Privacy fence post setting
Good Morning, Has any research been done on setting fence posts in gravel, as opposed to cement? This is a 6' privacy fence that has a good amount of wind exposure. My existing fence posts rotted down into the concrete and my feeling is there just wasn't sufficient drainage due to poor initial design. I suspect that no gravel was used at the bottom of the hole which then trapped moisture in between the concrete, and the wood post. My suspicion is that our saturated high-clay soil will simply not wick moisture away as necessary to prevent rot. My solution is to dig a 3' hole and fill the first foot with gravel, and then set the post, filling another foot up with additional gravel. The final foot would be concrete for stability. After all that, do you have any recommendation as to what type of gravel to use? Or, any general thoughts on this common problem. Thanks so much, Rick W.
Marion County Oregon
Thanks for your excellent question about setting fence posts in gravel vs. concrete. I have not been able to find any research on the subject, but there is a good deal of debate about the best method. Here are the key points:
Concrete provides weight and stability. If your fence is subject to high winds, this might be valuable. However, there will always be a bit of a gap between the concrete and the wood, as the wood ages, and water will get into that crack. This is what accelerates rot of the wood at that interface, so eventually the post may snap at the concrete. Then you are left with a huge, heavy, deeply buried hunk of concrete to deal with.
For this reason I would NOT recommend you use concrete at the top - that is exactly where the problem usually occurs.
Gravel drains much better, so it pretty much eliminates the problem of water being held against the post. It may not hold the post quite as solidly as concrete, but if you use the right procedures it can come close. The buried part of the post should be half the above-ground height. The gravel should be crushed rock, NOT round gravel, and needs to be very well compacted as you set the post.
Here are links to a couple of good articles that detail the process.
And if you want to be thoroughly confused, here is a lot of discussion of the pros and cons of both.
Thanks so much for your thoughts, I've discovered that I can't talk to anyone else about this because I get accused of over-analyzing!
I guess what this boils down to is what is the best way to manage the inherent moisture, and at the same time creating a structurally sound privacy fence that can withstand our annual windstorms that we get in the Willamette Valley? If the fungi that creates "rot" needs moisture and oxygen to survive, then we can safely assume that the problem of rot will always originate at or near the surface. The question then becomes how do we manage that moisture? (We can't eliminate oxygen) I agree that concrete probably isn't the best material to use, but it does offer some pretty measurable stability characteristics. The problem as I see it is that the water that always gets into the wood/concrete interface simply has no where to go. I feel that by putting the concrete high, and the gravel low, I'm creating an environment that encourages the moisture to be wicked out of the interface. If I put the concrete low, and the gravel high, then we still have the same problem of the moisture having no where to go. The high-clay soil down below the surface doesn't seem to have the absorption characteristics necessary to extract the moisture that will inevitably get in next to the wood post. Maybe there is no perfect solution!
My next idea is to cut a 3x3 square of heavy plastic sheeting, cut a hole in the middle, and slip that down over the post prior to assembly. That would help to keep the soil a little drier immediately around the base of the post. I guess my thought is that dry soil would do a better job of absorbing any water that got down into the wood area. I just don't know if a 1.5 ft. radius would really be all that effective.
Ok, that's it. I promise! Thanks again for the information.
PS: I find it interesting that there are a gazillion products available to repair rotten posts, but nothing to prevent it from occurring. I guess we'll have to work on that!
I don't know what the best solution is - there are a lot of variables, and even among contractors who build fences, you will find disagreement. To some degree it depends on the height of the fence, and how solid it is going to be (a more solid fence would catch more wind, and need stronger support).
Here are a few other ideas to consider. I am just speculating, and don't know how relevant these are.
Under some circumstances, water won't move from clay soil into gravel - even though this is counter-intuitive. In a flower pot, for instance, it is not a good idea to put gravel in the bottom "for drainage" - it is hard for the water to break the barrier between fine soil where it is tightly held in small gaps, to the relatively huge gaps between rock. I don't know if this would be true in a post hole, however. And...once the water does break through the barrier, then the hole the gravel is in could serve as a drainage sump for the soil around it - though the overall water level would tend to be the same whether it is in soil or gravel, since water wants to be level. Sooo...one of the things to consider is what the ground water level is in your yard, especially in winter. If it is fairly high, then your wooden post will be standing in water at some times, no matter what material it is surrounded with.
When you've considered all that, you may conclude that the plastic sheeting won't be all that effective. Also it wouldn't completely stop water from getting in next to the post.
Hey, over-analyzing is fun!
Ha! Now we're getting way down the rabbit-hole, I love it!
After all this, I think I'm going to stick with crushed rock at the bottom and partially up the sides. I really want the water that gets in between the concrete and wood to have somewhere to go. We're 300 feet above sea level so I'm assuming that rock will remain relatively dry. And, I would tend to agree with you that the plastic might not really do too much. That's not going to address the moisture at the interface, which is where the microorganisms assemble for battle. Probably my best strategy is to be as diligent as possible at keeping the base of each post as clean and clear of debris as possible. If I can keep the fungi spores away from their food source (wood), then maybe I can make this fence last longer than the previous. Too bad their wasn't some kind of enzyme that could be applied that would consume the fungi. I think you all need to get on that!
Anyhoo, thanks for the great advice, and feedback. I feel better now that I've beat it to death! Thanks for the therapy session.