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How to stiffen subfloor for tile inst…

| Posted in General Discussion on May 29, 2000 01:22am

*
I have a 2nd floor bathroom roughed-in and ready for finishing. I want to install ceramic tile on the floor, but there is one section of the subfloor which , I believe, needs to be stiffened before it’s ready for tile. The subfloor is 3/4″ t&g plywood and the section in question has a span of approximately 40″ between floor joists (the “missing” joist was cut off and tied into the joists next to it in order to provide headroom for a stairway below the bathroom). The area in question is approximately 8 square feet.

Short of taking up a section of the plywood and trying to add bracing under the subfloor, any suggestions on how to stiffen the floor? Would another sheet of plywood over the existing plywood be enough? How much strength will Hardibacker add? Thanks in advance for your input.

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Replies

  1. Guest_ | May 25, 2000 05:38pm | #1

    *
    Yes, you're going to have to stiffen the floor, even if you were installing vinyl tiles.

    Is this a renovation or a new construction?

    I can't even imagine the thought process or lack of that would sanction a 40 inch spacing between two joists, under a bathroom.

    The problem with your situation is that your subfloor is so weak that you have to effectively build a platform over top.

    Gabe

  2. jkd | May 25, 2000 06:02pm | #2

    *
    This is new construction (an addition to an older home).

    Yes, I agree that the thought process was lacking. I would go back to the contractor who did the work for a remedy, but he's skipped town and claimed bankruptcy. Shame on me for hiring him in the first place.

    Any ideas on how to cost-effectively fix this problem are appreciated.

  3. Guest_ | May 25, 2000 06:38pm | #3

    *
    If it were me.... I'd pull up the floor and fix it. Otherwise, I'd be really nervous about the finish floor. It's more work, but it is much better than trying to address the issue by means other than fixing the real cause.

    1. Guest_ | May 25, 2000 06:39pm | #4

      *Open the floor from above and cut the plywood back to the center of the two joists, so that you can install cross blocking on hangers between them.Install them at every foot and that should bring your floor back to a reasonable stiffness.Gabe

  4. jkd | May 25, 2000 09:45pm | #5

    *
    Thanks for the help. You have confirmed my gut feeling, that the best fix is to open up the floor. So much for shortcuts!

  5. Guest_ | May 25, 2000 10:59pm | #6

    *
    Gabe is right on.

    Just got through doing this, with a 100 year old home with spongy 2x10's 16" oc. I used 1 and 1/8" t&g. Can't imagine 40". Wow. This guy should be put in jail.

    Cut that baby open. Go back as far as possible to where the original joist was cut. I assume there is a header or cross-piece there. I'd tie into that, and go parallel to the other joists to the header on the other side if you can.

    Not an option? Then go perpendicular as Gabe suggested.

    Still a bit spongy? Rip the whole subfloor out and install 1 and 1/8" t&g. That stuff is really heavy, but has almost no deflection, even 24" oc. Block all seams, too, for even more stiffness.

    Hardi backer has no strength. None whatsoever. You might as well be relying on 1/4" sheetrock.

    If you want to add some further strength, try a good, old fashioned, 1" deck mud job. It will level off all uneveness on the joists, and some 16 ga. wire in the middle will make that bath really strong.

    1. Guest_ | May 26, 2000 12:22pm | #7

      *Don't forget to dbl up those joists that you are adding load to. Maybe you can use smaller joists in that area and hang them to some dbld up 10's in the existing frame. Time for some creative thinking, or rather, rethinking. Best of luck.

  6. jkd | May 26, 2000 02:31pm | #8

    *
    Thanks, everyone.

    Yes, Scooter, this contractor should be behind bars. The subject of this post is one of the smaller workmanship problems that have surfaced, not to mention the huge financial bag he left us holding. But here in sunny Florida, he'll probably go on to do this to the next guy, under a different corporate name. However, as I said before, I have no one to blame but myself for hiring him.

  7. Guest_ | May 26, 2000 10:34pm | #9

    *
    New sub floor, fully blocked, glued and nailed with ring shank nails.

    Then to really stiffen it, you can staple expanded metal over the whole floor and cover it with a 1/4" coat of Acrylic modified cement from Excellent Coatings (excellentcoatings.com).

    You can then thin-set your tile to the Excel Crete.

    Good luck!

    1. Guest_ | May 27, 2000 04:39am | #10

      *If floor thickness isn't a problem, why not just build up to 1-1/8 or even 1-1/4 by glueing and screwing a sheet of 1/2" to the existing 3/4"...hell use 3/4". That oughta span 40".David

      1. Guest_ | May 27, 2000 04:53pm | #11

        *The ultimate problem for a 40" span is deflection and broken tile and/or grout. That's why blocking is required.A second problem is stubbing bare feet on a 3" change in elevation. Better to block the plywood ans use thin set.

  8. josh_miller | May 28, 2000 07:07pm | #12

    *
    i agree but i would use a 3/4 " piece of concrete backer board that are made for tile. glue and screw the hell out of it. then tile

  9. Guest_ | May 29, 2000 01:15am | #13

    *
    Josh,

    The underlying problem is the 40" span and too much deflection. The backerboard won't help.

  10. jkd | May 29, 2000 01:22am | #14

    *
    I have a 2nd floor bathroom roughed-in and ready for finishing. I want to install ceramic tile on the floor, but there is one section of the subfloor which , I believe, needs to be stiffened before it's ready for tile. The subfloor is 3/4" t&g plywood and the section in question has a span of approximately 40" between floor joists (the "missing" joist was cut off and tied into the joists next to it in order to provide headroom for a stairway below the bathroom). The area in question is approximately 8 square feet.

    Short of taking up a section of the plywood and trying to add bracing under the subfloor, any suggestions on how to stiffen the floor? Would another sheet of plywood over the existing plywood be enough? How much strength will Hardibacker add? Thanks in advance for your input.

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