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shiane_gw

Concrete for wood fence posts?

shiane
14 years ago

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)

Comments (48)

  • brickeyee
    14 years ago

    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.

  • mikie_gw
    14 years ago

    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.

  • bill_g_web
    14 years ago

    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.

  • brickeyee
    14 years ago

    "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).

  • MongoCT
    14 years ago

    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.

  • tryinbrian
    14 years ago

    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.

  • macv
    14 years ago

    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

  • bill_g_web
    14 years ago

    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: "ÂPost 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

  • tryinbrian
    14 years ago

    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...

  • tryinbrian
    14 years ago

    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.

  • kudzu9
    14 years ago

    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.

  • macv
    14 years ago

    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?

  • brickeyee
    14 years ago

    "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.

  • shiane
    Original Author
    14 years ago

    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!

  • brickeyee
    14 years ago

    "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.

  • kudzu9
    14 years ago

    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.

  • shiane
    Original Author
    14 years ago

    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.

  • kudzu9
    14 years ago

    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.

  • rjd59
    14 years ago

    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.

  • shiane
    Original Author
    14 years ago

    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.)

  • sierraeast
    14 years ago

    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.

  • paul21
    14 years ago

    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.

  • brickeyee
    14 years ago

    "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.

  • paul21
    14 years ago

    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.

  • sierraeast
    14 years ago

    " painted the 6x6 with roofing tar "

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

  • brickeyee
    14 years ago

    " 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.

  • sierraeast
    14 years ago

    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.

  • robbob321
    11 years ago

    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.

  • brickeyee
    11 years ago

    "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.

  • bubblah
    11 years ago

    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?

  • brickeyee
    11 years ago

    "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.

  • kisu
    11 years ago

    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

  • kisu
    11 years ago

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

  • brickeyee
    11 years ago

    "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.

  • Muskoka2013
    11 years ago

    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

  • yourfencpro
    9 years ago

    Actually the question is age old. As a fence contractor for over thirty years We daily run into this question from homeowners. " We just replaced the fence a few years ago and the fence boards are perfect, can't we just replace the posts and re-use the fence boards and rails?" No because when you take the fence apart the boards split and crack. Yes if you spent many hours you can but the time outweighs the cost of just building a new fence.

    Now for years and years I have been seeing the same answers on dozens of forums that are totally incorrect and it baffles me. The same answers just reworded from writers that do not do good research.

    Post Rot can not occur underground. Protecting the post at the base is what needs to be done. Two of the four elements which rot needs to live- proliferate and exist can not physically happen underground. One is oxygen and the other is fuel- microbes and fungus. http://ucanr.edu/sites/forestry/files/172688.pdf

    Rot starts at the base of the post where soil comes in contact with the wood. Moisture and fuel rot the wood at that place where the post exit the earth, as a homeowner think and you will see where the posts break and snap... Now we pull posts out daily and the bottom of the posts still often have the tags on them from the lumber yards.

    Yes one thing is accurate. The concrete needs to be crowned (sloped) away from the post so water and soil do not touch the posts. But there is also a problem there. In time with wind and mostly because a contractor or homeowner doesnt seal the concrete properly, water will seep in the tiny gap between the post and concrete and sit in there like a pool. In the long run doing more damage- constant moisture and oxygen and fuel. So the posts rot even faster. The base of the post need to be protected. There is a new product on the market called The Post Collar that solves that problem. Or be certain when you crown the concrete that you actually float the concrete away and form a tight seal. There is only one product on the market that does this, The Post Collar. We are using them on all our fences now.

    t

    Here is a link that might be useful: The Best Product

  • PRO
    Skyview Building & Remodeling
    8 years ago

    Hi,

    One of the best wood fence posts you can use is aromatic cedar. I lived in a house in St. Louis where I replaced an old fence which had 6x6 posts made of aromatic cedar. They were practically as good as the day the were installed 60 to 70 years prior. No serious rot, no termite damage, nothing! And they were set about 36" in the ground. I was amazed at the condition of these posts after so many years, an recommend their use if you can find them.

    I have built many fences and lost them due to rot at the base. My current best practice is to use no concrete at all for PT wood line posts, and use concrete at gate posts only, installed as described elsewhere in this post.

  • Dan C
    8 years ago

    Hi, I'm building a fence here in sunny southern California. I'm using con heart redwood 4x4 posts. I've found some conflicting theories about how to set the posts. Some say concrete, some say gravel... and I'm looking for some advice on what wood protector to use for the base of the posts.

    Because my soil is heavy clay and the drainage is poor, I don't think gravel is a good idea, it'll just create a cauldron of water for the post to soak in after it rains. Even though concrete doesn't seem much better, I think that's the way I'm going to go. I'm definitely going to use about 6" of gravel at the bottom of the
    hole, and make sure the base of the post is set in an inch or two of
    said gravel before the concrete goes in.

    Since the fence is going to be unpainted and I want the natural look of the wood, I'm avoiding using PT wood for the posts, and I've had recommendations to use either asphalt emulsion (Henry 107) or Woodlife Copper Coat green to coat the part of the post that's going into the concrete. The Henry seems like a waterproof coating, while the copper is more of a wood preservative.

    It doesn't rain very often here, but when it does the posts will get a good soaking, and the ground will take a while to dry out due to the poor drainage. Some say that the Henry 107 will cause water to get trapped inside the wood below ground, accelerating rot. The copper coat would at least allow the wood to "breath" a little and allow it to dry out during our long periods of no rain.

    I know no matter what I do these posts may only last 10 years or so, but I'm looking to give them the best protection to start with, so they'll last as long as possible. Thanks for any input!

  • mikrisinger
    7 years ago

    What I have just recently done is to put the post on top of a 6" bed of crushed gravel in a 11" square hole x 1/3 fence post height deep. I bought a bunch of plastic sleeves from a company that make a product called "post protector" which was originally made for pole barns. I cut off the bottom from the sleeve and slide it over the post. level my post and put in about (3) 80# bags of strong concrete/hole. I taper the concrete up in a pyramid fashion to the sleeve perimeter to keep water away and caulk after drying. Then I cut the polyethylene sleeve off at the top a few inches above the concrete to help keep dirt and mulch from getting in it. The sleeves are convoluted to allow air to circulate around the post and keeps the moisture from saturating the post and rotting it. The additional beauty of this is if I need to remove the post later on, I just put a slight sqeeze ( I use clamps) on post and lift the post out. If it is too tight to pull out by hand, then I use a scissor jack to help give it a straight up nudge. The sleeves are made for many sizes of post as well.

  • kudzu9
    7 years ago
    I'd be interested to get more specifics on the plastic sleeve...who makes them?
  • mikrisinger
    7 years ago

    Put in post protector in your browser and find postprotector.com There is an overview of the product. Keep in mind that you will need to remove the bottom of the sleeve or moisture/ water will trapped in a fence post application. They were intended for pole barns, but work for fence posts with slight modification to keep the concrete and soil contact away from the post. There are a few other companies that make something similar to them for deck post, but I didn't like them as well because I felt the type of plastics used would have a tendency to crack later with cold temperatures whereas polyethylene is much more flexible. I live in western Oregon and the clay soil is wet for 75% of the year. You can order these thru Home Depot for about $16 each if I remember. It's been a couple years. Think of it as a slight added value to your home if you were ever to put it up for sale or the ease of replacing a fence post should it ever rot. No concrete or gravel to dig out, pull out, or replace.

  • kudzu9
    7 years ago
    Thanks for the info and link.
  • mikrisinger
    7 years ago

    Most the fence builders (not all) typically put dry cement in a small hole...squirt some water in the hole and call it good. This is done rather quickly to keep labor and material cost down to try to underbid the next guy building fences. They probably figure that when the fence problems start that you will have sold your house by that time or enough time will have passed . The concrete cracks, fence post rots, fence blows down or leans too soon. I've seen fences leaning as soon as 5 years after a new development goes in. Our society and business culture is too indifferent to long term durability and quality for customers. They would rather make something that has to be replaced or expensive or difficult for us to maintain to keep revenues flowing. Example: new "energy efficient" refrigerators last from 5-10 years without problems. Older appliances were typically 2-4 times that. I really don't "need" ice and water coming out the front of my refrigerator to have some. Ice cube trays and faucets are the un-lazy way to do it. Still have my working GE toaster from 1968. Thank you GE :)

  • Vith
    7 years ago
    last modified: 7 years ago

    I have seen a product not mentioned that also protects posts and utility poles at the ground surface area where the rot happens. Otherwise, an application of roofing tar in the same area that this is installed would do the same trick.

    Use a gravel bed for the footer, it allows drainage and doesn't retain water that can wick up the endgrain of the post.

    http://www.postsaver.com/Postsaver-Fence-Sleeves.html

    http://www.postsaver.com/Videos.html

  • mikrisinger
    7 years ago

    I had already seen those sleeve. I don't think that polystyrene is to brittle and won't hold up with time

  • scamtracker
    6 years ago

    Canadian Wood Preservation Association Proceedings, 2007, pp 80-88
    This study demonstrated clearly how plastic wrap, but even better, booting, the bottom of fence posts prolonged the life of both treated and untreated woods. The boot tests debunk the folklore concerning "trapping" moisture, as the lower unwrapped sections of wood rotted, just at a slower rate versus the ground level where rot was worst. Ten years on and American experts have never heard of this? Come on, people, we used to be # 1. I'm just going to use a Henry's coating including the bottom before cementing in a treated post, even with ground level and beveled to shed water. Done.

  • paul21
    5 years ago

    I agree with Brickeye and the others advocating posts resting on gravel then cement up just above ground level with the top of the cement tapered away from the post. I installed a 6 foot high board fence in our back yard 30 years ago and it's still standing and resisting our winter winds up here i the Great White North !