# Is a mud bed recommended for acrylic tub installation?



## jayp

I just bought a Kohler Bancroft acrylic bath tub and when I read the installation papers it said I could either set the tub into a mud bed or just glue the leveling blocks to the subfloor. A plumber I met at Home Depot said not to do the mud bed but I'm wondering if I should do it just to stabilize the thing? It's a tub with an apron made to sit in a 3-wall alcove. Thanks!


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## majakdragon

I always use a mortarbed for installation of anything but steel or cast iron tubs. They will "flex" which raises the possibility of cracking and can also pull the caulking loose around the tub edges. I just use a 5 gallon bucket of premixed mortar laid down to make sure the bottom of the tub fully contacts it.


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## Boston Plumber

Hi JayP:

If you can be sure the tub sits perfectly level on the floor, or if you can shim level, then glueing will work fine.

However, if like most remodel jobs....things are out of plumb/square....so I would set this unit in a bed of modified thinset mortar or on a bed of structolite gypsum plaster (make sure you have a plastic sheet (garbage bag) on floor before pouring these substrates).

I follow the FOOTPRINT of the tub base and then add 1-2 inches more than I think I need, then lower the tub into the substrate, level front to back and side to side, then I screw the unit into the studs.

Then I fill the tub 3/4 of the way for 24 hours or until substrate dries (simulates person getting in tub for bath...so less fluctuation/flex later).

I am a big fan of the structolite gypsum plaster...dries in 24 hours or less (NOT REGULAR PLASTER or joint compound here...can breed mold). It is available at most home supply stores or at a plaster supply house!!

Anyway...I would set it into some substrate...unit will make less noise when stepping into it, water will stay warmer longer (insulates tub base), and noise from showers will be reduced through the walls/ceiling!!

Good luck!!


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## Boston Plumber

Hey Majak...apparantly I take a very long time to type!! You weren't here a few minutes ago!! Nice to see we agree!!

And JayP...note that any of the bases mentioned work fine!! Good luck!!


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## majakdragon

Boston Plumber. Your comment about filling the tub with water while the mortarbed dries is confusing to me. If the tub flexes back to the original shape after the motarbed is dry and the water is removed, seems likely there would be a gap between the tub bottom and mortarbed. Thus defeating the purpose. Only time I fill a tub with water is while redoing a bad caulk job. This allows for the caulk stretching when someone gets in the tub after the caulk is cured. Hope this makes sense.


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## jayp

*Thanks guys*

I'm going with the mortar bed just to make sure. I have a few bags of Customfloat Bedding Mortar from Custom Building Products that I bought at home depot to bed my floor for tiling. I'm thinking I can just use a little of that for the tub bed. I've already put down a layer of roofing felt but I'm thinking since this mud bed doesn't really have to adhere to the tub, just create a "cradle" for it to sit in, I won't need to pull up the felt. Is it ok to just pour a little right on top of the felt and set the tub in? The installation sheet recommends putting at least a 2 mil liner on top of the wet mortar before setting the tub. I assume that's to prevent staining or having the hard rock right up against the acrylic.


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## jayp

*yes true*

Majak -- I see what you mean. I've heard of people doing it both ways but it's true that the tub could snap back thus negating the usefulness of the mortar bed. I'm reading the mortar bed installation instruction for this particular bath and it doesn't mention filling the tub.


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## jayp

*Here's a view of the project*

Here's a view of the project in question...


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## majakdragon

jayp, the only reason I can see for the plastic inbetween the mortar and tub is to prevent the two from sticking together in case you ever need to change it out. Plastic on the floor is just to prevent moisture from starting to rot the wood from the moisture in the mortar. If you feel the felt will prevent this, go for it. You actually want the "rock hard" surface to prevent any flexing later. The plastic won't hurt anything so go ahead and use it to avoid any warranty problems.


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## Alan

We only mortar drop in tubs. This way, they are not resting on the lip of the tub. If they have an apron and a screwing flange, we just secure it like a normal shower or tub/shower.

And NO, we never fill with water until the mortar is set. All of that water will push the tub through the mortar, and defeat the purpose of the mortar.


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## nettnut

*holding the motar in*

How do you keep the motar in place and not have it run out from under the tub when you lower it into the mud. Do you make some type of frame to hold it in?


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## Alan

nettnut said:


> How do you keep the motar in place and not have it run out from under the tub when you lower it into the mud. Do you make some type of frame to hold it in?


You just mix it a lot drier than you normally would....... :huh:


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## majakdragon

nettnut. The mortarmix is the thickness of peanut butter or thicker. You won't find it running anywhere. You just put plastic down to protect the wood under the tub and then pour enough mortar to about 1/2" higher than the bottom of the tub.


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## nettnut

Thanks for the info. I wasn't sure.


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## TMIV

majakdragon said:


> I just use a 5 gallon bucket of premixed mortar laid down to make sure the bottom of the tub fully contacts it.


I've read this a lot. "I use a 5 gallon bucket of premixed..." My question is how is 5 gallons enough material to fill the void in every case?

I've calculated the volume underneath my tub to be at least 7 gallons. Please see drawing. So you really can't apply the 5 gallon bucket to every job, right?


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## majakdragon

TMIV, you are not actually "filling every void". You are merely adding support to the bottom of the tub to prevent flexing. As long as you have a solid base across most of the bottom, it will do what it is supposed to.


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## buletbob

Boston Plumber said:


> Hi JayP:
> 
> If you can be sure the tub sits perfectly level on the floor, or if you can shim level, then glueing will work fine.
> 
> However, if like most remodel jobs....things are out of plumb/square....so I would set this unit in a bed of modified thinset mortar or on a bed of structolite gypsum plaster (make sure you have a plastic sheet (garbage bag) on floor before pouring these substrates).
> 
> I follow the FOOTPRINT of the tub base and then add 1-2 inches more than I think I need, then lower the tub into the substrate, level front to back and side to side, then I screw the unit into the studs.
> 
> Then I fill the tub 3/4 of the way for 24 hours or until substrate dries (simulates person getting in tub for bath...so less fluctuation/flex later).
> 
> I am a big fan of the structolite gypsum plaster...dries in 24 hours or less (NOT REGULAR PLASTER or joint compound here...can breed mold). It is available at most home supply stores or at a plaster supply house!!
> 
> Anyway...I would set it into some substrate...unit will make less noise when stepping into it, water will stay warmer longer (insulates tub base), and noise from showers will be reduced through the walls/ceiling!!
> 
> Good luck!!


 We use the structolite gypsum plaster all the time, with the same principle as stated above with excellent results, BOB


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## majakdragon

Sorry, but I have a problem with filling the tub (acrylic type) with water while the mud is drying. When you release the water, the tub will go back to the original shape and leave a gap between the mud and the tub bottom. This would just make an area where the tub would eventually crack since it would move each time someone stepped into it. The only time I add water is when I redo a bad caulk job around the edges.


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## jayp

*yes*

Yes -- exactly what he said. Never add water while the mortar is drying. You're defeating the purpose which is to not allow the tub to flex. As for the bed itself, you don't really need that much mortar. Trust me, if you add too much you'll find out how big of a pain it is to flatten it out underneath the tub. 

Just scoop it into a pile that is sort of long running from around the drain opening to the back where the tub starts to curve upward. Then put the tub in and rock it around till the tub is sitting flat. MAKE SURE you don't get any mortar underneath the feet if your tub has any sort of feet that are supposed to touch the floor. You'll want to adhere them with construction adhesive.


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## TMIV

The instructions to my tub say to "completely fill the space between the tub and the floor" and "Put eight to ten inches of water in the tub to hold it down." So I guess in some cases it is required. You can check out the instructions for yourself. Check section 4 - Setting the Tub


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## majakdragon

TMIV. Your instructions seem to be for a whirlpool tub. It also mentions supports on the bottom of the tub. Many tubs do not have these. Your tub company also describes two different types of installation. Drop-in, and 3 point with flanges. Your tub could be one of few exceptions if it is the drop in type, since there would be nothing to hold the tub in place while the mortar cures. The 3 point is secured to the walls through the flanges and supported by boards attached to the walls for keeping it level. This would negate the requirements for filling the tub with water. Since you would be voiding a warranty by not following the instructions, you need to do that. I was not thinking of a drop in tub when I replied to the post (my fault).


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## buletbob

majakdragon said:


> Sorry, but I have a problem with filling the tub (acrylic type) with water while the mud is drying. When you release the water, the tub will go back to the original shape and leave a gap between the mud and the tub bottom. This would just make an area where the tub would eventually crack since it would move each time someone stepped into it. The only time I add water is when I redo a bad caulk job around the edges.


Your correct, I should read through the post I quoted, I fill the tub when I'm caulking the tub. I stand to be corrected. Thanks guy's:thumbup:


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## SPIDERBRAD

Is the bath supposed to stick to the mortar, or just sit in it. I put a mortar base in, but within 24 hours the bath seperated - therefore has a hollow feel to it. Why not sandwicch expanding foam between mortar base and tub? ( my plumber insists that the bath doesn't need to stick to the base)


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## wrangler

It should not have separated unless weight was added to it before it was allowed to set. When we set tubs/shower pans, we use 20-45 minute gypsum, level the pan/tub and screw it into the studs and allow it to set up before walking on/in it. This is important if not to ensure that the tub or pan is level for doors and for the very problem that you stated... if you walk on it before setting, it will push out the underlayment and create cavities. Expansion foam can create uneven spots or raise the tub/pan out of level and cause even harder problems to deal with later(such as leveling doors).
Acrylic tubs should be test fit first, with the ledger board installed, and plumbing verified. If satisified, then pour the bed, set the tub, level, screw off and allow to set for the curing time. Same for acrylic pans, less the ledger board. Then finish drain connections (screwing compression fitting for pans, over-flows and shoe for tubs) and work your way on up the walls. This varies a bit if doing whirlpools or jetted tubs as they often require the electrical work first, and added carpentry to set the tub into. 
If working with code inspectors, some of this requires step-by-step inspections but, after repeated jobs, some inspectors will let you get by with detailed pictures of each step of your remodel as long as there is still access to wiring and plumbing connections (in some jurisdictions). 
But I digress... your bed was not, nor should not have acted as an adhesive, but more of an obstruction to downward movement of your tub. You could leave it as is and hope that it lasts as long as your use requires, pull it up and redo it, try to add more bedding or even 'expansion foam' as you suggested, but personally, unless it was moving alot or is a really cheap tub, I think I would leave it as is.


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