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Thread: Sub-Floor under Alcove Tub

  1. #1
    DIY Junior Member weekend_warrior's Avatar
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    Default Sub-Floor under Alcove Tub

    Stripped the old 1962 vintage pink 5x8 bathroom down to the studs. I have plumbers coming tomorrow to install new hot and cold shut-offs (sink and toilet), move the shower head up the wall a bit, install a new valve, and complete the bath tub hookups.

    Though my question may be more structural in nature, as opposed to plumbing-related, I thought I'd start with this forum.

    The original sub-floor in this bathroom is diagonally-run 1/8 planks over 2x12 joists, 16" on center. We're laying down 15/32" OSB over this, to provide the ideal surface for the incoming Schluter Ditra membrane and ceramic tile. It stands to reason that the OSB should be extended to the area where the tub will go, to further reinforce this section and provide adequate support for the added weight.

    The tub is a jacuzzi acrylic 3-wall alcove unit with a built-in apron. The manual states that the flooring underneath the tub must be able to support ~56 pounds per square foot. Impossible to tell whether or not the planks and joists will support that level of weight, as is, but am I right in thinking that an added layer of OSB can only improve the amount of support?


    Thanks,
    Seth

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Nominal 1/2" material on top of the planks should be adequate for tile - the tub could go on the planks, but extending the subfloor underneath would likely make the tub's apron fit better. I'm assuming you meant 1" thick, 8" wide planks?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    OSB sucks.

    There I said it.

    Install a layer of 5/8" plywood instead.

    or install some 1/2" cement board.

    You do not want to block the ship lap (1"x8") boards from 'Breathing'. IE they need to move somewhat. If you use glue to tie the to layers of wood together this glue can block the gaps on the ship lap and prevent it from moving. How well is the old ship lap installed?

    Did you re-check the nails? Add in any extra nails? Does the old ship lap split when nailed or screwed?

    I would cover this with building paper and install new 5/8" sheathing right over top. Angle your fasteners some what so you don't drive to many into the tops of those old joists. To many fasteners can split a joist down the center and you will lose your bonding strength.

    Make sure your research and understand 'Tenting' in sub floor and tile installations before proceeding. Once you understand this you will be more careful with your install and have a stronger grasp of the dynamics of floor preperation.

    JW


    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    No approved method of using cbu directly on planks! You need at least 1/2" nominal ply as a stable decoupling layer between the seasonal movement of the planks and the tiled structure. OSB wouldn't be my first choice, ply, especially in a potentially damp area works better. Now, not all are created equal, I do like Advantech, but, do NOT install cbu directly on planks!
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Quote Originally Posted by jadnashua View Post
    No approved method of using cbu directly on planks! You need at least 1/2" nominal ply as a stable decoupling layer between the seasonal movement of the planks and the tiled structure. OSB wouldn't be my first choice, ply, especially in a potentially damp area works better. Now, not all are created equal, I do like Advantech, but, do NOT install cbu directly on planks!
    Thanks Jim.

    I meant the 1/2" cement board in lue of Ditra. Even 1/4" would work after the plywood goes down.

    I hate OSB - I would never allow it on any job site.



    JW


    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    OSB is structurally unsound if it gets wet, so it would be a poor choice for a floor in a bathroom, even with a waterproofing barrier.
    Licensed residential and commercial plumber

  7. #7

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    i'm on the OSB bashing wagon....wouldn't even consider it for ANY use in a bathroom

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