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Concrete for wood fence posts?

Posted by shiane (My Page) on
Fri, Oct 16, 09 at 21:31

We need to replace our fence and plan on using pressure-treated privacy panels along with our existing pressure-treated wooden posts. Our posts are not concreted into the ground and some are leaning. Everything that we read concerning wooden fence posts tells us that they should be cemented into the ground. When we have had fencing companies come out and tell us what they would do they all say that they don't use concrete with wood fence posts. They say that the posts will rot and some even claim that the the posts will break off at ground level in a major storm! We have read that the correct way to do fence posts is to first have 4 or 5 inches of gravel and then place the post on the gravel and use cement to about 2 inches beneath the surface of the ground. Which way should we go? Concrete the posts or not? (BTW we are on Long Island, NY)


Follow-Up Postings:

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RE: Concrete for wood fence posts?

I built a large fence several years ago and researched this topic myself. After talking with a lot of fence builders and doing online research, I concluded that the best way to go was: pressure-treated posts installed on top of a gravel base, and concrete all the way up to ground level at the edges of the hole, with a taper in the concrete surface so that it is above ground level where it meets the post. That way, water will tend to run off and away from the posts where they enter the concrete. Don't stop the concrete below the ground level as it will encourage rot at that point.


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RE: Concrete for wood fence posts?

The rot problem occurs if the bottom of the post is inside the concrete.

There is no way for water to escape and the bottom of the post sits in damp all the time.
Real ground contact PT can take it for a while, but the typical home center PT is not treated heavily enough.

Dig hole.
Add a few inches of gravel.
Put in post.
Add another few inches of gravel.
Fill remainder of hole with concrete.
Slope top away from the post to not encourage any extra water to soak into the wood.

I usually use pea gravel since it is easy to handle and packs well.
Larger stuff does not settle in as nicely.


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RE: Concrete for wood fence posts?

Typical here in my area, rainy humid florida & really fine beach like sand - it is only the corner or gate posts that have the concrete. And its gravel/dirt/sackrete, no need for water - it pulls enough moisture from the soil - and lazy easy. Even old PT with no concrete rots off at ground after 20 yrs. The ones with the concrete seem to last the same.


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RE: Concrete for wood fence posts?

Additionally, even with PT posts, you might also buy some wood preservative, pour it into a bucket and set the post ends in the liquid to sit and soak for a few days. The end grain will soak up the liquid and increase the rot resistence. I've done this on various garden structures in addition to fence posts and it seems to have worked well. Like brickeyee says, there are grades of PT, so you might alos look for a professional supplier supplying wood with the heavier treatment.


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RE: Concrete for wood fence posts?

"Even old PT with no concrete rots off at ground after 20 yrs."

Sounds like wood not treated for direct ground contact.

The USDA Forest Products lab has a long term test going with PT wood.
The old CCA stuff was going over 30 years without issues, and the last update I heard was that they might have to end the test since they are running out of posts.
They pull the post and test it, but apparently do not put it back (they are probably sectioning them to see how solid the wood really is).


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RE: Concrete for wood fence posts?

shiane,

I do fence posts the same way brickeye detailed. Instead of putting the posts in a concrete "bucket" where the base of the post can end up saturated with held water, you end up with the base of the post surrounded by a concrete "collar".

This past spring I had to pull and replace 14 year-old PT fence posts set with collars. We pulled them with a backhoe and chain, the posts were fairly pristine when pulled. No rot or decay, the wood still looked fresh.


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RE: Concrete for wood fence posts?

If you're a little tight with the buck, like I am, you may want to try my modification of what is basically Brickeyee and Mongoct's method:

Put rocks or gravel as a base, put in the posts and a couple inches more gravel, then a concrete collar for about six inches, then more gravel up to near the top, then another concrete collar of about 6 inches, sloped away as they described. I also like to sink a few galvanized spikes halfway into the posts where that bottom collar is so the collar grips the post better to prevent heaving.

Having the concrete collar both high and low will help insure the posts remain plumb because the concrete is pressed against undisturbed ground. I had originally planed on using Brickeyee's method but was getting tired of mixing bag after bag of cement to fill up the hole. I think my "poor man's" method uses about 1/3 of the full collar method.

It's been about 8 years, and so far, so good.


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RE: Concrete for wood fence posts?

What goes into the ground should be concrete. That can be achieved by a concrete precast base with a metal bracket for a wood post or a solid concrete post.

Here is a link that might be useful: concrete post support


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RE: Concrete for wood fence posts?

Hey macv - Is that hardware you linked to designed for non-top supported structures, like fences? Simpson post bases all seem to have this description: "Post bases do not provide adequate resistance to prevent members from rotating about the base and therefore are not recommended for non top-supported installations (such as fences or unbraced carports)."

Thanks,
Bill


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RE: Concrete for wood fence posts $

Looks good, Macy, but I'll have to call your method the "rich man's method" for using an $82 bracket to support a ten dollar post. Should last for almost ever, though...


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RE: Concrete for wood fence posts ?

Good point, bill g web. If I was going to use those, I think I would at least put them the other way, with the plate and bolts at right angle to the plane with greatest stress on the fence.


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RE: Concrete for wood fence posts?

I considered using steel bases planted in concrete when I was building my fence, but found that they were not allowed by code -- at least in my area -- unless the fence is connected to some other structure, like a building. That's because, depending on the design of your fence, the wind-loading can be enormous (think about maneuvering a 4' X 8' piece of plywood in a breeze, and then imagine that type of force applied to the overall surface area of your fence in a wind storm).

One alternative to the problem of wooden fence posts that I've seen is to sink standard 2" galvanized cyclone fence posts into concrete and then drill through them so you can bolt 2X4's to them. Then you attach the stringers, etc. to the 2X4's. You can completely hide the galvanized posts by boxing them with wood if you want.


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RE: Concrete for wood fence posts?

Why would a fence not allow wind to pass through it and why would concrete not be stronger than a post set in the ground?


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RE: Concrete for wood fence posts?

"Why would a fence not allow wind to pass through it and why would concrete not be stronger than a post set in the ground?"

Almost any wooden fence except maybe split rails has enough area for the wind to generate a rather large force trying to tip over the fence.

The most common fence brackets do not go into the concrete far enough, and are themselves not strong enough to reisist overturning.

They are intended for things like deck support posts that have a lot of weight and smaller lateral area for the wind to act on.


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RE: Concrete for wood fence posts?

Thanks everyone for the great responses. I especially liked Brickeyee and Mongoct's method and will probably use that one.

Our existing pressure-treated wooden posts are the old CCA PT wood. Assuming they are in good condition when we pull them out, do you think we could re-use them?

Again, thanks for all the great help!


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RE: Concrete for wood fence posts?

"Our existing pressure-treated wooden posts are the old CCA PT wood. Assuming they are in good condition when we pull them out, do you think we could re-use them?"

If they are in good shape they probably are actually rated for ground contact.

Using them over would be fine.

I have been known to save posts removed from jobs and use them months later.


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RE: Concrete for wood fence posts?

shiane-
If on visual inspection the old posts look sound, it would still be a good to use a regular screwdriver as a probe to make sure that you don't have soft/squishy spots, which would indicate they are starting to deteriorate.


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RE: Concrete for wood fence posts?

Thanks again brickeyee and kudzu9. One last question...Should we put the concrete in dry and let the concrete absorb water from the soil or use wet concrete? I have heard of doing it both ways.


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RE: Concrete for wood fence posts?

I've heard about the dry method, but I prefer normal concrete, even if it takes a little work to mix it. Regular concrete will obviously set up faster, and you will be sure it's uniform. Also, if you want to start nailing stuff to the posts without worrying about cracking partially setup concrete, you'll want to go with mixed concrete. Finally, as has been pointed out, you want to have a sloping crown on the top surface of the concrete, and that's going to be easiest to do with mixed concrete.


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RE: Concrete for wood fence posts?

I use brickeyes method with "wet" concrete, mixed according to manufactures specs. after the concrete has cure for a couple of weeks I go around and caulk the joint between the post and the concrete, just as a bit of added insurance against moisture and rot.


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RE: Concrete for wood fence posts?

Thanks rjd59 for your suggestion 'after the concrete has cure for a couple of weeks I go around and caulk the joint between the post and the concrete' sounds like a good idea. We hope to get started on this project in the Spring (it's already getting too cold here in NY and we are busy with remodeling other areas in our home.)


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RE: Concrete for wood fence posts?

Along with all the fine advise above, one thing to consider in your area is post heaving. To prevent this, you chamfer the hole at the base to form an "anchor". That way when filled with the gravel and concrete, it will force those materials against the chamfered walls and not allow the post to rise. The chamfered area should be well below the frost line.


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RE: Concrete for wood fence posts?

My fence is 44 feet long , 6 feet high constructed of 6x6 PT posts sunk in concrete. the stringers are 2x6 lag bolted to the 6x6's and the vertical boards are 5/4 x 6 inch deck flooring, laid edge to edge. The frost line here is 3-4 feet , so I dug 5 foot holes, roughly 2 foot diameter( the last 2 feet are fun) , dumped in a couple of inches of crushed stone and a couple of buckets of wet cement to form a pad. While the cement was starting to form up , I slid the 12 foot post and an "O" form into the hole , plumbed them and stabilized them. I cut the cardboard "O" form off 8 inches above ground level and tapered the concrete to allow runoff. The fence sits just on my side of the property line beside my neighbor's driveway, and runs SW to NE, prevailing winds are from the North . Half of the fence is protected from wind by the two houses that are roughly 30 feet apart, the other half is not protected at all . That was 20 years ago. You may of heard that we had a couple of weather "events" a few years back, Juan and " white juan " , trees were uprooted, roofs blown off and no power in the city for days, very unusual weather for here. My 60 foot maple is gone but the fence is fine.


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RE: Concrete for wood fence posts?

"While the cement was starting to form up..."

If the bottom of the post forms any temp of indentation in the concrete, it will rot out much faster.

The indentation WILL fill and hold water.


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RE: Concrete for wood fence posts?

True, never thought of that! Maybe that is happening down there! I did forget to mention that I had painted the 6x6 with roofing tar before I put it in and I believe that the cement had begun to set up, by which I meant it wasn't sloppy like oatmeal, but quite a bit stiffer than that, as I remember from 20 years ago. Anyway, so far so good and if it lasts another 10 years I'll be happy, if I'm still around then.


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RE: Concrete for wood fence posts?

" painted the 6x6 with roofing tar "

I've had good luck with asphalt emulsion. Easier to spread than roofing mastic.


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RE: Concrete for wood fence posts?

" painted the 6x6 with roofing tar "

"I've had good luck with asphalt emulsion. Easier to spread than roofing mastic."

All you are doing is trapping water inside the wood.

The water WILL get in.

The idea is to provide an easy path for it to exit so the wood does not remain saturated long term.

Even the old CCA will rot if it was not treated to a high enough retention for burial.

The posts at the big box and many lumber yards are at best ground contact rated, not burial rated.

Even with the new treatments you need to watch the level of treatment.


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RE: Concrete for wood fence posts?

Should have stated in more detail, you dont want to seal the bottom of the post. It lies on the gravel bed and is your drainage plane. Sealing the sides with asphalt emulsion can slow the base sides of the post from gathering up moisture, but as Brick states, it will still get wet under enough of a soaking. The emulsion can help slow that down or even prevent it in small, infrequent rains.


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RE: Concrete for wood fence posts?

100 year fence post!

i've been thinking about this a lot. concrete is like a wick when in contact with water. however you set the post water will soak into it and rot the wood.

i just set some posts today. i used hemlock (natural rot resistance, much tougher than pt, eco-friendly) and wrapped the base in blue skin foundation membrane, then set it in concrete. no water is ever going to touch the would below ground level. hemlock is good wood, these posts will last a hundred years.

an alternative is to bevel down the corners of the 4x4, fit a sleeve of 4" abs rising to a few inches above ground/concrete level, put an abs cap with glue on the bottom, then seal the top with silicon. the problem with this though is that the posts will eventually shrink and begin to rattle a bit inside the abs, though i doubt this would weaken them.


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RE: Concrete for wood fence posts?

"however you set the post water will soak into it and rot the wood. "

Yes, but the idea is not to create a trap that will hold water accelerating rot.


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RE: Concrete for wood fence posts?

I've been planning a trellis to add some privacy to a deck and encourage birds.

I was thinking of
- Start with a ~3' deep x ~12" hole.
- Fill the bottom with approx. 3" of pea gravel, total height ~3"
- Insert my 4x4 PT post
- Fill another 3" of pea gravel, total height ~6"
- Slip a 12" diameter & 2' long sono-tube over the post and down into the hole, total height ~2'-6"
- Fill the tube with concrete and plumb the post (adding a bit of taper to it at the top as the concrete sets)
- Back fill around the tube with soil

At this point the top of my concrete would be within about 6" of the surface, give or take since rocks/etc are not exactly precision...

I'll put some edging around the ~12" hole and will up with pea gravel--this will keep dirt off the post at the surface, will keep me from weed-wacking against the post every week, and I hope will also encourage drainage a bit.

Pythagorean theorem (and a tape measure) says that 4x4 post measured diagonally will about 5-5/8" leaving about 3" of concrete space within the tube (as long as I keep things reasonable centered).

The idea of the sono-tube to try to avoid frost heaves which are caused, I think, when the ground swells around something jagged (like unformed concrete) and lifts the object. So using the sono-tube as a collar I am hoping will reduce the chance of a heave (which is more of an issue for a trellis then a fence I think)

My only concern is that my trellis will about 6-7' tall and about 6' wide. Once it has lattice nailed up and vines growing it will catch wind like a sail. I'll have 200% more surface area above ground vs. below ground, so it might spell trouble?


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RE: Concrete for wood fence posts?

"Once it has lattice nailed up and vines growing it will catch wind like a sail. "

Even wind speed data is available for many areas.

You still have to be careful since it may not be specific enough over many areas.

The 'average' (or even percentile group) for an area can easily be exceeded by topography, trees, and even other structures.


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RE: Concrete for wood fence posts?

is there a reason why no one in this thread is using the Simpson Stongtie brackets for 4x4 posts. one method that keeps the post completely dry and above ground is using a Simpson bracket - half the bracket is embedded in the concrete (also tooled to slope slightly at the top for drainage away from post). They have a version with a "stand-off" that keeps the wood post from touching the ground. See link.

The top half of the Simpson is above ground and holes for bolts and nuts to go through to hold the post in place.

You still use concrete in the hole - preferably - you have made the hole wider at the bottom so that the concrete flares out at the bottom and is narrower at the top side - this helps with prevention of heaving in frost.

The brackets are about $10-25 each and available at big box stores and building supply stores. They come in different grades - thinner cheaper ones and thicker heavy duty metal.

Are you worried about the wind blowing them and making them sway in the metal brackets?

I live near a HD Pro store and they do carry the ground burial type PT wood - it is 1/3 to twice more the price of the regular above ground PT wood and you have to ask them where it is because they don't advertise it - it's in the back as opposed to the cheaper stuff.

When I am in the store - I see a lot of contractors just buying all the cheaper PT stuff even though I can see they are going to bury the 4x4 posts in bags of cement they are also buying - it is very clear they are building a fence from the materials purchased. I guess it doesn't matter to them since they only give a 1-2 year warranty and it saves them money. I always buy the ground burial PT even if it is above ground - it's not that much more money all total for a project - maybe only $50-100 more overall.

Here is a picture of one version of the simpson bracket.
http://www.drillspot.com/products/418209/Simpson_Strong_Tie_CBSQ44-SDS2-WEST_4X4_Standoff_Base

Here is a link that might be useful: Simpson Stongtie Post Base


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RE: Concrete for wood fence posts?

sorry - i gave the wrong link - here is a link from the official company site: http://www.strongtie.com/products/connectors/PB-PBS.asp


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RE: Concrete for wood fence posts?

"Simpson Stongtie brackets for 4x4 posts"

They are not adequate for fences or structures that can have large wind loads.

They are intended for deck posts that have a large weight and a small wind cross section.


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RE: Concrete for wood fence posts?

We have been installing fence posts for 25 years. What is important is you dig your holes 6 inches past the frost line. For a 4x4 post PT dig your hole 8 inches in width. Put a bucket of clear stone in the bottom of your hole then use sona tube and fill it with cement. We have found over the years that the sona tube allows for the concrete to cure properly. To protect your top of the post we recommend not using wood caps they simply rot over time due to rain/snow/ice and wind. The best post cap is available in Black or copper Cast Aluminum which is very decorative and won't rust. I have installed many in the last 10 years and no problems. Available in 4x4, 5x5 and 6x6 pt or cedar. Visit our web site and view our gallery and you will see them on the posts

Here is a link that might be useful: Muskoka Wood Decks & Fencing


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