Advice for leveling alcove subfloor

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Hi, I am new to this forum and have never performed any major bathroom renovations. My intent has been to have a plumber install a new bathtub and related plumbing but have surprisingly encountered multiple plumbers who do not do bathtub installs. I’ve also encountered an array of differing approaches which led me to contact a general contractor/home improvement company who has suggested that I’m getting the runaround given size of my job and request for N95 mask to be worn while inside my home. The general contractor/owner has assured that the project is simple although I would appreciate advice if you all are willing and able given part of this project may very well turn into a DIY effort.

The alcove in my 3rd floor (of four total floors) 35 sqft secondary bathroom is 60 1/8” long. A 60”x30” Kohler Elmbrook, Left Drain, Acrylic bathtub will be installed. Per specifications, the bathtub weighs 75 lbs and holds a maximum of 42 gallons of water.

My OSB subfloor is likely 19/32” - a stamp is visible in the ceiling of first floor utility room which is the only unfinished space in my home. The bathroom’s joists are 2”x9”, span ~10’, and are spaced 16 o.c. (some closer). The left (drain) side of the alcove sits ~ 5/8” lower the the right side. The right (high) side is an exterior wall for my house and a common wall with neighbors. The previous bathtub was also acrylic. I had owned my home for five years before pulling it out and we never experienced any leaks when using it despite discovering that the alcove is sloped downward from right to left (towards the drain side). The alcove is generally level depth-wise from front to back. An immediately adjacent bathroom’s floor is covered in ceramic tile and supports a 24” high acrylic drop-in soaker that sits in a mortar bed. I point both out to highlight that there’s been a significant amount of weight on the subfloor for 20 years.

The subfloor outside of the alcove is also sloped but I would prefer not to level the rest of the floor provided it would lead to a significant step-up from the hallway once 1/8” Ditra or 1/4” underlayment (Hydra Ban Board or Cement Board) and tile are added. Given the to-be-tiled section of subfloor is also generally flat along the slope, I have been told that it is unnecessary to level it. I’ve read multiple threads and miscellaneous material which suggests the same. I mention this given I am aware that there will be a gap that tile and tile underlayment must cover on the drain side if the entire floor isn’t leveled.

Below are the general options mentioned to address the alcove’s slope:

Option A - use self-leveling compound to level the alcove’s subfloor. Not preferred given additional weight, there aren’t many options compatible with OSB, most of the minimal options require use of lathe which seems it will require the right (high) side to be raised to meet minimum required thickness of compound. A ledger board would also be used.

Option B - use some variation of wood to account for slope. A ledger board would also be used.

Option C - do not worry about leveling the alcove’s subfloor at all...to simply install bathtub using ledger board, mortar and plastic both above and below mortar.

Option A Questions:
  1. There are a few OSB-compatible self-leveling compounds offered by Custom Building Products, one that claims a feathered edge is possible. Do you recommend their products? If so, which one?
  2. Are their alternative manufacturers that you would recommend? I called Mapei and Laticrete who confirmed that they do not offer an OSB-compatible compound.
  3. Since the compound would not be supporting tile and an underlayment, is it even necessary to attain a feather edge? For example, for OSB-compatible compound that has a 1/4” thick minimum pour, could it be used to simply cover the alcove from the point where the slope is ~ 1/4” thick up to ~ 5/8” thick? This would provide support from the left (drain) side up to ~ 48” of the total 60” length of the tub, meaning the wooden left (drain) side feet would be supported to attain level. I admit I am not sure where the feet on the right (high) side will fall (feet distances are not noted in spec sheet) but would hope that any shimming of them would be minimal. Where the slope is less than ~ 1/4” thick, I would think that nothing further is absolutely necessary beyond standard shimming. Mortar bed would still be used throughout the alcove over both the compound and the slope where less than ~ 1/4” thick along with plastic both above and below mortar. Is this viable?
  4. How does one keep the self-leveling compound on a sloped surface from running downhill given its consistency?
Option B Questions:
  1. Would it be acceptable to use different thicknesses of plywood panels to account for the slope? For example, a 5/8” panel for area where the slope is 5/8” thick; then step down to 1/2” panel for area where thickness of 1/2” thick to less than 5/8” thick (and shim where necessary to attain level); then step down to 3/8” panel for area where thickness of 3/8” thick to less than 1/2” thick (and shim where necessary to attain level); then step down to 1/4” panel for area where thickness of 1/4” thick to less than 3/8” thick (and shim where necessary to attain level); then shim the wooden feet on the right (high) side of the bathtub if necessary? The plywood would be both glued and screwed. Mortar bed would still be used throughout the alcove over all of the plywood panels and the slope where less than ~ 1/4” thick along with plastic both above and below mortar. Is this viable?
  2. In some forums, others have stated plywood does not glue well to OSB. Is that true? I’ve seen construction adhesive that explicitly calls out OSB as compatible with it. Is there a particular type that either of you would recommend beyond Loctite PL Premium, PL Premium Max, or Power Grab Ultimate, or Liquid Nails Heavy Duty?
  3. I’ve also seen others state that screws don’t hold well when screwing plywood to OSB? Is that true or should I be able to simply screw the plywood to the OSB for a sufficient hold? This would be a problem given the wall where plumbing is installed (left) does not sit directly over a joist to allow the left-most end of the 5/8” thick panel to be screwed to the joist. I suppose the only alternative would be adding blocking although there is certainly plumbing from the adjacent bathroom nearby and ceiling below is finished so I’m not sure if there’s even space for blocking to be added.
  4. How large should the gap be between the plywood panels and the bottom of the wall framing? Is 1/8” sufficient or should it be 1/4”?
  5. Alternatively, would it be acceptable to use smaller panels of a 5/8” thick panel to only shim the two left (drain) side wooden feet, both glued and screwed to the OSB subfloor to level the bathtub? Should the feet also be glued to the smaller 5/8” panels serving as shims? I’m not sure how difficult or easy that would be given a mortar bed would still be used throughout the alcove along with plastic both above and below mortar - possibility that mortar could cover adhesive.
Option C Questions:
  1. Given the ~5/8” slope over the alcove, is it acceptable to install the bathtub without specifically doing anything to level the floor?
Tub specs:

https://images.thdstatic.com/catalog/pdfImages/56/5646c24d-8d3f-4f15-9720-da65d1301b75.pdf

Note:
  1. I do not have a basement or a crawl space - my home is a townhouse that sits directly on concrete and is an interior unit of a multi-unit row.
  2. I do not have access to the backside/underside of the bathtub given the master bathroom shower and vanity backs to the bathtub’s left (drain) side and back, respectively; and the right (high) side backs to the exterior wall which is common with neighbors. I mention all given there will be no chance to hand shovel in mortar or add adhesive after the fact.
3339DB2F-78B9-44A8-8999-F85B14199EA6.jpeg
 
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wwhitney

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Jeff H Young

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I set my tub level regardless of the floor if you're ok with floor out of level I'm ok with it. Thats from the plumbers stand point .
notch out where apron and feet sits so it will dig down below the floor a 1/2 inch on the high side the low side will be up an 1/8 inch above subfloor no problem use mortar in piles under tub
 

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notch out where apron and feet sits so it will dig down below the floor a 1/2 inch on the high side
If it's 5/8" subfloor (19/32"), you don't want to notch that 1/2" deep. If the foot's not over a joist, it could fall through.

Depending on which way the joists run, I guess you could cut out a largish piece of subfloor to get access to the floor framing, install blocking on and between the joists, and end up with a level, depressed subfloor for the feet and apron.

Cheers, Wayne
 

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I set my tub level regardless of the floor if you're ok with floor out of level I'm ok with it. That's from the plumbers stand point .
notch out where apron and feet sits so it will dig down below the floor a 1/2 inch on the high side the low side will be up an 1/8 inch above subfloor no problem use mortar in piles under tub
Although I appreciate the suggestion, I would not want for my subfloor to be notched to accommodate insertion of the apron. I have been advised that the build up of flooring - especially if 1/4” underlayment is used - should cover the gap if the entire floor is not level, so I prefer that route.
 
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Clarification Update: with a bathtub installed, the floor joists run parallel to the short sides of the bathtub or perpendicular to the long sides of the bathtub.
 

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In which case you could simply mark those areas on your subfloor and preshim them to provide 4 support regions that are level with each other. Along with a level ledger and shimming the front apron.
Assuming that the Elmbrook's wooden feet on the left (drain) side are on either side of the drain as is the Kohler Archer in the thread that you previously provided, there is no joist that will support the wooden feet. So with no option to screw the "preshim" to a joist, should there be any issues with plywood sticking to the OSB subfloor via construction adhesive? Similarly, are there issues that you are aware of with respect to OSB sufficiently holding screws? I ask both given I saw statements in other forums that it is difficult to both glue and screw plywood to OSB subflooring.
 

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Although I appreciate the suggestion, I would not want for my subfloor to be notched to accommodate insertion of the apron. I have been advised that the build up of flooring - especially if 1/4” underlayment is used - should cover the gap if the entire floor is not level, so I prefer that route.
I'd rather not notch into the subfloor as well its just more work in which case just set tub like we always do Level.
I put mortar in piles under tub and shims under feet screwed or glued in is fine the mortar is carrying it and where are the shims going? I run screws either through the flange on tub into studs or put screw in just above the flange with the head of screw kind of wedging in place to flange. Between the solid shims under feet of tub the mortar and the screws in flange it becomes rock solid
 

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You could shim the feet so the top rim of the tub was level, but without seeing the bottom of the tub, in general, you'll like the install better if you end up using some piles of mortar underneath to level it. The more of the bottom that is supported, the less flex there will be in the tub while you stand in it which also equates to longer life. The more support, the odds are you'll also hit a few joists underneath that marginal subfloor. Having the feet shimmed so things are level also would help when pressing the tub down into the mortar piles, as it would give you a solid stopping point, but the addition of a bunch more support points around the bottom of the tub.

There are various profiles you could use if necessary to cover the tile/tub joint, depending on the shape of the skirt. Note, the skirt on an acrylic tub isn't weight baring...the tub is supported by the bottom of the tub basin to the floor and via any ledgers used.
 

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You could shim the feet so the top rim of the tub was level, but without seeing the bottom of the tub, in general, you'll like the install better if you end up using some piles of mortar underneath to level it. The more of the bottom that is supported, the less flex there will be in the tub while you stand in it which also equates to longer life. The more support, the odds are you'll also hit a few joists underneath that marginal subfloor. Having the feet shimmed so things are level also would help when pressing the tub down into the mortar piles, as it would give you a solid stopping point, but the addition of a bunch more support points around the bottom of the tub.

There are various profiles you could use if necessary to cover the tile/tub joint, depending on the shape of the skirt. Note, the skirt on an acrylic tub isn't weight baring...the tub is supported by the bottom of the tub basin to the floor and via any ledgers used.
good explanation just a better way of doing it
 

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You could shim the feet so the top rim of the tub was level, but without seeing the bottom of the tub, in general, you'll like the install better if you end up using some piles of mortar underneath to level it. The more of the bottom that is supported, the less flex there will be in the tub while you stand in it which also equates to longer life. The more support, the odds are you'll also hit a few joists underneath that marginal subfloor. Having the feet shimmed so things are level also would help when pressing the tub down into the mortar piles, as it would give you a solid stopping point, but the addition of a bunch more support points around the bottom of the tub.

There are various profiles you could use if necessary to cover the tile/tub joint, depending on the shape of the skirt. Note, the skirt on an acrylic tub isn't weight baring...the tub is supported by the bottom of the tub basin to the floor and via any ledgers used.
I absolutely agree that the more support the better. I just hope that installers are willing.

I may actually cut the drywall open in the ceiling below myself and install blocking (if no plumbing or electrical in the way) to be on the safer side. The idea of the shims not being screwed to joists doesn’t sit well with me.
 
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I absolutely agree that the more support the better. I just hope that installers are willing.

I may actually cut the drywall open in the ceiling below myself and install blocking (if no plumbing or electrical in the way) to be on the safer side. The idea of the shims not being screwed to joists doesn’t sit well with me.
I'd tell them what you want because I never seen any screws going through shims. Wouldn't hurt unless the screws split your shims. I've used steel shims before too as some tubs have small feet on cast iron tubs but good size wood feet on fiberglass
 

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Id tell them what you want because I never seen any screws going through shims. Wouldnt hurt unless the screws split your shims. Ive used steel shims befor too as some tubs have small feet on cast iron tubs but good size wood feet on fiberglas
In this case when referring to shims I am referring to ripped segments of a 5/8” thick plywood panel for use beneath the drain side wooden feet. They would minimally be multiple inches by multiple inches - larger than the area of the wooden feet. They shouldn’t split.

Where can I order metal shims? I don’t think I’ve ever seen them in a big box store.
 

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You could shim the feet so the top rim of the tub was level, but without seeing the bottom of the tub, in general, you'll like the install better if you end up using some piles of mortar underneath to level it. The more of the bottom that is supported, the less flex there will be in the tub while you stand in it which also equates to longer life. The more support, the odds are you'll also hit a few joists underneath that marginal subfloor. Having the feet shimmed so things are level also would help when pressing the tub down into the mortar piles, as it would give you a solid stopping point, but the addition of a bunch more support points around the bottom of the tub.

There are various profiles you could use if necessary to cover the tile/tub joint, depending on the shape of the skirt. Note, the skirt on an acrylic tub isn't weight baring...the tub is supported by the bottom of the tub basin to the floor and via any ledgers used.
The alcove’s slope is ~5/8” from left to right up to ~30” of the alcove’s length. The bathtub is 60”x30”, so if a 30”x30”x5/8” plywood panel (with cutout to accommodate bathtub overflow pipe) is used one joist would be crossed and the apron would be supported for half its length by one panel. With the panel glued down and screwed into one joist, would simply screwing the leftmost end of the plywood panel to the OSB subfloor be sufficient or would you recommend that I still explore adding the blocking?
 

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It's hard to laminate large sheets without lots of clamps (screws in this situation), plus, screwing into 5/8" ply doesn't give much purchase for the screws and if you don't use something like a liquid wood glue. You can end up with voids in the bond like if you used construction adhesive you caulk out of a gun since you're not applying a lot per square inch. It works on top of a joist because the area is small. Also note that sometimes, the attached feet may not actually let the tub sit flat or introduce a little twist, so trying to make the area flat may or may not actually make the tub solid without twist or rocking. You don't want squeaks either from things moving, so more support than the four feet can be useful.
 

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It's hard to laminate large sheets without lots of clamps (screws in this situation), plus, screwing into 5/8" ply doesn't give much purchase for the screws and if you don't use something like a liquid wood glue. You can end up with voids in the bond like if you used construction adhesive you caulk out of a gun since you're not applying a lot per square inch. It works on top of a joist because the area is small.

So would you recommend scrapping the 5/8” thick shims if blocking cannot be added? I believe that’s what you may be conveying. So essentially, rather than shims and mortar only mortar if shims cannot be screwed into blocking?
Also note that sometimes, the attached feet may not actually let the tub sit flat or introduce a little twist, so trying to make the area flat may or may not actually make the tub solid without twist or rocking. You don't want squeaks either from things moving, so more support than the four feet can be useful.
I completely understand with respect to the feet. Even if my floor was level from left to right, an install without mortar is definitely not an option for me given the bathtub is acrylic.
 

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I don't see any problem using plywood shims screwed to 5/8" OSB. Just drill pilot holes and size the screws to extend 1/4" through the bottom of the OSB. Or if you prefer, make the shims large enough to span from joist to joist and screw them into the joists.

Cheers, Wayne
 

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Mortar is used all of the time with an acrylic tub. NOw, if your contractor doesn't want to, you either live with what you get, do it yourself, or get a different contractor.

When finished, you want the tub deck to sit level and at least the four feet firmly sitting on the subflooring along with any ledger board(s) called out in the installation instructions.

Many acrylic tubs either require or allow for mortar underneath in their instructions. Can you post a link to yours? Sometimes, putting a layer of roofing felt or plastic on the subfloor prior to the mortar helps, along with another layer on top of it. THat allows the cement in the mortar more time to cure without water being sucked out by the subflooring and air. Cement cures, it doesn't 'dry'. Curing is a chemical process where the water is literally incorporated into the cement- excess eventually evaporates. The only way to get it out after curing is extremely high heat as you might get in a kiln.
 

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Mortar is used all of the time with an acrylic tub. NOw, if your contractor doesn't want to, you either live with what you get, do it yourself, or get a different contractor.

When finished, you want the tub deck to sit level and at least the four feet firmly sitting on the subflooring along with any ledger board(s) called out in the installation instructions.

Many acrylic tubs either require or allow for mortar underneath in their instructions. Can you post a link to yours? Sometimes, putting a layer of roofing felt or plastic on the subfloor prior to the mortar helps, along with another layer on top of it. THat allows the cement in the mortar more time to cure without water being sucked out by the subflooring and air. Cement cures, it doesn't 'dry'. Curing is a chemical process where the water is literally incorporated into the cement- excess eventually evaporates. The only way to get it out after curing is extremely high heat as you might get in a kiln.
Kohler’s instructions for the Elmbrook states that mortar is optional but I will not accept a bathtub installed without it. I will also be insisting on use of both a ledger board and plastic both above and below mortar. Are you asking for the a link to the installation instructions or the spec sheet? Spec sheet is linked above.

Installation instructions:
 
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I don't see any problem using plywood shims screwed to 5/8" OSB. Just drill pilot holes and size the screws to extend 1/4" through the bottom of the OSB. Or if you prefer, make the shims large enough to span from joist to joist and screw them into the joists.

Cheers, Wayne
Unfortunately, I wouldn’t be able to size them from joist to joist provided from left to right, there is only one joist, and that joist is not left of the drain. Blocking would have to be added to enable shims to be screwed to framing and support the bathtub’s feet although I am not sure just yet whether there is room for blocking.
 

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