Mortar Bed for Bathtub


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Old 09-16-10, 03:00 PM
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Mortar Bed for Bathtub

I'm about to set an acrylic tub in my master bathroom. Looking for some advice, suggestions, tips, etc. with regards to creating a mortar base (e.g., what kind of mortar to use, how much, do I place a plastic sheet between the bottom of the tub and the mortar, etc.). FYI, already poured self-leveling mortar on the wooden subfloor, which didn't level out for some reason so the floor is unlevel about 3/4" on the left side. Just wanted to mention this because I'm assuming I'll have to use a bit more mortar.
 
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Old 09-16-10, 04:36 PM
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All I can suggest is follow manufacturers directions. But I am not a fan of plastic tubs/showers, unless the base is a solid reinforced unit that requires no prep base.
 
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Old 09-16-10, 05:00 PM
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Manufacturer doesn't require a prep'd base, but they do recommend that the tub lie flat on the subfloor, whether it be wood or concrete. My subfloor is un-level, so I am going to lay it in a mortar base because shimming is not an option for obvious reasons.
 
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Old 09-16-10, 05:16 PM
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Originally Posted by mossman View Post
I'm about to set an acrylic tub in my master bathroom. Looking for some advice, suggestions, tips, etc. with regards to creating a mortar base (e.g., what kind of mortar to use, how much, do I place a plastic sheet between the bottom of the tub and the mortar, etc.). FYI, already poured self-leveling mortar on the wooden subfloor, which didn't level out for some reason so the floor is unlevel about 3/4" on the left side. Just wanted to mention this because I'm assuming I'll have to use a bit more mortar.
Levelling the wooden subfloor before you pour the mortar base under the tub is very important. Do this 1st and assure the tub is level and don’t use the mortar intended for underneath the tub to level the subfloor.

You don’t say what size is your tub but in the past I used for a 60” long tub one bag of dry pre mixed cement / sand. Measure what is the height from the bottom of the tub to the floor, add some water to the mortar mix and try to get a “mountain” in the center of the tub lengthwise and about 12” from either end. You don’t want the mortar to be too liquidy, better to be kind of dry. Then put a plastic sheet over the mortar and set the tub over it. Push the tub down to squeeze the mortar and assure that the tub sits properly. If one bag of cement / sand is not enough get 2 bags.

Acrylic tubs are not very strong but if properly installed will last forever. The center of the tub can take a lot of abuse when you walk on it and by installing the mortar under it you kind of reinforcing that area.
 
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Old 09-16-10, 07:21 PM
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Thanks for the advice. It is a 60" tub and leveling the subfloor (again) is not an option, as that would elevate the tub too much. Exactly what purpose does the plastic serve? Is it simply to prevent the tub from sticking to the mortar? What thickness do I need? I'm confident I can pull this off. My only concern is pushing the tub too far down at which point I will be screwed. Definitely not going to get quick setting mortar. I want it to dry as slow as possible.
 
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Old 09-17-10, 03:20 PM
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I would suggest levelling the subfloor or you are asking for trouble down the line. I would even remove what you have done and start over again to have the floor level

I have used all kinds of plastic on top of the mortar. I guess it prevents the tub from sticking to the mortar (that is only my opinion)
 
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Old 09-17-10, 04:00 PM
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What kind of "trouble" are you referring to? The tub I just tore out has been in there for 23 years with the floor unlevel and I didn't see any problems. After all, the mortar bed is going to conform to the tub and subfloor regardless if the underlying subfloor is out of level. That's like saying you can't use shims under a kitchen cabinet. Please clarify.
 
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Old 09-17-10, 04:32 PM
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If they don't require a bed, don't do one. But use leveling cement to level the subfloor. It only needs to be the thickness of the out-of-level, I assume less than 3/4"??? Leveling cement will flow as needed to level, but it will find all holes, even very small ones, plug or dam all holes. And it dries fairly fast, <24 hrs.

What you are describing is 'flat', not necessarily level. Level for a tub is needed to make the walls correct.
 
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Old 09-17-10, 05:31 PM
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I tried the self-leveling mortar and it didn't work for god knows what reason. It only added to the height of the subfloor. At least its flat (not level). Therefore, I will be adding a mortar base, which the manufacturer DOES call for. This is the plan:

Add 2" of mortar in a pile spanning down the center to about 12" within each end, placing the tub on top, pushing it down until level, anchoring it to wall with clips. Hopefully everything will go smoothly. Only thing I'm worried about is pushing it down too far and having to take it back out. I'm going to do everything I can to avoid that.
 

Last edited by mossman; 09-17-10 at 05:55 PM.
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Old 09-18-10, 10:41 AM
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One of the troubles down the line I may see is that the old tub probably was steel or CI and the new one is acrylic. If you secure to the studs the acrylic tub and the floor is not level. It may slide and break the flange by the studs.

The mortar under the tub is NOT to secure the tub in place but rather to provide strength on the long axis of the tub.
 
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Old 09-18-10, 11:42 AM
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Mossman - The easiest way to fix this is to level the floor under the tub with strips of scrap plywood of differing thickness. Make the strips a bit shorter than the skirt. It just takes a few minutes and will do the job of supporting the tub.

Once that is done you can put a mortar bed on top of the plywood to bed the tub. I used plastic VB between the mortar and the plywood subfloor. My understanding is that the plastic keeps the wood from stealing moisture from the mortar before it has a chance to cure.
 
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Old 09-18-10, 01:35 PM
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To my opinion, no seams are allowed under the skirt of a tub. In addition, the top of the mortar should be covered with 1/16” polyethylene sheet or any other plastic sheet. You don’t want to glue the bottom of the tub to the mortar.

If you don’t have installation instruction for your tub go to maax.com , select the tub of your choice and then you will see a download section to get the installation instructions for your tub (20 page pdf file). In there you will see all the required info in details and photos.
 
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Old 09-20-10, 12:22 PM
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Tub is designed to rest directly on subfloor and laying it in a mortar bed is optional and recommended, so that's what I did. The installation instructions didn't mention anything about leveling the floor first or placing a plastic sheet between the tub and mortar. Seems pretty darn solid to me. I'm sure it will be fine.
 
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Old 01-16-15, 01:55 PM
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Hi mossman,

Hoping you can help me with some advice.

I am in exact same situation as you were in - can't level the floor with self leveller, can't add more height, and the floor is not level.

I have placed perfectly level lip support (this is alcove install), and the tub sits nicely on the lip supports and it's level, but it's between 1/5" and 3/8" above the floor. I placed 1x2 around the tub feet to keep the mortar in once the tub pushes into it. I also put double layer poly on the floor.

I plan to position the tub, then lift one side and fill the mortar. I will then cover it with another sheet of poly, and drop the tub in, pushing it so that it sits on lip supports.

Mortar I'm using is some 1 hour set construction mortar, and I'll add set mix to make it set in more time, and also another mix that enhances the compression strength. I plan to mix it so that it's thick as dough.

Two questions:

1. Do I need the poly between the subfloor and the mortar? This is not a polymer modified mortar, so it will probably not stick to plywood very well.

If I leave the poly under it, will it cause any issues? It will be held in place (from sliding) by the "borders" I made out of 1x2s.

2. How is your installation holding up 5 years later?
 
 

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