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Community and Q&A

Wood fence post foundation in Wisconsin

Kristopher Steege-Reimann | Posted in Energy Efficiency and Durability on

I’m interested in wood fence durability in cold climates. I am planning on installing a 5ft picket fence using 10ft cedar 4×4 posts this weekend. The posts will be buried 5ft deep and spaced 5ft apart. Will be using a skid steer with a 10″ auger to make short order of holes. I want to stay away from setting posts with concrete or foam because I may be moving the fence in a couple of years (and want to minimize materials I am using). I live in Madison, WI where the frost line should be 3-4 ft.

My questions are:
1. Will burying a fence post a foot or two below the frost line allow me to forgo using concrete to prevent frost damage? Most articles (like this: http://www.gatewayalpacas.com/alpaca-farming/fence-building/setting-fence-posts.htm) assume your fence post is not below the frost line. It seems like frost wouldn’t push up a post if it the post extended below the frost line. I am not too concerned about concrete providing lateral support for the fence above ground because of the posts’ depth and 5ft spacing.

2. Is there any treatment you can apply to the portion of fence post that meets the ground to make it last longer? I’ve seen telephone poles treated with creosote and heard guys of using used motor oil to make posts last longer. I’m guessing ideally one would back fill with gravel so water doesn’t collect near post in the first place, but I wasn’t planning on using gravel (unless someone convinces me that it would significantly increase the durability of the fence). Just planning fill hole with tamped dirt.

3. Any other tips that could increase the fence post durability?

Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Kristopher,
    You hear all kinds of advice about these issues, some of it contradictory. I hold with those who advocate no concrete and no crushed stone. Frost heaves are unpredictable, but an inch or two up or down during the thaw in April or May usually doesn't matter much for most types of fencing.

    White cedar or red cedar will last longer than other species. Pressure-treated 4x4s or 6x6s are another option, but are quite expensive.

  2. Mark Fredericks | | #2

    I recently watched the video below from This Old House, where they wrapped the below grade portion of a fence post with self adhesive flashing like Grace Ice and Water Shield. I've never seen this done before but seems like it might work. I expect the slippery outer surface of the membrane would also help allow the surrounding soil to move up and down freely without 'grabbing' the bare wood post as easily.

    https://www.youtube.com/watch?v=28zUJpDL7qM

  3. Flitch Plate | | #3

    You must go below frost line
    If you are in clay, minimally you need a punch pad … but if you are in clay, don’t do it at all
    If you are in drained soil, tamp the bottom of the holes and insert the post without a pad
    Size the holes large enough to get a heavy tamping tool down each side of the post – oversized holes are better than tight holes (can’t tamp a tight hole)
    No gravel – it forms a well that pools water and will deepen your frost line; and gravel prevents a tight fit on the sides when back filling/tamping and acts ball bearings, underneath
    80% of weight is carried by the friction on the sides of the poles, not the bottom, provided you tamp the posts as you backfill … this can’t be overemphasized
    Ground contact rated pressure treated posts are best
    But cedar, hemlock and black locust are good – to ensure fungal and insect protection, wrap with Cu-Bor post wraps - http://www.coppercare.com/Products/CuBor.aspx
    Pre-coat each post section below grade with basement waterproofing tar

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Interesting posts of how others do posts. I always use PT posts the rest rot for me. And I set them tamping gravel around posts never concrete. Cedar posts in concrete have rotted everywhere I have seen such an installation. And Cedar posts go bad above ground after twenty years anyway. We also cap the top of posts to keep moisture out of the top end grain.

  5. Flitch Plate | | #5

    Black locust will last forever. But ground contact grade PT poles and timbers are best, I agree. I wrap every important structural pole, treated or not, with Cu-Bor products. Its industry standard for PT poles in electrical transmission.

    http://www.coppercare.com/Products/CuLam.aspx

    I never back fill with gravel (anymore), it keeps the post lose and invites water to pool, ice to get under the post.

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