it is too thin and would probably be destroyed on installation.

I covered the same area in roofing tar and they have been in for over 25 years. I came up about six inches above ground level. It's been through earthquakes and high winds and the posts have never been replaced. My next door neighbor did creosote and he has had no problems either.

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This is a horrible idea IMO. Check out The Fence Bible from you local library. The cement does not rot your posts, the moisture does. Concrete holds moisture. If you don't allow the concrete to wick moisture out to its surrounding dirt, it will rot your posts quick.

I think the best way to do posts is to use just gravel, no concrete. Tamp every 6 inches and you could get it as solid as concrete and this would keep you posts as dry as possible. I was going to do this for my new house, but I decided to use aluminum fencing to avoid the issue entirely.

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This is what I like, or you can use class 2 base rock, but we have no clay out here. –  dbracey Aug 21 '12 at 20:21
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Major work compared to just pouring bags of quickrete and hosing in some water, but so many advantages: drainage, post removal... –  Evil Elf Aug 22 '12 at 13:03

An Oregon project. I soaked my 6"X6"X18' deck roof support posts with two coats of Flood preservative stain. The posts had been kiln dried then sun baked before treatment. The stain dried in the hot sun for a few days. The surfaces were then flooded with Helmsman Spar Urethane - 3 coats. The urethane extends up a foot above concrete to shed water. The concrete is mounded, finished and painted. The post sits on 3/4 minus at the bottom of the hole. I've used this UV protecting urethane coating to protect solar panels on a mountain top repeater installation. After ten years of extreme weather exposure the urethane coating was intact. It is tough material. There is no doubt that the contact surfaces on my posts are impermeable.

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While polyurethane might count as plastic, the OP was specifically asking about plastic wrap. –  Niall C. Sep 13 at 14:32

I have been a fence contractor for 25 years. It is important to keep the soil away from the base of the post. Wood rot starts there because of moisture, microbes and fungus. Post rot does not start underground because there is no oxygen there or fuel.

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It is important to crown or slope the concrete above the grade to keep soil away and so water runs off and doesnt sit at the base and pool up. Very simple. Tar wears off, so do other products. There is only one product out that works and it is called The Post Collar. It seals the base of the post and has a galvanized metal barrier to protect the sealant. We are using them on all our jobs.

So crown the concrete, no tar, creosote is banned in the US and so is Cromated Copper Arsinate (CCA). Crowning concrete
The Post Collar

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There are commercial products on the market designed for exactly this purpose.

One example can be found at http://www.postprotector.com, which manufactures a plastic sheath for 4x4 posts, intended to be buried in the ground.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

    
While there's nothing wrong with the link, it would be best for you to briefly summarize the linked article so that the information is visible here. –  Chris Cudmore Aug 22 '12 at 17:57
    
This is to prevent soil contact, not cement contact. Soil microbes and fungi attack wood. Cement isn't very conducive to their existence. –  Fiasco Labs Jun 28 at 17:15

I've tried wrapping the submerged part of the post in Blueskin foundation membrane, including the butt end. I think this should work excellently, so long as the Blueskin rises above the ground/concrete level, and doesn't peel back from the sun.

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I have been told to use a dry ballast/cement mix using minimal water to avoid too much moisture getting to the post, I have also treated them all before installation

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And what about the plastic wrap that the OP asked about? –  Niall C. Jun 28 at 12:40

I have heard of wrapping the bottom of wood fence posts with rubber flashing, or (better) "packing" their end grain with epoxy, to reduce the rate at which the wood takes up water and hence discourage rotting.

I haven't heard of plastic wrap being used for the purpose.

And I have no idea how either would interact with concrete. In fact, the advice I've gotten has been that concrete is not currently recommended for fence posts in most soils, and that when it is used it should be a concrete footing that the posts sit on top of rather than being embedded in. (Wish I'd read that before I built my rain-barrel platform. Oh well.)

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Where did you hear about concrete no longer being recommended? –  wallyk 15 hours ago
    
Wish I could give you a citation... Concrete under a post, as footing, is fine; concrete cast around it, I've been told, risks trapping moisture and encouraging rot. I've been meaning to check with the UW Forest Products Lab, who are the most authoritative source I know of on wood as a construction material. –  keshlam 6 hours ago

The main reason to wrap the in ground portion of a wooden post in plastic is not to prevent water getting in but to provide a slippery surface so that the post can not be pushed up by frost in the ground. Gravel at the bottom of the posthole is a must for drainage.

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