sealing the buried part of posts with tar?

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Forum topic by Tim_456 posted 08-05-2010 12:16 AM 26807 views 0 times favorited 13 replies Add to Favorites Watch
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164 posts in 2461 days

08-05-2010 12:16 AM

Topic tags/keywords: question cedar

i’m building a wooden screen for some privacy around my deck. Part of this will include two 4×4 Cedar posts that will be set 3-4 feet in the ground. I’ve had some success by putting in the crushed limestone around posts rather than concrete. I saw a This Old House or New Yankee workshop where they dind’t use concrete they used the crushed stuff.

Anyway, I just sealed my driveway with some tar-paint substance that looks fairly waterproof and I’m wondering if slathering some of that on the bottom and lower sides would a good way to seal the part of the post that’s below ground.

Any thoughts on if this is worth the time and aggrevation and if it will make things better or worse in terms of rot.


13 replies so far

View Dan Lyke's profile

Dan Lyke

1500 posts in 2991 days

#1 posted 08-05-2010 01:01 AM

I don’t know what the current thinking is, and I don’t have any actual facts to bring this conversation, but…

When I was growing up out in the middle o’ nowhere, the common wisdom was that you never set your fence posts in concrete because that’d form a cup that would hold water; the reason for using gravel was to let the water flow away from the bottom of your fence post.

So, in any attempt to try to keep the water away from the bottom of the post, I’d be concerned that I’d be creating a cup that would collect any water, even if it’s just higher humidity flowing internally to the post itself, and keep the bottom of the post wet all the time.

However, I don’t have any tests that’d tell me one way or the other, and I don’t know how one would get actual facts in a process like this ‘cause… well… I need to replace the fence at my house and so far as we can tell it’s 50 years old. And there’s lots about it that was done in ways that I think are wrong. So I don’t know either.

-- Dan Lyke, Petaluma California,

View jusfine's profile


2341 posts in 1792 days

#2 posted 08-05-2010 01:06 AM

Slather away!

I have installed quite a few posts, and you will do no harm in using a water resistant finish.

I would suggest cutting the bottom of your posts so they are not a square cut, then slather them.
Bevel all 4 sides at 45 degrees so you have a smaller flat spot on the bottom.

Old timers told me this over 25 years ago, I installed all my fence posts for our farm with tamping them and no concrete, and they are still holding up well.

-- Randy "You are judged as much by the questions you ask as the answers you give..."

View Gregn's profile


1642 posts in 1849 days

#3 posted 08-05-2010 01:40 AM

It was a practice before pressure treated wood to cover the in ground portion with tar to prevent rot.

-- I don't make mistakes, I have great learning lessons, Greg

View Knothead62's profile


2363 posts in 1827 days

#4 posted 08-05-2010 02:03 AM

This might sound goofy but my father was a farmer for many years. He swears that this is true. Never set posts in the full moon. A local guy experimented with some fence posts. The ones set in the full moon actually sank a bit in the ground. Your inquiry is a valid one and I think Gregn is right on the money. Lots of tar.

View pawpawsteve's profile


16 posts in 1856 days

#5 posted 08-05-2010 02:27 AM

The primary place that posts rot is at ground level and slightly below. I know this from experience of having posts that I set anywhere from 10 to 15 years ago. Some I set in concrete and some I just tamped in. The ones set in concrete have, every one of them, broken at ground level leaving me to dig up a whole sack of concrete and post so I could put in a new post.

Now when I set posts I put a collar around the post about six inches above and six inches below grade. This can be paint or tar, I use roofing tar as it is cheap and I usually have some on hand for repair jobs. I don’t try to cover the bottom of the post as (in theory anyway) this will trap moisture in the post.

I agree with using the crushed stone as fill rather than concrete as it allows drainage and it should be under the post as well so that water does not collect in the bottom of the hole and it saves a lot of tamping.

All of this is a whole lot more important if putting the posts in clay soil that doesn’t drain well compared to sand where the water just runs straight through. I have the sandy soil so I mainly just tamp them in well. One of things I remember from working with Grandpa was “tamp the bottom, the top will take care of itself.

Another thing I do is put a “cap” on top of the post to keep water from entering there. These are coffee cans generally because that is what I have that won’t rust. My wife hates them but if it keeps the rain out I like them. Sloping the top of the post so water cannot pool there is probably as good as covering.

Having said all of that: most of the “barn builders” around here are using a full pvc sleeve that the post sets in from the base to above the splash board. Since these are inside the building and water cannot penetrate the post of from the top, that should prevent any rot from occuring below grade. However for a fence or deck where the top of the post can get wet I don’t think they would work as well.

-- Steve ... determined to die working and believing that God is good!

View CampD's profile


1240 posts in 2352 days

#6 posted 08-05-2010 02:32 AM

Only time you use concrete to set a post is on the hinge side of a gate, helps to keep it ancorded, till it rots that is (like mentioned above). Go ahead and tar with gravel or stone at the bottom for drainage.

-- Doug...

View TopamaxSurvivor's profile


15553 posts in 2542 days

#7 posted 08-05-2010 03:43 AM

WE always soaked them in penta. Some have been in the ground for 60 years, but that is an arid climate.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View richgreer's profile


4541 posts in 1940 days

#8 posted 08-05-2010 04:56 AM

Back on the farm we always had some barrels of used motor oil around. Before we put wooden fence posts into the ground we would take them to a barrel of oil and dip the ends in about 3 feet.

That was our standard practice as directed by my father and his father before him.

The logical question is “did it do any good?” and the honest answer is “I don’t know.” I can only attest that it was messy.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View Tim_456's profile


164 posts in 2461 days

#9 posted 08-05-2010 05:20 AM

whoa, great advice! I’m going to cut the bottom on a bevel, slather it with tar, AND use the gravel; thanks for all the great tips! My two measly posts are going to out last everyone :D

View spclPatrolGroup's profile


232 posts in 1760 days

#10 posted 08-05-2010 05:59 AM

If you seal the boards, how will moisture get out? The wood will asorb moisture, above the tar, and probably wick it in behind the tar, if that happens, then the tar seal would be holding moisture in the post. I would pour concrete, have it come a few inches above the ground, then once it has cured, anhcor yoru post to it, they have little brackets that you anhcor to the concrete and then your posts bolt to those.

-- Dave, from ND "The mind is an infinitely long workbench, and its cluttered with half-finished thoughts and ideas, sometimes we need to clear off the workbench and start again from step one."

View swirt's profile


1978 posts in 1838 days

#11 posted 08-05-2010 05:57 PM

I’d go without the tar. I look at it like this way. When you cut a log down and store it to later turn to lumber, you have to get the bark off, because the bark seals the moisture in and lets the bugs and other organisms get to work.
Example: Hornbeam is a very hard wood, but if you cut a hornbeam tree down and leave the bark on, if you come back a year later, the wood will be pithy and useless.

By putting the tar on it you are essentially coating the timber with artificial bark. Tar is not the same as treating it with something that will completely penetrate and allow water to move in and out of the wood.

-- Galootish log blog,

View spclPatrolGroup's profile


232 posts in 1760 days

#12 posted 08-05-2010 08:57 PM

Also if you live within a city, I have never seen a city code that did not include a whold section dedicated to how to install support posts, so you may want to check that out if you think it effects you.

-- Dave, from ND "The mind is an infinitely long workbench, and its cluttered with half-finished thoughts and ideas, sometimes we need to clear off the workbench and start again from step one."

View Abbott's profile


2570 posts in 2169 days

#13 posted 08-05-2010 10:39 PM

I coat the part of the post that will be in the ground with old motor oil, if I don’t have any around then I coat them with Thompsons Water Seal. Posts last without rot for many, many years that way. I bet tar would also work very well.

-- Ohh mann...pancakes and boobies...I'll bet that's what Heaven is like! ♣ ♣ ♣ ♣

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