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!

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!!

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!!

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

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.

jayp

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.

jayp

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

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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Journeyman Plumber

nettnut

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?

Alan

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

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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.

TMIV

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?