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Fence Post Rot In Concrete Prevention Help

Fence post typically rot near the ground, i believe because the repeated wet dry cycle. I have read if using concrete to slope it away from the post and above grade, so this is what i have done. (Actually a 6x6 free standing pergola post i am building, but same concept)

Here is my idea. Does anyone think it would help if i used a product like drylock on the concrete to stop the post from the wet dry cycle near ground level? Will the drylock waterproof the top portion of the post or will it do the opposite and hold the moisture in. How often would this need to be applied outside? Every year? What are everyones thoughts?

Also if it does rot, how long does do you think i can get out of a 6x6 pressure treated post? i am never going to be able to dig up the post which is 4 feet deep wiht concrete so am hoping it last a good 30 years?

Re: Fence Post Rot In Concrete Prevention Help

Must you "bury" the post? If it's like a fence, or pole barn application, then "yes". But if it's a building of sorts then you could tie the post to a concrete footer. If you have to "bury" it, then here are my off the cuff ideas -- others likely have better ones:

- paint on some roof tar or similar, a 1 gal bucket goes a long ways, let it dry some
- use a rot resistant wood, there are special "ground contact" treatments
- slope the concrete/soil away from the post
- do you need to put concrete around the post? Generally "no", unless you need the added weight. Compact the soil very well with a tamping bar, and forego the concrete if you can. Put a few inches of gravel at the bottom of the hole.

Re: Fence Post Rot In Concrete Prevention Help

There are two levels of pressure treating. If you use a PT post rated for ground contact, then it can last up to 40 years.

Do NOT use any waterproofing like tar on the post at the base. It is not the wet/dry cycles that cause the most rot, it is the wet/wet cycle. You need the wood to breath so that it can dry out after a rain.

Whether incased in concrete or just put in the ground, the post needs something that is slightly above grade and slopes away from the post. The water has to shed away from the base of the post. If it gets trapped there, it will penetrate the wood deeper and stay wet longer, therefore it will be more likely to rot.

One option for your pergola is to pour a footer about 8" in diameter up to ground level. At the top, imbed some threaded rod a foot or so into the concrete and extending a couple of inches above. Use a galvanized or SS all thread. Then use one of those galvanized boxes that is used to attach a post to the footer. The footer will never need to be replaced and the post will last longer and be easier to replace when needed.

You can hide the box at the base with some PT 1x8 to form a baseboard around the bottom of the post. Make sure the top has a diagonal brace to prevent racking.

Re: Fence Post Rot In Concrete Prevention Help

How I would do it:

  1. Dig hole deeper than depth of post. Place drain rock (round gravel) in the bottom, bringing it up to the level of the post.
  2. Insert post into hole, and add a few more inches of drain rock. This will provide a place for trapped water to go -- the bottom of the post should not be sealed in concrete.
  3. Pour concrete around the post to above the level of finished grade; taper the concrete away from the post down to grade level. This will redirect rainwater away from the post.

Rot requires both moisture and air. That's why posts rot more quickly at ground level than below ground: the moisture from the ground wicks up a little ways to where there's air, but below ground there isn't much air to facilitate to rot.

Even with all that, it won't last forever. As others have suggested, make sure you use PT lumber rated for ground contact. With this type, the treatment has been forced deeper into the wood. Be sure to apply treatment to all cuts and bored holes, as the treatment typically doesn't reach to the core of the wood. This will help prevent rotting from the center.

Re: Fence Post Rot In Concrete Prevention Help

Concrete around the post is a poor idea. Once dried it holds its form and shape but the wood expands and contracts. The concrete then forms a bucket to hold water. Either pour a footer with a threaded rod as Keith3267 suggested or set the post in the ground an thoroughly tamp the dirt around it by adding and tampo a few inches at a time.


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