Plywood under Cast Iron Tub

Discussion in 'Shower & bathtub Forum & Blog' started by winger27, May 9, 2014.

  1. winger27

    winger27 New Member

    Messages:
    18
    Location:
    Pa.
    This may seem like a stupid question, but here goes. I am installing a Villager cast tub. My floor joists run left to right (drain to end wall).I have 3/4 subfloor running diagonally and installing 5/8 plywood over top. If I was to run the plywood perpendicular to the joists I would have to seam in under the tub (60 in. width). I really didn't want to do that. What does everyone do? Thanks
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    Plywood has a very definite grain to it, and you MUST install it perpendicular to the joists to gain the strength you want (to, I assume) tile outside of the tub area on the floor. If you screw the new ply down (to the planks, not into the joists) well, it should not be an issue. Prior to installing the plywood, refasten any loose planks and replace or repair any bad boards. You may want to use screws to ensure the planks are well fastened.
  3. winger27

    winger27 New Member

    Messages:
    18
    Location:
    Pa.
    Are you saying then if I screw the plywood to just the planks, without it catching any floor joists, it won't be an issue? Should I be concerned with a seam running back to front (apron) at approx. the middle? I will be tiling the floor also.
  4. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,246
    Location:
    Land of Cheese
    What is your concern? If the subfloor is solid and you are screwing plywood over it, I would rather have the seam under the tub given the option.
  5. dj2

    dj2 Member

    Messages:
    411
    Location:
    California
    Don't forget a 2x4 across the back wall and under the tub lip for support.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,636
    Location:
    Cave Creek, Arizona
    quote; Don't forget a 2x4 across the back wall and under the tub lip for support.

    There is not enough room behind the Villager, at the rim, for a full 2x4 ledger. Either use a 1x4 or bevel the edge of a 2x4.
  7. winger27

    winger27 New Member

    Messages:
    18
    Location:
    Pa.
    Cacher, I wasn't sure if a seam under the middle of the tub would be a support issue.
    As far as the ledger board on the back lip goes, it seems to be be a big debate.
    Rest on the four legs as manufactures recommend or ledger like old school.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    Plywood, properly installed over your planks, will be fine, regardless of where that joint occurs. NOw, if you were really paranoid, you could get thicker ply with a T&G joint, and make it stronger, but that is only needed when it is the first layer, since there would otherwise be nothing to keep the edge from deflecting. WHen you install ply as a second layer, those vulnerable edges are fully supported by the first layer. The only thing I'd look at is to potentially avoid having that joint exactly line up with what's beneath it, but since your first layer is on the diagonal and is T&G (?), it's not an issue either.

    The idea of not hitting the joists with the second layer is to help decouple things some. Your planks and joists will want to change dimensions with the seasons (humidity changes), and the ply will be much more stable. With the planks and joists running in different directions, who knows which way things will try to warp to compensate...best to only have to deal with one. Using an uncoupling layer, like Ditra on top, can help with that seasonal movement much more than a rigid cbu board which primarily only offers tiling compatibility, and only a little decoupling.
  9. SHR

    SHR Member

    Messages:
    99
    Location:
    Minnesota
    Why? I have successfully installed almost 100 Villager cast iron tubs and never used a ledger board. The Kohler installation instructions clearly show no board is recommended. Leveling the Kohler Villager tub is accomplished by shimming under the cast iron feet of the tub. Even I could jump up and down on the tub ledge and not deflect the cast iron material.

    http://www.homedepot.com/catalog/pdfImages/ea/ea34031f-b504-4b0c-ba0b-bc8c4289710f.pdf
  10. winger27

    winger27 New Member

    Messages:
    18
    Location:
    Pa.
    The subfloor is t&g. As far as the plywood goes I bought some APA 19/32. It seems to have rough surfaces on both sides with knot holes it is not exterior. Is this acceptable for the underlayment for 1/4 cbu. If not what should I get?
    I think I will just level the tub with the feet, since that is what it is designed to do.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    No, your plywood is not acceptable for use under tile. The industry guidelines call for the plywood to have exposure I or exterior rated glue, and have no side worse than that rated 'C' (C has any holes patched and filled - B, allows tight knots and no holes, A has no knots or voids anywhere). Unfilled knot holes indicates a 'D' surface, which means a void, which means a potential deflection, which can lead to tile failure. The glue holding it together is important, too...you do not want it to come apart if it accidentally got wet. And, think about it under the tiny foot of the tub filled with water and a person or two in it...lots of weight potentially sitting on a void...not a good idea!

    From a tiling standpoint, cbu is NOT considered structural on a floor. The floor must be suitable for strength prior to installing the cbu.
  12. winger27

    winger27 New Member

    Messages:
    18
    Location:
    Pa.
    jadnashua, thanks for the heads up on the plywood. Took it back and got some B/C exposure 1 looks alot better. Glad I checked here.
  13. RedShoecounterbalance

    RedShoecounterbalance Member

    Messages:
    304
    Location:
    Minnesota

    Good move, Jim is pretty spot on here. The best way to go is by measuring deflection first and proceeding accordingly.... Class rating of tile, and joist system at hand.....BC exp 1 is the best way to go. Leave expansion gaps of 1/16 between all sheets and backfill with silicone before thinsetting membrane down. cbu even embedded in thinset not smart. And of course there is way more to it.....screws glues cross grain etc
  14. winger27

    winger27 New Member

    Messages:
    18
    Location:
    Pa.
    By backfill you mean caulking the joints? What was meant by cbu embedded in thinset not smart. I am not hip to the lingo
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    An uncoupling membrane provides lots more benefits (at least to me) on a bathroom floor verses cbu. The smarter choice is an uncoupling membrane verses cbu.
  16. winger27

    winger27 New Member

    Messages:
    18
    Location:
    Pa.
    This is a first time project for me and probably my last. lol. I feel more comfortable using cbu. Seems like there's a bit more learning curve with a membrane.
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    Cbu works...it's a pain to install (relative to a membrane) and cut (some is easier than others), it's heavy, if you use power tools to cut it, the dust is toxic. Screws are time consuming to install (this is all relative), and you must then tape the seams (you can do this during tiling - requires alkali resistant fiberglass mesh tape).

    There's more than one membrane out there, but it goes down with thinset. Then, you spread thinset on top of it, and set your tiles. If you want your seams to be waterproof, you add a band to it. The one I have most experience with is Ditra http://www.schluter.com/media/DITRAHandbook-ENG-2013.pdf?v=201405100602 .
    Laticrete also makes one, but it cannot be made waterproof, and there are others out there. Ditra has been around for a long time.
  18. winger27

    winger27 New Member

    Messages:
    18
    Location:
    Pa.
    It seems like the only difference is the way the it is installed, with both having the same results.
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    Not if you understand what an uncoupling membrane does! Lots of tile have been installed with no issues using cbu...it is a viable product. It's just that an uncoupling membrane has a lot of advantages: better protection for the tile (cbu is primarily a tile compatibility layer, and only offers a very small uncoupling), ability to be made waterproof easily (some models, brands), much lighter to carry, easy to cut with a knife or scissors, less height buildup...take a few minutes and watch this video, then decide. https://www.youtube.com/watch?v=f6FyWs2WZ1k&feature=player_embedded They are not the only one available, but the concept is the same.
Similar Threads: Plywood under
Forum Title Date
Shower & bathtub Forum & Blog Plywood thickness under tile Oct 6, 2013
Shower & bathtub Forum & Blog Plywood vs. OSB: Which Is Better? Feb 6, 2014
Shower & bathtub Forum & Blog Installing Kohler cast iron tub (alcove) on plywood floor Dec 25, 2013
Shower & bathtub Forum & Blog Walk in shower with plywood walls Oct 30, 2013
Shower & bathtub Forum & Blog New tub installation: is 1/2" plywood OK? Jul 27, 2013

Share This Page