Undermount tub install questions

Discussion in 'Shower & Bathtub Forum & Blog' started by mtn_mac, Jul 1, 2016.

  1. mtn_mac

    mtn_mac New Member

    Jul 1, 2016
    I have two areas of questions on an undermount tub install and can't seem to find the answers in past posts.
    Looking at a Kohler underscore tub in an undermount location (back side of tub against a wall, but not an alcove). Tub only--no shower. Deck surface will be something like a caesar stone product.

    The acrylic tub has about a 3" wide lip on all sides so the first question is about a deck mounted faucet/tub filler. Assuming the finished deck surface overlaps this 3" wide lip so that the finished edge is close to flush with the opening of the tub, do I need to make sure I mount a deck mount tub filler more than 3" back from the finished edge of the final surface so I avoid going through any part of this 3" tub lip? (Aka make sure I mount the tub filler through my wood frame built around the tub rather than catching any part of the edge of the tub.)

    The second questions are about the wood frame to build up around the tub and the final surface edges. From what I can gather, I want to build up my deck to have a surface height flush with (or just a hair proud of?) the top of the tub. The finished surface like caesar stone or whatever would then mount to my deck and also go over the lip of the tub. If that is correct, does the finished surface actually mount at all to the tub lip or just to my deck and then essentially cantilever over the tub lip and just get caulking/whatever to seal to the tub?

    Again assuming the tub lip is ~3" wide on all sides and the finished surface would overlap that, what is the minimum amount of wood deck width beyond that 3" that would make you feel comfortable for a secure enough mounting of the finished surface? For example, is 3" of wood deck width sufficient (so ~6" wide finished surface would mount to 3" of wood deck and also span over the 3" of tub lip)? Maybe the back edge (against the wall) could get by with less width than the front edge that is more likely to have somebody sit on it/step on it? From pictures I see there seem to be some installs with a pretty thin/narrow amount of finished surface on the front or back edge but I'm a little suspicious of it being flimsy.

    Lastly, while it's clear that the floor supports the tub, not the lip, it seems some try to stiffen up or otherwise strengthen the lip of the tub--presumably to make the finished deck surface more solid. Is there something I should consider doing to effectively try and beef up the lip like mounting an equivalent of a stringer on all four sides interior sides of my wood frame to catch the bottom of the lip of the tub or ?

    Thanks in advance
  2. JHJ

    JHJ New Member

    Sep 8, 2014
    Sorry to revive this old thread that never made it past the OP, but I have every single one of these questions, and searching leads to other threads that get close to answering them, then finish up without *quite* being as specific as I want.

    +1 to mtn_mac for articulating these problems as concisely as he did!

    And, thanks in advance.
  3. Sponsor

    Sponsor Paid Advertisement

  4. Judd Maltin

    Judd Maltin New Member

    Mar 17, 2017
    New York
    I wouldn't worry too much... just don't build too light. Get it solid.

    I'm just finishing up an undermount tub install, similar to yours. It's for my home. Deck will follow the wall, and three sides stand free, following the curve of the tub. I built the surround with 2x4 dimensional lumber, and used decking screws, 3 at every joint, on all the studs. I measured 2 1/2" proud of the lip of the tub with a plumb bob and framed to that curve. A good portion of the 2x4 top plates are under the lip of the tub. I was also considering using construction adhesive on the joints, but that really would have been overkill. I've already cut the 3/4" CDX plywood for the deck. I affixed it to the framing with decking screws, just in a few spots to hold it steady. I snapped center lines, and found the center, and taped the big cutout template that Kohler shipped with the tub to the plywood. I taped the cut line down in areas so it wouldn't come up as I cut out the plywood. Some of my framing was inside the line of the cutout, but the shallow depth of the circular saw barely touched it. The plywood cutout came out in one piece. Nice. I cut the plywood to length of the framing, leaving a 1 inch overhang on the three sides that are not against the wall. Then, I cut the plywood in three pieces - two against the wall, and one big one following the curve of the tub. I'll be able to slide those all in place once the plumbing is all done. I weigh 220 lbs and can jump up and down on the frame without it even creaking.

    I've finished the plumbing, shimmed the tub, marked the shims to the floor, marked where the bottom fins of the tub hit the subfloor, and lifted the tub out. I used construction adhesive, very generously, under the shims, and followed the lines that marked the fins of the tub. I've set the tub back in place, and it set two days while I fussed with the drains. I've been sitting on the edge of the tub without support (again, I'm 220lbs) and it kinda flexes a little, but barely, and doesn't crack.

    I'll be following the vertical curve of the framing with 1/2" plywood, cut to allow curve. Then I'll cover all the plywood with Kerdi and thinset. I'll thinset tile and grout with 1/8" left for silicone between tub lip and tile. What pressure and flex the tub might sustain at the lip will be resisted/absorbed by the silicone, and the very hearty deck.

    I'll report back. :)
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    You've set yourself up with built-in problems...first, CDX plywood is not a suitable underlayment for tiling...there are voids in that type of ply which is made for siding, not underlayment...then, Kerdi is NOT designed to be installed directly to plywood (while Ditra is). The fleece and structure of Ditra is different than Kerdi, and you may have long-term bonding issues. At least with the CDX plywood, you're not as likely to be walking on it as you might while on a floor as underlayment, so those voids might not come back to hit you, but the Kerdi directly to ply IS a problem. You might get by, but may be better off with either KerdiBoard (you realize, you could have built the entire deck out of KerdiBoard?!) or a layer of cbu and it would have been more stable, waterproof, and ready for tile. You could throw a 1/4" cbu on top, and then you'd have an approved substrate for Kerdi. How are you going to manage the edges of the substrates for the undermount tub that would be exposed (verses a drop-in which would cover them)? It can take some special work to get those edges ready to tile, and again, Kerdi is NOT designed to bond directly to plywood, regardless of the type of thinset you choose. Schluter has a good tech support department...you might want to call them for advice.
Similar Threads: Undermount install
Forum Title Date
Shower & Bathtub Forum & Blog Kohler K-899-G Undermount Tub Installation: Template(s) 892 899 Nov 23, 2014
Shower & Bathtub Forum & Blog How to install an undermount sink to your bathroom counter top. Apr 13, 2012
Shower & Bathtub Forum & Blog Undermount installation option??? Mar 20, 2006
Shower & Bathtub Forum & Blog Undermount sink clip recommendation Oct 22, 2016
Shower & Bathtub Forum & Blog 4 slabs or two seams for undermount soaking tub? Jul 8, 2016

Share This Page