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Replacement of Porch/Foundation Segment with Pier & Post Support

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  • Replacement of Porch/Foundation Segment with Pier & Post Support

    Originally posted 6-23-2017 12:15 PM
    By: mdoleman

    I have an older (1924) home, onto which a small entryway vestibule was added later. The main house sits on a subterranean/basement foundation, which is in fine shape. The small, additional foundation section which was added for the vestibule, however, was not constructed well, and suffered a pretty major collapse recently. I'm in the process of effecting repairs, the obvious route for which is to put-in a simple system consisting of a couple timber beams supported by pier-and-post construction. As of now, the old foundation is cleared, and the vestibule is temporarily supported with a single support beam.

    I've looked and looked at local building codes, several how-to books, and other sources of information, but cannot seem to find exactly the right reference that will instruct me as to exactly how to approach the problem. The building department can't even necessarily give me a definitive answer as to whether or not I even need a building permit for the work.

    At any rate, I've come-up with the best plan I could muster... It's drop-dead simple, but I can't see any reason it wouldn't work and be perfectly adequate (probably overkill) for supporting the tiny extension. The vestibule is about 9' 6" wide and extends from the main structure by 5' 6". I've included some photos and very simple front & side elevation schematics that show what I plan to do. I'm going to install 4 concrete piers, with footings, down to 18 inches (which is the code-required depth for pier-supported foundations). Onto each pair of footings I'll span a 9' 6" beam (6-by-6 pressure-treated), supported in the center by a pier block (which may not even be necessary, since a 6-by-6 beam can span up to 10 feet, from everything I'm reading. But it feels like it'd be better to support it in the middle).

    What I'm looking-for, here, is for some sort of either confirmation or red-flag as to why my concept either is or is not solid and appropriate.


    Here are the images, as things stand right now:

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    And here are my elevation sketches:

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  • #2
    Originally posted 6-23-2017 3:27 PM

    What makes you think a 6x6 can span 10'? What load are you assuming on that beam? Width isn't buying you much (i.e. a 4x6 carries nearly as much load as a 6x6, and a 4x8 will carry more than an 6x6).

    Do you need the piers at the rear (that is, are the joists under this entry vestibule attached to the house rim joist and being carried by the house foundation? If you are staying with x6 material for the beam under the entry door, I would use 3 piers (one near each end and one in the middle). With a 4x8, you may be able to get away with an 8' span and only 2 piers.
    Kent, WA


    • #3
      Originally posted 6-23-2017 9:11 PM
      By: mdoleman

      Well I derived info from a number of sources. I used the guidelines on the City's building codes page to calculate the overall weight of the vestibule portion, and then looked at their recommendations on that. Thing is, though -- yes, the vestibule is definitely tied into the house's rim joist and its weight is being carried by that. I am definitely going to use three piers, yes -- two on each edge and one in the middle. For the middle one, though, I intend to just use a pier block.

      The idea for the 4X8 rather than 6X6 is sound. In some ways that makes things easier. So I will do that for sure.


      • #4
        Originally posted 6-24-2017 7:59 AM

        Don't skimp on the middle block. Make it below the frost line like the other two.