Sub-Floor under Alcove Tub

Discussion in 'Shower & bathtub Forum & Blog' started by weekend_warrior, Aug 6, 2012.

  1. weekend_warrior

    weekend_warrior New Member

    Messages:
    1
    Location:
    Pleasant Hill, CA
    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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,932
    Location:
    New England
    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?
  3. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

    Messages:
    4,028
    Location:
    North Vancouver, BC
    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
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,932
    Location:
    New England
    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!
  5. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

    Messages:
    4,028
    Location:
    North Vancouver, BC
    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
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,519
    Location:
    Cave Creek, Arizona
    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.
  7. mikeplummer

    mikeplummer Plumber

    Messages:
    190
    Location:
    Ontario, Canada
    i'm on the OSB bashing wagon....wouldn't even consider it for ANY use in a bathroom
Similar Threads: Sub-Floor under
Forum Title Date
Shower & bathtub Forum & Blog Rust Stain Under Your Shower Arm: WHY THIS HAPPENS AND HOW TO STOP IT Jun 5, 2014
Shower & bathtub Forum & Blog Plywood under Cast Iron Tub May 9, 2014
Shower & bathtub Forum & Blog Shower Construction Terminology: Pookey - Understanding what it is and how to use it Apr 27, 2014
Shower & bathtub Forum & Blog Understanding a Steam Shower Sealed Slip Joint: Photo's, Sketches and Video Tutorial Dec 7, 2013
Shower & bathtub Forum & Blog Under Mount Tub - Deck and Mounting Oct 12, 2013

Share This Page