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  1. #1
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    Mar 2007
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    Default alternating joist sizes

    42 year old home. In the front half of the basement are joists that alternate in size. In the back half they're all the same size, same as the smaller ones in front. What's up with this?

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  2. #2
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    Default Re: alternating joist sizes

    Please explain what you mean by different size? All I can think of is a crazed builder who either suffered from myopia or ran out of material?

    Jerry McCarthy
    Building Code/ Construction Consultant

  3. #3
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    Default Re: alternating joist sizes

    As in the photo - 2x alternated with something thicker than 2x.

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  4. #4
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    Default Re: alternating joist sizes

    John,

    I guess you are asking if it is OK to alternate between 2X8 and 4X8 (or whatever the depth is) joists. I can't say that I have ever seen that done but I don't see anything inherently wrong PROVIDED the 2X8 is suitable for that span and loading. In other words, if the 2X8 is adequate then the 4X8 is more than adequate.

    The floor will be bouncier at the 2X4 joists than at the 4X8 joists (but not by a lot - in fact it may not even be noticeable).

    Why did the builder build the floor system that way? Who knows? Perhaps he had some 4X8s that he needed to get rid of.

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  5. #5
    Jon Randolph's Avatar
    Jon Randolph Guest

    Default Re: alternating joist sizes

    John,

    It appears to me that the larger floor joists are boards of actual dimmension. They used to ese rough cut boards and a 2X4 would actually be 2" X 4". Now they are planed to provide a more smooth surface and the resulting 2x4 is now 1-1/2" X 3-1/2". The home may have been built with the "rough cut" lumber then additional joists added for additional support as the rough joists appear to be spaced about 24" and the standard joists put the spacing somewhere around 12" (according to my view of your picture).

    In the photo that you provided it isn't very clear, but it appears that the smaller joists may not be supported at the sill plate. My biggest concern would be insufficient end support for those joists. After that, the PVC does not appear to have sufficient support.

    The smaller joists in the other area may be due to the addition being built at a different time and that was what was available.


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