Advice for leveling alcove subfloor

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jadnashua

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The tub doesn't need to be bonded to the floor. You can use some shims on the existing feet to get the slope you want along with some construction adhesive, and then use the piles of mortar for final fine tuning. Do not try to make a flat bed of mortar, as you won't be able to get enough pressure to then level the tub (thus, the piles which will cause more pressure on a smaller area that will collapse as you press the tub down). The tub will feel more solid and last longer if you bed it into piles of mortar. Otherwise, over time, you'll end up with stress cracks, and it can fail. That will likely be outside of the warranty, so the company won't care, but in the meantime, as it cracks (crazing - small cracks), crud will get in those, discoloring things, and eventually, it gives up and fails. Happens faster depending on how heavy the people are that use the tub.

It's your house...use the advice or ignore it.
 

Jeff H Young

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The tub doesn't need to be bonded to the floor. You can use some shims on the existing feet to get the slope you want along with some construction adhesive, and then use the piles of mortar for final fine tuning. Do not try to make a flat bed of mortar, as you won't be able to get enough pressure to then level the tub (thus, the piles which will cause more pressure on a smaller area that will collapse as you press the tub down). The tub will feel more solid and last longer if you bed it into piles of mortar. Otherwise, over time, you'll end up with stress cracks, and it can fail. That will likely be outside of the warranty, so the company won't care, but in the meantime, as it cracks (crazing - small cracks), crud will get in those, discoloring things, and eventually, it gives up and fails. Happens faster depending on how heavy the people are that use the tub.

It's your house...use the advice or ignore it.
a good quality of tub might not require the mortar but I just think it a real good install . and always put the mortar I've seen plaster of some sort used as well
 

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a good quality of tub might not require the mortar but I just think it a real good install . and always put the mortar I've seen plaster of some sort used as well
I agree. Mortar is a must. I’ve had some plumbers push back against its use so I didn’t even consider their quotes. Some even questioned why I would want a ledger board.

I certainly appreciate everyone’s advice and explicit details to help with installation!
 

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The tub doesn't need to be bonded to the floor. You can use some shims on the existing feet to get the slope you want along with some construction adhesive, and then use the piles of mortar for final fine tuning. Do not try to make a flat bed of mortar, as you won't be able to get enough pressure to then level the tub (thus, the piles which will cause more pressure on a smaller area that will collapse as you press the tub down). The tub will feel more solid and last longer if you bed it into piles of mortar. Otherwise, over time, you'll end up with stress cracks, and it can fail. That will likely be outside of the warranty, so the company won't care, but in the meantime, as it cracks (crazing - small cracks), crud will get in those, discoloring things, and eventually, it gives up and fails. Happens faster depending on how heavy the people are that use the tub.

It's your house...use the advice or ignore it.
I understand that the bathtub does not have to be bonded to the floor. I’m more concerned with keeping the 5/8” shims in place when the bathtub flexes given there won’t be easy access to adjust them after installation. It has been suggested in this thread and on other forums that plywood doesn’t hold well to OSB via construction adhesive or screws. This is why I’m considering adding blocking - well that and also for general support given the distance between the two joists closest to the drain. I simply will not know what can be done for sure until the ceiling below is opened up.

I completely understand and agree with respect to use of mortar. My sincerest thanks for all of your feedback!
 

jadnashua

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I would think that you could use some brads or screws to hold the shims to the existing feet, with some construction adhesive. Once the adhesive has cured, those fasteners become mostly redundant. You might need to put it in/out a few times. But, if you use mortar, and got the shims close (perfect not needed), the mounds of mortar would be holding the tub up, and the feet would become not as important. Or, maybe a slow-setting epoxy so you could get the shims fine-tuned, but then leave it alone for the epoxy to set. The 5-minute stuff probably wouldn't work, but there are alternatives that aren't as speedy.

Not all OSB is created equal, but wide spread construction adhesive on a flat panel tends to leave gaps between the beads. If you did want to laminate the sheets, you'd want to use a good liquid wood glue, spread like thinset, then lots of clamps (screws) to hold it down. Some OSB has a wax coating, and things don't bond to it well. Advantec uses a plastic resin, and glue will bond to it.
 

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Differant tubs call for slight changes in installation On Fiberglass I normaly omit the ledger if there are 4 feet on the bottom also on cast iron builder tubs I almost never put one. pressed steel builder tubs always have a ledger.
 

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I just received what I thought would be my last estimate...and I’m a bit confused. I was told again that self-leveling would be used to level the alcove (not surprising) but this time I was told that cement board with a waterproofing membrane would be used in the alcove to stiffen the floor before self-leveling is poured over it. That seems like a lot of weight...thoughts?

Is it common to use cement board under self-leveling? I was also under the impression that cement board provides no stiffening or strengthening, that it is simply an underlayment that holds up to water (although not waterproof alone). I’ve also not heard of cement board being used underneath a bathtub.

I was also told that they will not limit leveling to the alcove, that if I want them to install the bathtub and plumbing, I will have to agree for them to level the entire floor despite expressing that I do not prefer the 1” plus step up - tile is 0.4” - up that will result at the door. This obviously means that the bathroom door will have to be cut down. So essentially, an unexpected scope increase to get the bathtub installed!

This home improvement contractor was insistent that if the slope remains the tiled floor will look bad. Do you all believe that a slope will be very noticeable - keeping in-mind that the slope is gradual for about ~ 30” until it reaches ~ 5/8” and then the rest of the alcove and bathroom floor levels out from right to left? It remains consistent into the hallway. I point you all back to one of my attached pictures to gauge the slope/gap that tile should cover given a 1/4” tile underlayment will most likely be used.

If either of you have pictures of an alcove tub butted to a sloping tiled floor, please attach or post a link. I’m curious how noticeable the slope is after all is installed and how much out of level the tub(s) were relative to their high side(s).
 

jadnashua

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Many SLC say they cannot be used over cement board...regardless, nearly all of them require a primer first. Many of them over a wooden subfloor also require metal lath. One the doesn't require lath is Ardex Liquid backerboard https://www.ardexamericas.com/produ...ard/?msclkid=dd3824e4c66211ec9f4c638d5650237a

There are ways to level the bathroom without using SLC, but it involves tearing out the existing subflooring and playing with the joists. Depending on what you have, you may be able to (slightly) plane the high spots, and put a cleat on the sides of the low areas to achieve all of the surface being flat, then a new plywood subfloor.

The exact placement of the feet on many tubs means that even on a perfectly flat/level floor, the tub may not sit tight on all feet. That's why the instructions say you can use mortar underneath...that lets you fine-tune the exact height and level of the tub rim which is usually the best way to achieve the desired result. That mortar is also LOTS less expensive in the materials needed for the primer and SLC, if you go that route.

Putting down SLC is more involved that some think. 'Self' leveling is a misnomer...it rarely gets flat without some expert help, especially if the pour is thin...and, it runs everywhere, so keeping it out of the drain area or through cracks, or under walls can become a major problem if not properly addressed. Piles of mortar are a lot more forgiving.
 

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Many SLC say they cannot be used over cement board...regardless, nearly all of them require a primer first. Many of them over a wooden subfloor also require metal lath. One the doesn't require lath is Ardex Liquid backerboard https://www.ardexamericas.com/produ...ard/?msclkid=dd3824e4c66211ec9f4c638d5650237a

There are ways to level the bathroom without using SLC, but it involves tearing out the existing subflooring and playing with the joists. Depending on what you have, you may be able to (slightly) plane the high spots, and put a cleat on the sides of the low areas to achieve all of the surface being flat, then a new plywood subfloor.

The exact placement of the feet on many tubs means that even on a perfectly flat/level floor, the tub may not sit tight on all feet. That's why the instructions say you can use mortar underneath...that lets you fine-tune the exact height and level of the tub rim which is usually the best way to achieve the desired result. That mortar is also LOTS less expensive in the materials needed for the primer and SLC, if you go that route.

Putting down SLC is more involved that some think. 'Self' leveling is a misnomer...it rarely gets flat without some expert help, especially if the pour is thin...and, it runs everywhere, so keeping it out of the drain area or through cracks, or under walls can become a major problem if not properly addressed. Piles of mortar are a lot more forgiving.

Yeah, I’ve watched videos and read thoroughly about SLC. It definitely does not seem simple with all of the prep and effort to get “slush” to cooperate. If my bathroom were on ground level, I would likely be all for it given the concrete slab. But I’m not at all inclined to allow cement board and self-leveling to be used underneath my bathtub. I’m concerned with the significant amount weight especially given my floor is 19/32”, it’s on a third floor, and I’ve been assured that my bathtub installation is simple. What puzzles me with repsect to SLC, is how to keep it from rolling downhill on a sloped floor. A floor that has some valleys, however, seems straight forward.
 

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Do you believe that a slope will be very noticeable if I do not have the subfloor outside the alcove leveled with the alcove? Can you point me to pictures of a finished slope floor against a bathtub where tile covers the gap below the bathtub?
 

jadnashua

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While there are numerous brands of SLC, there are basically two types (speed of setting, compressive strength, and maximum depth are other differentiator): 'normal' stuff, and those referred to a thixotropic. A thixotropic material flows when aggitated, and stays when you stop, so, if you were to use one of those, you could build a slope that wasn't level.

A 'normal' one, other than the meniscus (think a bead of water on a waxed car, or a pancake), does level itself some, but you have to break the surface tension first. Fail to stop a crack in the floor or prevent it from going under the walls or out the door or down into the ceiling below.

A modern home is typically built to handle things like a bathtub, and a combined live:dead load of 50#/sqft. If you're thinking about tiling things, you should evaluate the joists. My go to place for that is www.johnbridge.com

You might notice it, but it may not be an issue. Functionally, though, the tub must be level and the toilet as well. You'd want to level the cabinet if you have a vanity rather than a wall-hung one.
 

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While there are numerous brands of SLC, there are basically two types (speed of setting, compressive strength, and maximum depth are other differentiator): 'normal' stuff, and those referred to a thixotropic. A thixotropic material flows when aggitated, and stays when you stop, so, if you were to use one of those, you could build a slope that wasn't level.

A 'normal' one, other than the meniscus (think a bead of water on a waxed car, or a pancake), does level itself some, but you have to break the surface tension first. Fail to stop a crack in the floor or prevent it from going under the walls or out the door or down into the ceiling below.

A modern home is typically built to handle things like a bathtub, and a combined live:dead load of 50#/sqft. If you're thinking about tiling things, you should evaluate the joists. My go to place for that is www.johnbridge.com

You might notice it, but it may not be an issue. Functionally, though, the tub must be level and the toilet as well. You'd want to level the cabinet if you have a vanity rather than a wall-hung one.
Someone turned me on to John Bridge last year. My deflection should be about L / 615, good for ceramic. So hopefully my chosen porcelain floor will be fine. Thanks again for your insight!
 
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jadnashua

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Note that joist strength is dependent on the length of the joist between supported ends, and is not the size of the room involved. A support could be a rim joist, a beam, or a load-baring wall. It is not the dimensions of the room.

It can happen, but it's rare that a house would be built to more than the code minimum of L/360.
 

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Note that joist strength is dependent on the length of the joist between supported ends, and is not the size of the room involved. A support could be a rim joist, a beam, or a load-baring wall. It is not the dimensions of the room.

It can happen, but it's rare that a house would be built to more than the code minimum of L/360.
I understand. The joist span below this particular bathroom is ~10 feet with two beams supporting them.
 
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