Confused about mortar under tubs...

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jw123

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I just installed a kohler acrylic tub that suggested two alternate methods of support. You can either use a mortar bed or put construction adhesive under the "feet" (2x4's on end). For the life of me I can't see where these two methods are comparable, but this is manufacturer's instructions. They also don't recommend any additional framing, i.e. a sill support, although they do suggest screwing through the tub flange into the studs. I can't see any flex when I stand in the tub and the weight of water doesn't concern me since it's evenly distributed over the bottom of the tub, but a point load like a heavy person could cause a problem. That said, it seems to be a strong design with interior armrests and contour that make it more rigid, so maybe this explains their confidence level.

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William47

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motar after tub set?

tub has been installed. was going to foam under, but now want mortar. have access from end of tub. how is best way to add mortar after tub has been set into place permanently? this tub is installed perfectly level (right on the bubble all directions!) on a new level double 3/4" plywood floor and the install instructions said additional support was not necessary.
 
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Don Gwinn

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Here's what I'm not getting, and maybe I'm overthinking things. I have a Sterling Ensemble tub, which is built with small round "feet" all over underneath. There are probably 15-20 of these small pillars built into the bottom of the tub. I'm well over 300 pounds and I know I want to use the mortar bed (though the instructions call this an "optional" method.) What I'm not sure about is whether I need to make piles around the feet while making sure the feet have no mortar under them. I wouldn't think that would be necessary, but there's that one line that mentions "directly on the subfloor."

On the other hand, it also says each part that would contact the subfloor should "have mortar." Never done this before and I want to do it right.

Install a Mortar Cement Bed (Optional)
Remove the bath and pad from the alcove.
Verify that the bath is level.

Shim if necessary.

Studs

NOTE: Do not use plaster, gypsum cement, or drywall compound for this application, as they will not provide adequate structural support.
NOTE: The bath supports must rest directly on the subfloor.
Spread a 2″ (5.1 cm) layer of mortar cement on the subfloor, spreading it so that each area of the bath that will contact the subfloor has mortar cement.
Do not spread the mortar cement around the drain area. Allow room for the mortar cement to expand from the weight of the bath.
Immediately move the bath into position.
Verify that the bath is level and resting on all supports.
Clean the bath to reduce the risk of damaging the surface.
Place a clean drop cloth or other similar material into the bottom and over the
outside edge of the bath. Be careful not to scratch the surface of the unit.
 
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Sounds like Sterling wants those feet touching the subfloor. And have the 2" of space filled in with mortar as well for support.

When in doubt, do as the manufacture instructs.

I have had plumbers and installers physically get upset with me when I told them that sand, drywall mud and foam are unaccpetable bedding materials to set our tubs with. I get the usual angry responses of "I have been setting tubs for 20 years..."

The sad thing is a majority of manufacturers will void their structual warranties or sometimes the entire warranty of the tub because of no or improper support. :( not without good reason:

I have seen a plumber raise a whirlpool bath out of the deck 3 inches with foam.(too much)

I have seen someone put their foot thru a lasco tub/shower combo. (well the after effects)

And stress cracking on a Jacuzzi, that leaked from upstairs to downstairs and ruin $10,000 worth of an entertainment center below.

Just my $0.02
 

Jadnashua

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Trying to put mortar underneath there now will be nearly impossible, at least and expect full coverage. Mortar (when allowed by the manufacturer) does two things: fully supports the bottom and compensates for any slightly uneven floor allowing you to get it level.

If the tub doesn't have an apron, you could do a good approximation of full coverage, but it will be tough.
 

Terry

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Allow room for the mortar cement to expand from the weight of the bath.

This is why I use piles of mortar.
When you drop the tub down, it will push the piles of mortar down, but it has to have somewhere to go.

If you try to go for a full spread, you are likely to either have too little, or too much.

Allowing the weight of the tub to do the spreading works best for me.

If the tub is in place, you can just push some in there.
It doesn't have to be under every bit.

Setting tile is the same way.
There is a notched spreader, and then the tile is pushed into it.

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Master Plumber Mark

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moarter is easy to do

Here's what I'm not getting, and maybe I'm overthinking things. I have a Sterling Ensemble tub, which is built with small round "feet" all over underneath. There are probably 15-20 of these small pillars built into the bottom of the tub. I'm well over 300 pounds and I know I want to use the mortar bed (though the instructions call this an "optional" method.) What I'm not sure about is whether I need to make piles around the feet while making sure the feet have no mortar under them. I wouldn't think that would be necessary, but there's that one line that mentions "directly on the subfloor."

On the other hand, it also says each part that would contact the subfloor should "have mortar." Never done this before and I want to do it right.


Dont be intimated by this, its very easy..

in the last month I have had to installl 2 Sterling
48 inch shower bases.....and one whilrpool tub


I have found that a 60 lb bag of rather extra wet concrete (not soupy) works best spread out in a overall bed like fasion under the tub... you can figure where the edjes of the paltform are under the tub and end the bed as close to the end as possible...

when you set the base in the concrete it simply floats down to where is should be and the moarter fills in the gaps,
you might have to shake and shimmey the base to get it to set down, then you basically stand in the shower base and walk around getting the unit to sink in good...


I went back on one of them today and I was rather
impressed on how solid the bottom actually was....


it was like standing on a concrete floor with no give what-so - ever...
 
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Don Gwinn

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I'm going to try it on the piles. My concern was how to locate the piles so I wouldn't set any feet into them, but it sounds like these little pillars (maybe 3/8" across) will find their way down to the subfloor if one or two gets a little mortar underneath.

Thanks!
 

Jadnashua

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Unless you mix the mortar up really stiff, the tub should settle into the stuff and don't worry about the feet - the whole thing will settle down if you help it out like Mark indicated. And, if the floor isn't perfectly level, you don't want the foot on the ground - you want mortar under it so it does sit even. Some people then fill the tub with water to help ensure it doesn't float until the mortar sets.
 

Edwardh1

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amazing

2000 + people have read this topic. The word must not be getting out about what "mortar" to use-
lets see
-plastic on the floor
-plastic under the tub
- 1/2 to 2 in of space between the two plastics
- motor in piles
- but what exact brand mortar is best? something HD, lowes or a lumber yard carries?
- and what about bedding compound - what the heck is that if different from mortar

=========
Izzi tub people say on their web site to NOT USE CONCRETE as it shrinks, but to use wood shims
=========
MTI says that no concrete is needed if you have a foam base under the tub (this is an extra cost option)
=================
Jason says that mortar should be used if the floor is "not level"
+++++++++
None of them say anything about when you set the tub finally with no bedding compound, what the allowed gap betwen the finished deck and the underside of the tub rim is-
what should it be 1/8 inch or less??? I think toto says 1/8 inch
Then you caulk it????

==================
 
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HoracioO

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I'm going to throw my 1.5 cents in... only from what I've observed...

I have 50 year house.. it has a fiberglass neo shower that I recently tore out. Original gear so Fiberglass shower bases have been used for at least that long. This one had a poured plaster (lime not gypsum) sub floor to tub support that did not crack and had no reinforcing—chicken or otherwise.
A shower drain set in poured tar like substance that had been poured hot... the shower drain was of course caulked to the pipe with poured hot lead & jute... this system worked for nearly fifty years and did not fail due to cracks.
The shower system failed because of the bad shower wall to plaster/shower glass surround transition; the shower wall's one seam also leaked and the killer issue was that the wall system was chip-board (trade name) covered in a formica like material rotted from sitting on the shower base lip and sponging up the water.
This house also had an outside bathroom with an old metal enamel shower base that was set on a a concrete floor with just straight dirt from outside in the garden... needless to say the bathroom was non functional as someone probably failed to winterize... but the base was there until I demo'd it

Now... a quote from the install instructions for a Sterling shower base...

Install a Mortar or Cement Bed (optional)

Remove the receptor from the alcove. Remove the felt pad if present.
NOTE: The receptor supports must rest directly on the subfloor. Build a dam to keep the flooring material clear of the drain area.
NOTE: Do not use gypsum cement or drywall compound for this application, as they will not provide an acceptable, durable bond. Use cement or mortar.
NOTE: Do not spread cement or mortar around the receptor feet.
Spread a 2′′ (5.1 cm) layer of cement or mortar on the subfloor.
Remove the dam.
Position a piece of plastic drop cloth material on top of the cement or mortar bed.
Move the receptor into position.
Verify that the receptor is level and resting on all supports.
Clean the receptor to reduce the risk of surface damage.
Position a clean drop cloth or other similar material into the bottom and over the outside edge of the receptor. Be careful not to scratch the surface of the unit.​

Cheers
 

Gasser

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I've been reading through this thread and I am unclear on whether or not it applies to my partcular tub...

I'm installing a Maax Picadilly tub (air jet not whirlpool but similiar construction). It has 2 - 2x4's molded in that run the length of the tub. They are on edge so the tub is raised about 3.5 inches off the floor at it's lowest point.

The instructions...which are somewhat generic and not specifically for my model say that you can use mortar or shims to help level and secure the tub. I get the use of mortar in my case to help level but the discussions about it supporting the tub I am not clear on. Is what's being said in this thread that I should have a layer of mud 3.5" thick under my tub? I'm not disputing it...just seemed like a lot so I thought maybe the tubs you are talking about rest more or less directly on the ground with the feet only serving to support where the tub bottom curves.

Along the same lines...if I use mud for leveling (and possible support) how do I account for the thickness of the mud when building the frame around the edges of the tub?

Thanks!
 

BimmerRacer

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Whats the reason for plastic on top of the mortar bed? Also, can the mortar bed be contructed similar to a shower, i.e. tar paper, lath, mortar?
 

Jadnashua

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Some people prefer plastic on top of the mortar to keep it from sticking to the tub. Something on the subfloor to prevent it from wicking the moisture out is a good idea. Generally, you don't need lath as you don't have any point loads, the body of the tub spreads it out. You don't want to pack your mudbed down like in a shower...you leave it loose, then settle the tub down into it until it is at the right level and plumb. In fact, you can use mounds of the stuff so when you do smush it down, it has somewhere to go more easily - the mounds spread out and provide the required support.
 

Terry

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If you could drop a tub into place and the bottom of the tub perfectly touched the floor at four points, you wouldn't need shims or addtional support.
However, what a shame if you tiled up the box, and the tub sets up even 1/4" of an inch. Then what?
What do you do with the gap?

I like to plan for about 1" under the tub.
Then I put a few piles of mortar down, and let the tub squish the piles.

If you try to make a perfect bed for the tub, it will not spead and let the tub conform as needed.
You may wind up with a tub setting up 1/4" or more above the deck.

The piles allow for spreading.
It's the same reason that trowels for tile have notches.

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Gasser

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Thanks Terry that really helps! I am confused on 1 point and want to make sure I have this right...

This is an alcove installation with a tub supported by 2 - 2x4's running the length of the tub...I will use some 1x3's to create a level surface on 3 walls to support the edges of the tub about an inch higher than the tub. Then put mortar down and I'm assuming if I make two rows of motar to match the 2x4's on the tub this would still accomplish the whole squishing thing.

Next I set the tub in place so that the edges rest on the 1x3's around the edge....and this is where I'm confused. The edges are not meant to support the weight of the tub. If I push the tub in until it solidly rests on the edges will the mortar underneath set up and not shrink when it dries? If it does that would put some pressure on the edges to support the tub. Also, I've read where people recommend filling the tub with water to make sure it is pushed down good in the mortar. Wouldn't that put more pressure on the edges by pushing the tub deeper into the mortar?

Also, using the method you describe, as long as I build the frame perfectly level then I don't really need to worry about leveling the tub so to speak do I? If the tub is resting on the edge all the way around it will be level and the mortar will take care of any issues with the base/floor not being level correct?

Thanks for your help!
 

Jadnashua

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I'd forget that those stringers attached to the tub are even there and put my mounds of mortar spaced around the bottom of the thing. No need to try to match their position with the mortar. Then, assuming your ledger boards are indeed level, push the tub down so the lip touches all around. Note, the tub might be skewed a little, and could rack or twist or could be already. Unless you use too much water to mix the mortar, it shouldn't shrink as it cures.
 

Gasser

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Thanks! So basically the weight of the tub itself will be resting on the ledger boards correct? Then when I fill the tub with water that weight is taken on by the mortar since of course it won't flex.

This really helps... starting to make some sense now!!
 

Jadnashua

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The weight of the tub should be resting on the bottom - that would be the mortar (or the embedded stringers). The more you can spread that load out, the less likely you will have any flex in the tub. The fact the edges are touching the ledger boards is good and they are there to support the edges if, for example, someone were to sit on the edge, not to hold the tub up. There should be no weight on the edges at the ledger board...the weight should be on the bottom. Depending on the person stepping into the tub, you could have a point load of many hundreds of pounds on say the ball of one foot. Flex is not good for plastic materials...eventually, they fatigue and can crack or craze. The support supplied by the mortar spreads that load out and prevents those fatigue issues. It's worse on fiberglass with just a jell coat, but will happen with acrylic as well, but probably take longer to notice.
 
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