Are ther alternative to Wooden Post on Deck Foundation

Discussion in 'Remodel Forum & Blog' started by herbolaryo, May 26, 2007.

  1. herbolaryo

    herbolaryo New Member

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    San Francisco
    The Deck "wood" post that is nailed on the cement foundation is rotten.
    I understand there are synthetic deck materials made of some kind of plastic for the deck flooring.
    Sometimes its hard to keep up with waterproofing and is costly.

    Are there similar materials for the deck "wood" posts?
    Is there a way to make the wood post resistant to rot?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Synthetic materials generally aren't strong enough for a support, so no. You could use steel. There is hardware that holds the bottom of the wood post up away from the concrete, and allows air to circulate. Using one of the pressure treated woods and the fixtures to hold it up off of the concrete should allow it to last a long time.
  3. Bob NH

    Bob NH In the Trades

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    You can get away with plastic deck boards by limiting the joist spacing but it is not good for structures. It is not stiff enough.

    The ends of posts should NEVER be put against concrete. The best is to put them on heavy galvanized supports that are cast into the concrete. The supports keep them about 1/2" off the concrete. You can also use "earth contact rated" pressure treated plates under the posts but you need straps for a good attachment to resist uplift.

    All wood material should be pressure treated. The safest for the posts is to get material that is certified for earth contact.

    You can use steel posts or even reinforced concrete. That is a lot harder for a DIYer to work with. I helped my son put in a low deck and we ran the concrete piers from below the frost line up to the beam heights with a 1/2" threaded rod extending down and up enough to both reinforce the pier and serve as an anchor bolt.

    The steel backing of old disk brake pads makes a nice big "washer" to hold down multiple 2 x 8s or 2 x 10s used for beams.
  4. herbolaryo

    herbolaryo New Member

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    Location:
    San Francisco
    Thanks jadnashua for the post. Can you tell more about the hardware at the bottom... I am a bit of a beginner in terms of deck...
  5. herbolaryo

    herbolaryo New Member

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    31
    Location:
    San Francisco
    Thank you Bob NH for the post. Can you explain further about the heavy galvanized support... How do I make that? Or is it bought ready to place?
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    Any big box store generally carries them...you don't make them, you buy them. It bolts to the concrete pier and then you bolt the post to it.
  7. leejosepho

    leejosepho DIY scratch-pad engineer

    Somewhere I have seen "highchair" post pads that look kind of like a small bed of nails/legs/fingers that sit on the concrete and keep the post up an inch or two ... but tie-downs would have to be added somehow.
  8. jimbo

    jimbo Plumber

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  9. molo

    molo New Member

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    Location:
    cold new york
    More deck post/material questions

    Hello all,

    This post leads to some more questions regarding installing deck posts. I live in Nw York St. where the frost depth can easily reach 4' below grade. For deck posts, I have seen people use sauna tubes filled with concrete, and I have seen some put the pressure treated posts directly into the ground and tamp every couple of inches as they backfill. I have seen the sauna tubes filled with concrete be lifted out of the ground by frost, and I would assume that eventually the deck posts will rot.

    1. What is the best way of installing a deck post that will last for 20 + yrs?

    2. Also, with this wet and cold climate, wooden decks are constantly being maintained on an annual basis. I have seen the synthetic deck boards and rail systems. Are these systems framed with traditional lumber (pressure treated), or are they framed with synthetic materials?

    3. Can anyone recommend a good synthetic deck brand/line?

    TIA,
    Molo
  10. jimbo

    jimbo Plumber

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    Location:
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    As already stated, synthetic materials are not structural. The best structural material is pressure treated lumber.

    If you put posts in the ground, the life expectancy would probably be stated at about 10 to 15 years. Life expectancy of any wood planted in cement is 5 to 10 years. The best bet is pressure treated wood posts placed on concrete footers of appropriate, using the post base which keeps it away from direct contact with the concrete. This probably gives you a life expectancy of 20 years. You just can'y really build a deck that is going to last forever. Too much exposure, too many variable.

    With pressure treated, remember that every time you cut it, you need to get a can of the "green stuff" and brush it on the cut.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    The key to any footing is that the base needs to be below the frost line. If you don't go far enough, or the soil is not good enough for that foundation, you'll get those results. If there is clay or a lot of ground water, you need to potentially ammend the soil.

    If you use one of the strong-tie products to hold the base of the PT wood up off of the concrete, it should last for a very long time.

    There was an article on synthetic decking in Fine Homebuilding in the last year. You should be able to find it at a library, or maybe a friend, or order it on-line. It compared many of those available.

    Many of the railing systems stand alone for the rail and stiles, but the posts are a cap for PT wood. Not all, though.
  12. dampness when it freezes can crack anything! as long as damp air or moisture can get into it and seep in.

    go real far north, like halfway to the North Pole, and you see they only use concrete for the lower half of the structure. Utilities antennas, goverment piers and docks, everything even houses all have concrete bases, or full basements, and the concrete goes up a few feet above ground.

    assuming the dock is built to last. governments always want to look like they are going to remain the authority, so they want to build to last. Also, in the long term they have much fewer problems with maintenance and responsibility concerns if it's made tough.

    There are hundreds of kinds of concrete products. Some have a lot of air in them, some have none. Damp air will let H2o get in and crack-freeze in winter and spring.

    A single long freeze is not a problem, so a long cold winter causes almost no damage. In the fall, as temperatures drop the moisture content in the air also lowers slowly, so there is less and less moisture in the ambient environment too, so the first big loong freeze causes no damage. The worst thing is if moisture is in the concrete in spring -- when daily cycles between thaw and freeze let water percolate (slide around) which lets it occupy (like "expanding") into the micro fissures, and then it applies brand new pressures overnight when it freezes. Repeat for 45 days and you have aged your concrete the equivalent of 100 winters. After ten springs (ten years) your concrete could have aged as if 1000 years old, or it could be fresh and new looking, and it all depends on whether you managed to keep it dry in February, March, April and May when the sun is strong during the day but the night time temperature is below freezing.


    david
    Last edited: Jun 19, 2007
  13. frenchie

    frenchie Jack of all trades

    As far as post straight into the ground go, be aware that there are different grades of pressure-treated, depending on how much of the chemical has been injected into the wood.

    Some pressure-treated is just rated for "outdoor exposure" - ie, deck framing
    some is rated for "ground contact" - retaining walls, etc.
    some is rated for "foundation" - pile foundation under coastal homes
    and then there's "marine-use" - piers, docks, pilings
  14. Backglass

    Backglass New Member

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    A picture is worth a thousand words!

    [​IMG][​IMG][​IMG]
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    There are other, similar brackets. The one I was referring to has an upside-down u-shaped channel with a bracket that holds the post. This raises it above the concrete and gives it a good chance of staying out of surface water, or if not, then drying out later since it is not in direct contact with the concrete. The main component of all is that the post is not burried in the concrete which often creates a cup to hold water against the wood for extended time and leads to rot.
  16. leejosepho

    leejosepho DIY scratch-pad engineer

    What about pole buildings where posts are set in holes that are then filled with concrete?

    Believing that is okay, I am presently setting some 6x6 PT posts on large foundation pads below the frost line and pouring 12" columns around them and a foot above ground level.

    Is my new workshop possibly going to lose its foundational integrity in 20 years?

    Attached Files:

  17. Bob NH

    Bob NH In the Trades

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    Location:
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    It depends on the quality of the pressure treating of the wood.

    Much of the pressure-treated framing material is now treated with lower quality or lower concentration biocides (no arsenic) because there was concern about kiddies chewing on deck posts and getting arsenic poisoning from deck boards. Therefore, most of the material that you see in the Big Box stores is not adequately treated for foundation structures.

    Power transmission lines and pole barns are constructed with heavily treated poles that will last a long time because they have more material to rot and have more of the most effective treatment materials. They are usually NOT set in concrete. If they are backfilled with gravel or crushed stone they can dry out and decay more slowly. A structural post system is designed to take the vertical and horizontal loads but the sway bracing is provided by sheathing on buildings or by diagonal bracing.
  18. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Personally, I much prefer never to encase wood in concrete...I think you are just asking for problems down the road. If wood can stay dry, or have a chance to dry out, it can last a very long time. If it can stay constantly wet, it can last a very long time. Many of the old buildings in Boston's back bay are on wood pilings, a couple of centuries old. They've been fine until the water table dropped...now, many require $100K to $1M to fix so the building doesn't fall down. When given the chance, keep it dry. Any moisture that does get next to the wood will be held there for a long time by the concrete.
  19. molo

    molo New Member

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    840
    Location:
    cold new york
    `
    Last edited: Jun 20, 2007
  20. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    To get the minimum (least cost) solution using the minimum materials, you may need to talk to a structural engineer and probably dig a test hole. What you put in the hole depends on the soil. Clay is lousy for support, it expands and contracts with the moisture content...you really want something different there, since it really does compress, and that doesn't provide the support you want.
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