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Existing 2″x8′ joists between residential units need rating…

user-966377 | Posted in Building Code Questions on

Hi there,

I am working on a project where we are converting a single family dwelling into two separate units. It is quite a narrow house; and the existing floor joists at the rear, narrower section of the house are 2″x8″.
The Building Plans Examiner is requesting a 1-hour fire rating and STC-50 between the two units. Our building code has typical assemblies for 2″x10″ construction, and are therefore it is not acceptable to just slap a sample detail down and call it a day.

Any ideas how we can these ratings between the units?


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  1. gusfhb | | #1

    5/8 sheetrock is 1 hour rated no?

    The sound I know little about, but looked around for my last project

    somewhere there is a list of the stc rating of various fixes.....

  2. psarchitecture | | #2
    The link has listed assemblies that will meet your requirements, which you can select
    based on the construction type (wood frame, metal, etc.)
    They also publish manuals that are really helpful.

  3. ArthurPelskxjhky | | #3


    I notice that nobody seems to have answered your question. I also have the same problem with 2 x 8 in an ancient building that is being renovated.

    How did you end up solving this puzzle ?



    1. Expert Member
      MALCOLM TAYLOR | | #4


      The two previous replies both answered the question. Neither the fire or sound rating are altered by the size of the joists. They are determined by the presence of specialized materials, which can be found in the links Keith and Peter provided.

      1. Expert Member
        Dana Dorsett | | #5

        It's also a question that was posted more than seven years ago, and the link to the nationalgypsum site is no longer working (at least not for me, right now.)

        What are the odds Donald Peckover is still regularly visiting this site?

  4. ArthurPelskxjhky | | #6

    For what I understand :

    Actually, depending on the area and the building code, an architect or any builder is allowed to use an assembly that has been tested and is rated like the ones rated by the Underwriters Laboratory (UL). Most of the wood joist ceiling-floor assemblies are composed of 2x10 joists and they specify that 2x10 *or greater* joists are to be used to obtain the rated fire resistance.

    If no assembly is UL tested and rated with 2x8 joists, how can one determine the fire resistance of a particular assembly ?

    There must be a simpler solution than replacing the joists.

    Wich specialized engineer, institution or business could be contacted to approve this kind of assembly ?

    Thanks !


  5. Expert Member


    Sorry, I didn't quite understand your question. I see the problem now.

    Our building code has two paths to meeting the requirements. The first is to select one of the tested assemblies, the other is to build up an assembly by adding fire-rated (and acoustically rated) components.

    I think yo0ur code offers the same options:

  6. ArthurPelskxjhky | | #8

    Hi Malcolm,

    Yes, I think you're right. I'm in Canada, by the way, like Donald Peckover who posted the original question.

    I still have to find the appropriate part of the Code wich described this method but I'm pretty sure there's one.

    Thanks !

    1. Expert Member
      BILL WICHERS | | #9

      5/8” type X drywall, one layer on BOTH sides of the wall, is the standard 1-hour assembly.

      If you have some oddball area that needs a fire rating and can’t use one of the standard listed assemblies (drywall manufactures have data sheets with all the listed assemblies and their fire ratings, usually going up to 3 hours), then your best bet might be one of the spray-on intumescent fireproof coatings. These are commonly applied to steel trusses and the like commercially. I’m not sure if they’d work correctly on wood since I’ve never seen that tried, but I’m sure one of the manufacturers of the stuff could tell you.

      I don’t have the name of the stuff I’ve encountered handy. We all called it “smurf barf” because it was blue and foamy-looking. I wasn’t involved in that part of the project so I don’t have any more detail, unfortunately.

      BTW, Dana is correct. You really should have posted a new question instead of replying to a very old one.


    2. Expert Member
      Dana Dorsett | | #10

      Malcolm is an architect working in B.C. - trust his instincts! He's very familiar with the NBC, as well as B.C. specific code, might be able to cite chapter & verse on this one without having to look it up.

    3. Expert Member
      MALCOLM TAYLOR | | #11


      The relevant code sections are: where the alternate test methods are described , 9.11.1, and Appendix

      The first step I'd take is to talk to your Building Inspector and see what they will accept. There is a fair amount of leeway in how renovations are dealt with. Fire ratings for floor/ceilings are measured from below, so building up a I hour rating is usually as simple as adding layers of ULC rated materials (typically type X drywall) to the underside.

  7. stephaniezed | | #12

    There are two U.L. floor-ceiling assemblies that allow 2x8: L540 and L556

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