Installing Kohler cast iron tub (alcove) on plywood floor

Discussion in 'Shower & Bathtub Forum & Blog' started by lemmy, Dec 25, 2013.

  1. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    I followed the instructions for my Kohler Bellweather (K-876) tub and slid the tub in place and I have the plumbing attached and the drywall installed on the long side but not on the front and back yet. I was worried about the 4 fairly small legs sitting on the 3/4 plywood floor but that was what the instructions said to do. The instructions said to use metal shims if the tub was not level, but the floor was level and the tub did not rock. However the other day I noticed the tub was slightly rocking. I assumed being in and out of the tub hanging drywall on the ceiling and walls has resulted in at least one of the feet sinking into the plywood like I feared. I know many suggest using ledger board but I don't really know that I can do that now with the tub already installed. I do think I can get to the front (from below) and rear (from behind) of the tub and raise it enough to put a 1/32 or 1/16 piece of steel or aluminum under each foot to spread the load and keep the tub from sinking in, but wouldn't that cause the tub to be to easily moved post install? (metal feet sliding on a thin piece of metal) Also, the tub instructions said to use metal shims if the tub was not level, so even if I do put the steel under each leg the tub would still rock slightly. Where do you get metal shims? I asked around and nobody seems to have them. I really need to finish this bathroom (baby on the way any day!) and this tub issue has stopped my progress so I would be very thankful for any suggestions from the experts.

    BTW, Kohler says the minimum floor handling capacity if 52lb/ft^2. My flooring is 2x10 joists 16 inches on center and they span only 8 feet. So the floor joists are plenty strong enough.

    Last edited by a moderator: Dec 26, 2013
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    FWIW, the typical residential design load on a floor is 50psi, with 10# being dead (constant which includes the subfloor and any finish materials on it) load and 40# being transient load (like the people and water in the tub). Some bathrooms are built with it higher. But, a typical home handles a tub fine, even if it does exceed the design load by a bit. It may become a bigger issue if you then want to use lots of ceramic or stone in there unless it was designed for it.

    I've not looked at Kohler's installation instructions, but most call for a ledger board to help keep the tub from rocking or shifting. Once the walls are up, it is hard for it to move much. The ledger helps when you may say sit on an edge - if installed correctly, it will help prevent it from rocking onto two legs. If this tub is being used as a shower, and you have drywall up in there, you do have a problem, though, especially at this stage. If it's only a soaking tub, it may be okay, but is still not ideal.

    MOrtar is often used underneath a tub - it's more important on non-CI tubs (like fiberglass, and acrylic), to help support the bottom from flexing. It can also be used to help level. If used, regardless of the material, it prevents the tub feet from sinking into the subfloor because it spreads the load out much further.

    Keep in mind that deflection is critical not only along the joists, but in between them as well (from the subflooring). Depending on the type and thickness of the subflooring, excessive weight on an individual tub foot, particularly when it doesn't have the ledger boards to prevent it from sinking, it could be an issue.

    Maybe a few pictures would help suggest a solution to your situation for others to comment.
  3. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    Thanks for the suggestions.

    The instructions are here for rough in:

    http://www.us.kohler.com/webassets/kpna/catalog/pdf/en/1074694_4.pdf

    and here for installation:

    http://www.us.kohler.com/webassets/kpna/catalog/pdf/en/1084177_2.pdf

    I couldn't find any mention of a ledger board and does specifically say "make sure the bath is level, plumb, and resting on all four feet". I really don't see how a ledger board would really be much better. Wouldn't my floor support more weight than a ledger board? Also it seems that it would be almost impossible to wrestle a 330lb tub in place, pick it straight up, then sit it down on the ledger board. Then once on there, it seems that it would be just as likely to rock on the ledger board unless you had some seriously straight lumber. I also think it would void any warranty if the instructions say to put the weight on the feet and instead I put it on the ledge. I know the ledger board is the most common way to do this (from reading AFTER I installed according to Kohler's instructions) though. So I know it can work and work well.

    It isn't a shower, but if it was, have you changed your mind regarding kerdi and drywall? I thought I had read some of your posts saying that drywall is an industry approved method according to the TCNA when you have Kerdi over it.

    The mortar would be a good idea. Would I need to take the tub out or just keep packing it in there until it is up to the tub bottom and then stay out of the tub until it completely dries?

    So do you think sitting each leg on 1/16" steel plates is a bad idea?

    Yeah, I know that flex in the 3/4 plywood flooring can be an issue regardless of the joist strength, but I just wanted to state that joist strength won't be an issue. I know all kinds of bad information can be found on the internet, but I found this equation Max load = FBd^2/9L. For 1.5 x 9.25 spaced 16 on center and spanning 8 feet and assuming 1000 for F (which they said was a conservative estimate) it comes out to 1782 pounds per joist or 1782/ [(16/12) x 8] = 167 pounds per square foot. For 14 foot spans it would be 54 pounds per square foot.

    The joists run from the drain side of the tub toward the back. The two feet toward the long wall (the front left and rear left foot if you are standing in the tub) sit very close to a joist and the joists aren't 16 on center but about 8" there (2 joists about 3" apart are under that back wall and another joist runs just to the left of the drain). So I think with the steel or aluminum plates there would be little movement there. But the front right leg is on a section of plywood that while not far from the joist, it doesn't get as much support because the plywood had to be cut out for the drain. I think this is allowing the plywood to flex more in this area so I am thinking of putting another section of 2x10 directly under the foot and nail it to the joist so the plywood the foot is sitting on rests directly on something solid and not just on plywood that could flex.

    Attached Files:

    Last edited: Dec 25, 2013
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    Most tubs call for a ledger, and it is not normally bearing any weight, it is just there, up against the underside of the edge to prevent the whole thing from tipping.

    I don't have an issue with Kerdi over drywall (Schluter has that independently tested and certified), but in a tub situation, the diagrams I remember show Kerdi on the wall down below the tub rim (not necessarily to the floor), then Kerdiband applied, lapping over the tiling flange. Since most people put the tub in up against the studs, I think they should test and revise that with something they've verified works. Lapping it over the tiling flange should be reliable by itself, and if you ask them, they may approve it, but maybe not if you use drywall since that seam is iffy. When the Kerdi extends below the tub rim, then Kerdiband is installed, there would be no drywall sitting ON the flange or rim that might be destroyed if your sealing job to the flange was not perfect. THere's a LOT more overlap and confidence when seaming Kerdi to Kerdi, verses Kerdi to something else. Kerdifix is great stuff, but all it takes is a little gap, and drywall at a tub junction could be an issue - your workmanship has no room for error, where in a typical Kerdi-Kerdi seam, with at least the mandated minimum 2" overlap, you have a little flexibility.

    If the tub now rocks, you need to address that. Spreading the load would make the feet less likely to compress the ply. Since you are now seeing that happen, I'd have to think about it some. Those that do this on a more frequent basis, may have an easier retrofit rather than tearing it out to get metal shims or bedding it in mortar (even then, if it sits flat, it would still sit on the feet, but the bottom would also be supported in between, moving the likelihood of them trying to punch a hole in the ply to near impossible).
  5. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    Thanks. The drywall now is about 1/8" above the tub flange. I was planning on putting sealant between the flange and the drywall, then putting kerdi all the way down to the tub flange and sealing that with kerdifix. Then bringing the tile down and sealing that to the tub flange with sealant as well. Are the diagrams you are talking about on the Kerdi website?

    I can put steel or aluminum shims under the feet without taking it out (at least I think I can). With the rear I can fit something under to pry up the tub 1/8" or so to put a shim under both feet and then for the front I can go in the crawl space and use a bottle jack (with a block of wood between jack and tub) and push up on the area of the tub near the drain to lift the front 1/8" to get 1/16 shim under the front legs. Then I would just worry about the tub being able to slide too easily on the metal shims.
    Last edited: Dec 26, 2013
  6. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    I know a couple of people get really upset with the idea of using kerdi over drywall for showers, but this installation manual shows basically what I am doing:

    http://www.schluter.com/media/ShowerHandbook-2012-2013.pdf?v=201312260614

    Step one shows sealing the 1/4 gap between the wall and the tub with Kerdi-Fix, then putting Kerdi-Band over the transition from the wall to the tub. I only have an 1/8" gap though.
  7. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    Location:
    North Vancouver, BC

    That would be me - who gets upset. Good luck with your reno. Hope goes well. Maybe wait for Jim - the Defence Engineer. He is an expert on all things Kerdi and Drywall.

    I'd love to help with your questions but can't look past the drywall. It kills me that people read all the info on why drywall should not be used and then use it anyway.

    Why not just call Schluter? They will help you.

    Your in a group site where many of us build showers for a living. I would bet none of these men have drywall in their showers.

    Lots of the men on John Bridge use drywall. Go ask them.

    Installing Kerdi Band to your Tub's Tile Flange

    [video=youtube_share;lPIyG8v1ZhU]http://youtu.be/lPIyG8v1ZhU[/video]

    As a bathroom builder this is what we see most times when removing drywall from a shower's wall. Nasty Black Mold.

    The only Kohler Cast Iron Tub we have done of late is this free standing one.

    [​IMG]

    Kohler Cast Iron Tub
    Last edited: Dec 27, 2013
  8. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,815
    Location:
    Cave Creek, Arizona
    Most plumbers DO use a ledger board, but since you did not, but have access to both ends of the tub, just cut 2x4 "legs" and install them under the back corners of the tub to support it solidly. Cast iron tubs do NOT "slide easily" no matter what is under them.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    Drywall as the backer for a full shower isn't an issue with me. But, I do get a little squeamish when there's a tub involved. It works, but you have no room for error. Kerdifix is a really good sealant - sticks to pretty much anything, is a little flexible, and is impervious to water. But, your line of defense is shallow - a bead of sealant, verses when a full Kerdi shower, where you end up with huge, in comparison, overlap of the Kerdi material. The range of stretch of Kerdifix isn't huge, so you must ensure your tub is solidly in place.

    Just be vigilant in getting a good, continuous bead, and then embed the Kerdiband into it and you should be fine. I'd prefer you had Kerdi further up the wall as well, but splashing is nothing like a showerhead. If you follow their instructions, you should get their warranty, too.
  10. jm66208

    jm66208 Member

    Messages:
    99
    Location:
    KC
    For the kohler CI tubs I've installed, I take some 6" x 6" x 1/8" mild steel and screw it to the floor for the tub feet to sit on. Do a dry fit, then slide the tub out & adjust the tub with an angle grinder if necessary to get it to sit solidly & level. Usually takes just a minor adjustment, and I get it with one or two attempts. Might be overkill, but I tend to do things the hard way...
  11. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    Messages:
    4,393
    Location:
    North Vancouver, BC
    But Jim in a steam shower, shower or shower curb it's OK?

    I bet if anybody calls Schluter they will tell you - go ahead, use drywall. Marketing departments promote this fact. Foolish choice and leaving up a couple sheets to save $20.00 in materials is such a bad idea.

    Install concrete board. Then Kerdi.

    If you want to use drywall then just use Ardex 8+9. If you want to do the least possible, save the most money then Ardex 8+9 will cost you roughly 1/2 the price of a Kerdi job.

    [video=youtube_share;gLAGTafOqhM]http://youtu.be/gLAGTafOqhM[/video]

    Ardex 8+9 over drywall

    Approved for drywall, plywood and cement backer board. Good idea? Not. Allowed? Yup.


    I think the missing blocking is more alarming in the pictures than the use of drywall.
    Last edited: Dec 27, 2013
  12. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    1) I really wasn't asking for help with the drywall or tile (until it was brought up). Just the tub.
    2) My father's house was built (not by him) in 1972. We had four people use the same tub (the other bathroom was rarely used) nightly and it was tile on drywall with no barrier of any kind. I helped him take it out 2 years ago. Absolutely no signs of water damage or mold. I am not saying that is the way to do it, but if he had that kind of luck with very heavy usage over the years, then I don't see why I would have problems if I used a supposedly water proof barrier. If it lasts 40 years and then falls apart the next day I will be happy and probably dead. Anyway, unless are flailing uncontrollably in the tub, the wall of a tub (without a shower) probably will see less water than my kitchen backsplash, or the drywall near the vanity.
    3) 95% of the information saying not to use it comes from your posts on this forum.
    4) do you not believe kerdi is waterproof or do you think that you can't keep a good seal around the bottom (this would be a problem with any board because wouldn't the water just leak out and onto the floor and destroy the floor and framing under the tub)?
  13. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn

    That is a good idea. Thanks. I shimmed the tub with 1/16 galvanized steel plates today, and I made sure the plywood where the tub legs are sitting are directly over a joist or a 2x8 brace that is connected to the joists.
  14. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    Are you referring to midspan blocking? I rarely see that in houses and the middle of the wall isn't even in the pictures.
  15. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    Thank you. The tub is very sturdy now and I also plan on putting the "legs" in as hj suggested. Also, I do plan on putting the Kerdi higher up on the wall.
  16. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    Messages:
    4,393
    Location:
    North Vancouver, BC
    Mid Span blocking and missing wall studs for curtain rod on the right side of your tub.

    Sorry for voicing my concerns over your choice in backer board. You seem to know all the answers and Jim here has filled in maybe 40% of the missing info.

    You really need a strong plan for your build and understand the entire job.

    If using drywall and Kerdi I have been told (By Schluter) that you should involve your local Schluter rep if you want any kind of warranty. Jim refers to movement. I called you on missing blocking. But hey - if you did not have it before why do it now.... I'm kidding of course.
  17. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    You point is made. I am not trying to disagree with you because I know that your experience is far far greater than mine. Have you ever seen mold like that when kerdi was used over drywall? If so, where is the most common failure point? Is it cracks in the grout and then the Kerdi doesn't do a good job protecting the drywall, or is it almost always at the tub drywall interface?

    I am not so far along (as you can see from the pictures) that I can't easily change to concrete board. Would I just run the concrete board out near to the point where I would stop the tile then but up drywall to that?. Then bring the kerdi out over this joint between concrete board and drywall then tile out just over the kerdi? I am not concerned with increased price, I just don't have much experience working with concrete board and I had read from so many people that kerdi over drywall worked well. I know you may disagree, but if you have faith that the Kerdi is 100% water proof, you can see why someone would think that the material behind the water proof membrane wouldn't need to be water proof.

    BTW, I had also considered using the DensArmor but I didn't know how the Kerdi would stick to it. I was going to just get it for the entire bathroom but then I read it has a rough finish and requires a skim coat to make it look smooth. But if it was just going to be under the tile around the tub I guess it wouldn't matter.
  18. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    Messages:
    4,393
    Location:
    North Vancouver, BC
    Concrete board can handle some mositure and not fail. Drywall when it gets a little moisture turns to mush and the paper is a food source for mold.

    I have not ever repaired a Kerdi Shower that had failed. Same is true for any other shower that has had red guard, HPG, Hydro Ban, Ardex 8+9 and on and on. Ecvery shower I have repaired with mild and discoloured grout has had drywall behind it. Every single one.

    The local inspectors so no to drywall with kerdi. I know because I asked.

    Denshield can work fine. Kerdi band over the tub's flange and set the Kerdi band to the denshield with Kerdi Fix. Then use 8+9 over all screws and seams.

    When we come across Denshield I often skim coat the entire wall with a self curing product from Mapei called Grani Rapid.

    How much Kerdi do you plan to install? If you are not going up the walls 6' then you best do a traditional cement backer board design with the Kerdi going up the wall studs, poly or builders felt lapping over the Kerdi and then the cement backer board - tape the seams....

    Any split design has holes in the planning. If you plan to go topical - go all the way.

    Did you see my blog post on Kerdi Band? Simple design. You could go this route with ease.

    It's horrible the advice Schluter gives out about drywall. I hate this about this company. I do however like the waterproofing and Kerdi Fix more than I can't stand the advice they give out. For years I have blasted Schluter over this point.

    I think Kerdi Board is crap as well. I don't use any foam products.

    The TCNA and TTMAC list one backer board for steam showers - Cement Board. That's it. Ask yourself why...

    I have never fixed a shower with cement board in my life.

    We find showers all the time built with drywall that are perfect. Makes no sense - but it CAN be done. But. every failed shower we fix is also done with drywall. Why for the sake of a few dollars take the chance.

    Deflection (movement) cause lots of failures.

    Bond Breakers + number one reason for tile failure.

    If you can find Ardex setting materials first install your concrete board and tape the seams. Skin out the entire wall with a flat trowel of thin-set. Then the next day install your Kerdi. Ardex will allow you to use their modified setting material. This way you will have more working time and not have to fuss with spray bottles of water over drywall.

    One thing the yaw whos on John Bridge don't tell you about water application is that water can by itself be a bond breaker.

    A shower is so easy to screw up. Kerdi Fix. Ardex 8+9 together cost about $100.00. together with Kerdi you can build anything. While you have these items why not build one of my LED shower niches out of your scrap building materials....
  19. lemmy

    lemmy New Member

    Messages:
    31
    Location:
    knoxville, tn
    I was planning on doing Kerdi band around the bottom and the Kerdi membrane up to within 1.5 foot of the 9' ceiling (all the way to the point where the tile will end).

    I did see your blog post. The only thing I didn't quite grasp is where you have a gap between the backer board and the tub, then you say not to fill the gap between the tub and the back board. Are you saying you don't want to push down on the Kerdi Band too hard (in the area of the flange) so that the Kerdi Fix gets up in the area between the tub flange and backer board? Why do you not want the sealant to get in to that area? So does the 8+9 provide a water proof seal for the Green EBoard or is the Eboard water proof already?

    In all honesty I don't think it is cost (at least for me) that makes people want to go with the drywall and Kerdi membrane (not just the bands, along the seams and tub, but all the way up the wall with the Kerdi membrane). I think it is the simplicity. Attach drywall as usual, no need to finish the drywall (except for tapered edges), apply mortar and kerdi, then apply your tile. Maybe the other methods (i.e. concrete board) aren't any more complicated, but they seem to be.
  20. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    This is a TUB only install, NO shower, at least that's what was said, and is the basis of my thoughts on what is required. So, no shower head to get things wet on a regular basis, and the only splashing is unlikely to go very high , or be very consistent. If this were a shower, THEN, it would need to cover the drywall up to at least the height of the showerhead (per industry standards). To add a showerhead after completion would likely take a remodel, and then you could address the issue, although you could change the spout and put in a handheld. IF that is a possibity, reconsider your waterproofing and go higher. BIG difference in being a shower and a tub only in how things MUST be prepared. It is crucial to get the tub/wall joint waterproofed, and not allow any moisture to get in from there, and if there's waterproofing up a foot (probably higher than required) or so above the tub rim, it should be pretty bulletproof. A little splash first, won't penetrate far, and would then have an opportunity to dry in between tub uses.
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