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  1. #1
    Join Date
    Feb 2012
    Posts
    2

    Default Porch post: what's best construction?

    I'm planning to replace two posts that rest on a concrete pad and support a porch roof. The home was built in 1998. Each post is formed from four 1x8's that are nailed into a box cross-section. They are structural; there is no column inside them. They are rotting at the base because the end-grain wood sets directly on the concrete and wicks up moisture. In addition there is decorative plinth which has a hard time keeping up with the seasonal swelling of the posts. Each plinth is formed from 1x10's mitred cross-grain and joined as a box that surrounds the post. So, the cross-grain of the plinth is nailed to the long-grain of the post. I've oversized replacement plinths by 1/4" on each side and they are still forced apart by the swelling of the columns.

    I'm thinking there are two solutions. One is to use different materials, MDO perhaps. The second is to create a flat base from pressure-treated 2x10 on which the post will rest. What's the right track to solve both the rotting and the swelling problems?

    Regards,

    Don

  2. #2
    Join Date
    Aug 2007
    Posts
    6,486

    Default Re: Porch post: what's best construction?

    I believe I would opt to install a post to carry the weight of the roof, then install the decorative materials around it. This way you can isolate everything from the concrete patio.

    Go with a 4x4 and set a post base into the concrete, I've always used a Simpson AB44 anchored with an expansion bolt. The post base has a plate the stands the post off the cement by 1", preventing the wicking of moisture and rotting of the post. The post can be standard structural grade material, it does not need to be pressure treated or specialty species.

    Next attach spacers to the post that will support the decorative shell. Cut the shell and trim pieces and paint ALL surfaces and ends before installing on the post, this will help with stability issues. Keep the bottom plinth off the concrete by 1/4" - 1/2" then install a synthetic quarter round or shoe molding that will contact the concrete.

    Caulk it all up really good and fill the fastener holes, then give it a final coat of paint.

    If you want to avoid the problems of expansion/contraction and moisture altogether, you can install a synthetic column. These come in a multitude of designs and are structural, so no inner post is necessary in light duty situations. These columns can be a little expensive, but when compared to the problems that you've described and the time difference in the installation and maintenance, they may be the better choice for your situation.
    Last edited by A. Spruce; 02-11-2012 at 11:33 AM.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  3. #3
    Join Date
    Jul 2010
    Location
    SoCal
    Posts
    5,103

    Default Re: Porch post: what's best construction?

    Funny, I have 3 post replacement job next week. Good timing to talk about this subject.

    I'll be using, as Spruce described, 4x4 posts, but treated. What the heck. On simpson anchors to avoid water.

  4. #4
    Join Date
    Mar 2008
    Location
    nova scotia, canada
    Posts
    1,522

    Default Re: Porch post: what's best construction?

    replace the existing with a 4x4 or 6x6 treated post anchored via the simpson anchor mentioned. from there dress it up by cladding it with 3/4" thick azec stock. the azec will never rot and looks great unpainted
    fire up the saw and make some dust

  5. #5
    Join Date
    May 2011
    Posts
    251

    Default Re: Porch post: what's best construction?

    You could just have a base made out cedar and keep it painted. IF the concrete is sloped properly you shouldn't have an issue. Even better would be a 4" tall concrete pad, then the cedar column base on top of that.

    One of my column bases is original cedar and the other I replaced just last year that only rotten from lack of proper painting and gutter maintenance form the balcony above. But then again, old growth wood is far more rot resistance than the young fast growth trees that use for lumber today. I suspect the replacement will still rot out before the other original base.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  6. #6
    Join Date
    Jan 2008
    Posts
    1,109

    Default Re: Porch post: what's best construction?

    A cheap 'fix' would be to cut off the bottom 1 1/2 or 3" and build a base using treated lumber of those dimensions. Personally I wouldn't go there- for a little more money and a lot more strength add a 4X4 in the box. You can do that by removing one side of the existing box to reduce the cost and effort needed. Once that's in place use spacers to locate the 'box' section near the bottom above the rotted part and reassemble the box. Now cut off the rot and build a square collar to fit around the old column with the top slanted to shed rainwater. Keep the collar above the porch about 1/8 and it won't wick anymore This is one of my my fave cheapo fixes that works great. The collar is truly "thinking outside of the box"

    Phil

  7. #7
    Join Date
    Feb 2012
    Posts
    2

    Default Re: Porch post: what's best construction?

    Thanks to all for your suggestions. One question about 4x4's: what is the load at which a 10ft 4x4 will become unstable (i.e., fail in column buckling)?

    Don

  8. #8
    Join Date
    Aug 2007
    Posts
    6,486

    Default Re: Porch post: what's best construction?

    That will depend on the species of wood and it's structural value. Regardless of species, a single 4x4 will be stronger than your current set up.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

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