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Wood fence post question

J

John-MI

Member
In the southern states the fences along the hiways have wood posts with a cap on top. What is the purpose of the cap? Never seen this in northern states?
 
(quoted from post at 13:20:11 02/20/23) My guess is it keeps water out of the end grain.
Click to expand...

Some highway departments use mechanical drivers to drive wooden posts into the ground. Maybe the metal cap keeps the driver from splitting the end of the post. Just a guess but I have seen the turnpike guys in KS driving wooden posts but did not look at the end of the post.
 
To keep the post from rotting out from the top down. Some cot the top of the post at a angle if they dont want to cap it. You see all kinds of caps. Roof flashing formed to the post old metal cans ect.

With 60 inches of rain beating on the top of the post each year all it takes is for a small hole to form in the end grain and the post will rot out.

Years ago another poster here questioned my answer to this question. He said there is no way rain would rot the top of a post. I took a picture of a post I had just replaced and made him a believer.
 

Another big issue beside rotting is freezing. As the top of wood post soak up rain then subject to freezing temps they tend to start splitting. I have seen them split all the way to the ground. When we concrete wood post in the ground we put a little dirt at the bottom end so post isn't standing in water to try to prevent the concrete from busting.
 
In N MN most everyone uses cedar for posts, we have a lot of it. We usually get a lot of rain, but they don't rot from the top, they rot first just below ground level. I have pulled old posts out that were good on top and bottom, but almost rotted off just below ground level. I think to rot you need water and air?
 
(quoted from post at 19:20:39 02/20/23) In N MN most everyone uses cedar for posts, we have a lot of it. We usually get a lot of rain, but they don't rot from the top, they rot first just below ground level. I have pulled old posts out that were good on top and bottom, but almost rotted off just below ground level. I think to rot you need water and air?
Click to expand...

We use black locust, and it's the same here. Posts rot off just below ground level, though the ends do rot but not to the same degree.

If it took water and air to cause rot, all the wood ABOVE ground would be rotten. It's the bacteria, fungus, etc. in the topsoil that rots the wood.
 
Post caps are used all over the place. I have black locust posts that have been in the ground for nearly 100 years that still have the original sheet metal cap nailed on.
 
Wood needs 3 things to rot.
Moisture Oxygen and Fungus.
The fungus is in the air you breath.
Oxygen is also in the air.
So most try to prevent rot by keeping moisture out.
But using a chemical to kill the fungus also works. Think treated lumber.

Right at ground level is a perfect place for all 3 to come together because the ground holds moisture.
But a leaking roof or gutter can provide the moisture for rot to start so you do not need to have dirt involved.

With a fence post the end grain is not smooth like the planed side of a board.
This end grain is a perfect spot for drops of water to sit.
That is why some cut the top at a angle like I mentioned so the drops of water will run off.
Once these drops of water sit rot starts and makes a bigger spot for water to sit.

If you ever see one of these post rot from the top the thing you will notice is the center rots out yet the outside of the
post is still good. That is the chemical treatment of the outer part of the post preventing the fungus from moving into
this part of the post.
 
Here a post will rot through at ground level. The above ground and below ground will have some life left, but where the air, soil, an moisture meet the wood disappears.

But I sure understand your explanation as the top of the post holds any water it deteriorates and rots some frame the top down for sure. Keeping the water out of the end grain where it can wick in can help save some.

Paul
 
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