Everyone seems to think you need to put gravel under your fence posts to protect the post from water damage and help prevent rot. So today we test it. The results surprised me.
12 years ago I installed 30 cedar posts 4ft or more below grade. I'm up in Canada and we get frost from a foot or more in the ground in winter. I ran out of suitable soil to backfill 3 posts and used 3/4" clean gravel. Those 3 posts have never heaved but a lot of the others have heaved up a bit each year. We are in dark Grey clay soil. Gravel is expensive and not a way to go in most cases but I think it works well here in winter. I had 10 inch holes and 6 inch round posts. Cheers 🇨🇦
The chemical reactions between calcium corbonate(portland cement), water and carbon(wood) and the products of those reactions could eventually cause deterioration at the wood/cement interface. The gravel allows the interface between concrete and wood to remain relatively dry. This is pure chemical conjecture and hypothesis from an old chemical engineer. so take it with a grain of salt. LOL
I used gravel in all my four foot deep post holes and after 12 years no movement or rot. I tapped the first few inches of gravel as hard as I could until it would not tap anymore. Filled the rest with tapped gravel. BTW some of the holes had water in them due to saturation in a poor drainage area. I had to drain them out with a hand pump and mop. Despite this saturation area the posts are still straight and no rot. An old timer told me to use gravel because cement will rot the posts at the surface area and that also cement is like a wick that soaks in water. Gravel meanwhile dries more quicker after a rain like on a gravel road and the soil acts like a paper towel to absorb the water away. I also theorized that some of the water in the holes evaporated through the gravel into the air.
It works too. My dad built his brothers deck out of redwood in the mid 1970s, put gravel in the hole, and the posts are still in perfect shape. The Trellis over his driveway, about 10 feet away, didn't get the same treatment and every post rotted away at ground level.
Here in Michigan, I went with all packed gravel, no cement. That's 6 inches of gravel underneath and about for inches around the outside, right up to the surface. After 12 years, there has been no heaving nor any rot.
I installed 300’ of post & rail last year, gave it a 6” gravel base, set post with paver base, and tamped the excavated soils over top. I have heard enough stories about frost heave being prevented by having the gravel, that I felt like the expense and effort was worth it to avoid frost heave.
Because it allows the water to have room to expand in the aggregate/ gravel when it freezes to prevent frost heave and another reason to have your posts at or below frost line for your area. It also allows the water to continue its path to deeper depths below the post. You put a bucket under the post with a bag. Obviously, the water will not dissipate further down the ground... If you don't put gravel at bottom of your posts the water will stay in the upper layers of the soil / around your post / cement, and when it freezes it will grab the post with it. I think this is only necessary in colder climates with deep frost lines.
The gravel is more useful as a capillary break. If you don’t want to use concrete you can pack the whole post in gravel instead of dirt to let it survive longer as it will help it try quicker to reduce the time it will want to rot.
when my parents bought their current property it had a failing fence on it. we set a new fence directly in the dirt, and it was just fine for close to 30 years. all I've seen in my experience is that the only difference concrete makes is, it makes it harder to get bad posts out.
We do not put gravel in the hole either. Speaking of Victor's bucket trick, that came to mind yesterday when I was working with buckets and how handy it would have been to put a block on the ones I was using.
Just replaced my back privacy fence installed in the 80’s all post were drypacked with with soil only and are still good shape but i replaced them anyway
What you said makes sense me to me. Do yall have any videos or can make a video of putting a fence on boulders/bedrock? I have 16 acres of this and looking for ideas. Thank you in advance.
Here in south Florida it is not uncommon for deeper holes to get ground water in. Some coastal areas have salt water underground! The whole gravel in the hole is strange for sure as some old buildings had wood pilings as part of the foundation!
Once the water reaches the gravel it's going to sit there until it can percolate down through the ground which by the way on a rainy day is also saturated for hundreds of feet surrounding the post. I would think that the moisture surrounding the gravel will fill the void space with water and dissipate only when all of the nearby surrounding ground dries.
Curious, if mosts wood posts rot out right at ground or below ground, would there be any benefit to applying a pruning sealer from the point where the concrete ends to the top level of the ground?
The water is supposed to percolate through the gravel into the soil beneath it. Yes, it is helpful to put 6m inches of gravel underneath the the wood post.
I think I’ve dug out/jacked out most of the concrete slugs left in the ground in my backyard fence from rotted off wood posts. Some didn’t last 10 years. No wood in concrete in the ground.
Gravel maybe, if you do it right, might affect heaving from frost. Or not. Geopolymer cures over twenty times faster than OPC. Maybe try that for some six day sets?
I believe as the lumber will wick the moisture to the bottom and the gravel will let water go down and dry out the top of the post at ground contact location, slowing decomposition. But like you said it depends on soil type, ground water level and annual rain. South Florida has very sandy soil so, I am not sure gravel would help much as long as the bottom of the post is in contact with the soil and not encase by concrete. - Maybe that is why the recommend the gravel, so that the bottom of the post is not encased in concrete thereby allowing the lumber dry faster. - Another good video and something that has crossed my mind before.
Me as soon as I get paid from this commercial job I am going to go back to no dig. My rotohammer is going to get some work buy can't help but think it is better then auger/jack hammer and cement.
When I saw you prepping for the pour I was wondering if the bucket had drain holes. When you dug it back up clearly there were no holes. Leaves me wondering if you had drain holes if that makes a difference over years of use.
The key thing here is OXYGEN, not water. Posts will rot where there is plentiful oxygen and alternating wet / dry conditions = microbes will thrive. Why do you think old wooden ships are so well preserved for 100's of years at the bottom of the ocean - ZERO OXYGEN. Good video though, thanks for posting.
Well I guess I guessed right per this vid. Just set 14 PT posts in 36" D holes (35 total) without putting gravel in the bottom in hard packed GA red clay. I guess when whomever owns this place in 25 years when I am long gone from here will have to deal with whatever rot happens then. 😁
I believe poles backfilled with GA red clay will last a long time. How much pole is above the ground? Wondering if a 6 foot wooden privacy fence would stand up to strong wind damage better if the posts are weighted with concrete. The fiberglass utility "street light poles" in my north GA neighborhood are not set in concrete. You can shake one back and forth and watch it loosen. Guess is sets up again after a good rain.
@@chetmyers7041 I did use concrete, guess I forgot to mention that part. Posts are 8' above ground (using 8' pickets), 3' in ground, cut off extra foot for code. Holes are ~36"d x ~12"w.
Idk. I think the idea is purely for drainage. The only problem I've ever had is at grade where the post meets the dirt. Maybe this prevents build up there by allowing drainage.
