# Mortar bed depth for acrylic tub



## WJKovacks (Jan 20, 2011)

Hoping someone can help me out on an acrylic tub install I'm doing. The issue is pretty simple, I've got an acrylic soaker tub (no jets or anything) that requires a mortar base for added support. There is a sheet of 1/2 chip board under the bottom of the tub with wooden feet at the back, but that's obviously not enough to stop potential sagging and then cracking. 

So, I need to put in a mortar bed but I have a 3" gap between the floor and the chip board. That's a lot of mortar and a lot of weight. Am I able to make a wooden base (2x4 and ply) or something similar to drop the height down to 2" so that I only need an 1" of mortar? I've read about using foam board and then mortar but I'm concerned the foam might compress.

Any advice would be greatly appreciated. Thanks.


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## Bondo (Dec 8, 2007)

> I've read about using foam board and then mortar but I'm concerned the foam might compress.


Ayuh,... Use the blue or pink stuff used under concrete slabs,...

It won't compress...


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## Jim F (Mar 4, 2010)

Does the chip board with feet belong on the bottom of the tub during the install or is that packing material? A picture would be of help here. They typically have small adjustable plastic feet or some sort of plastic grid on the bottom.


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## WJKovacks (Jan 20, 2011)

The chip board is cut to fit the shape of the underside of the tub and molded into the fiberglass - it's support not packing material and the feet are 3" square wooden blocks at the edge. It is support. I'll try to get a picture up soon.

I'm just glad to know I can build a base first and then add the mortar bed. Now a mortar question - what kind of mortar is best? I'm thinking of using a dry mix like for a shower stall base. Does that make sense?


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## Jim F (Mar 4, 2010)

I wonder what would happen if you took the feet off and just set the chip board piece level in the morter bed. I've heard of thin set morter being used and thick set. I don't know which is better. Here is a recent thread that talks about the morter http://www.diychatroom.com/f7/sterling-kohler-vikrell-bathtub-60475/ .


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## WJKovacks (Jan 20, 2011)

Since I can use the rigid foam and I've got 3" to play with I was wondering if I should make a foam sandwich?

Layer of rigid foam, mortar, then foam. I still get the leveling and support but now I have an insulation against the bottom of my tub to help keep the water warm.

Does that sound sensible or am I talking out of my....?

Thanks.


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## Bondo (Dec 8, 2007)

> Layer of rigid foam, mortar, then foam. I still get the leveling and support but now I have an insulation against the bottom of my tub to help keep the water warm.


Ayuh,... If you were to do That, you might as well skip the mortar altogether...

The Idea of the mortar is it conforms to the pitch, 'n irregularities of the bottom of the tub...


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## WJKovacks (Jan 20, 2011)

Right, I guess I'm thinking of the mortar as compensating for any irregularities. Let's assume I do have to use the mortar still. My idea makes some sense?

And I'm still trying to figure out if a dry mix mortar is the best to use. Any recommendations?

Thanks.


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## Homerepairguy (Aug 1, 2010)

WJKovacks said:


> So, I need to put in a mortar bed but I have a 3" gap between the floor and the chip board. That's a lot of mortar and a lot of weight. Am I able to make a wooden base (2x4 and ply) or something similar to drop the height down to 2" so that I only need an 1" of mortar? I've read about using foam board and then mortar but I'm concerned the foam might compress.


Considering the weight of the water in a filled tub, your floor MUST be capable of supporting that amount of weight. That being the case, I wouldn't worry about the additional weight of the mortar bed.

But to minimize the amount of mortar used if that's still a concern, I would just lay two 3/4" sheets of plywood on the floor, cover the top piece with black roofing paper to avoid absorbtion of moisture, then use mortar on top of that. Cut a large, heavy duty garbage bag open and lay that on top of the mortar before you press the tub down on it. This will prevent the mortar from sticking to the bottom of the tub and give you a second chance in case things don't go well on try #1.

HRG


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## WJKovacks (Jan 20, 2011)

That's way I've decided to go.

I've got a 3" gap to fill so I'll make a base of ripped down 2x4 and ply to give me a 2" base (supported through the middle as well of course). I'll level and screw that into the subfloor and then I'll put down some 6 mil poly I have on top of the base then add a fairly thick mortar of unmodified thinset. Then more poly on top of the thinset and then the tub. 

That will give me a solid base that won't require 3 inches of mortar.

As for overall support, there's a wall below that run directly under the tub and has bracing in place to transfer the weight straight done.

Think I've got it covered?


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## Homerepairguy (Aug 1, 2010)

WJKovacks said:


> That's way I've decided to go.
> 
> I've got a 3" gap to fill so I'll make a base of ripped down 2x4 and ply to give me a 2" base (supported through the middle as well of course). I'll level and screw that into the subfloor and then I'll put down some 6 mil poly I have on top of the base then add a fairly thick mortar of unmodified thinset. Then more poly on top of the thinset and then the tub.
> 
> ...


I'm not a plumber but sounds good to me. Not sure about using thinset verses a Quickrete ready made sand mix. I have to decide what type of mortar to use to set our new tub in a few weeks.
HRG


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## Jim F (Mar 4, 2010)

Can I ask what brand of tub this is? I'm still not picturing the base. That way I can look it up online.


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## COLDIRON (Mar 15, 2009)

Use non-shrinkable grout. And make sure you either plug the drain and fill with water or add the weight of an average person to the tub as the grout is drying that way it is set properly.


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## WJKovacks (Jan 20, 2011)

Thanks for all the advice, I put the tub in last night and everything seems good. Well supported and level. If you want to see details, I've been blogging my reno strictly for friends and family but this post shows all the tub setting stuff that I did:

http://squeakysbathroomreno.blogspot.com/2011/01/light-at-end-of-tunnel-and-i-dont-think.html


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## Homerepairguy (Aug 1, 2010)

WJKovacks said:


> Thanks for all the advice, I put the tub in last night and everything seems good. Well supported and level. If you want to see details, I've been blogging my reno strictly for friends and family but this post shows all the tub setting stuff that I did:
> 
> http://squeakysbathroomreno.blogspot.com/2011/01/light-at-end-of-tunnel-and-i-dont-think.html


Nice job. Did you lay some plastic on top of the mortar before installing the tub to isolate the mortar from the tub?

I think your explanation of why you didn't fill the tub with water sounds valid. Also, it gives some leeway for any shrinkage of the mortar as it dries.

HRG


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## WJKovacks (Jan 20, 2011)

Thanks, I attached poly to the underside of the chipboard on the tub so it wouldn't stick - just confirm and give support. Essentially I have a sandwich of poly on the subfloor, mortar on the wooden support bed and then poly on the underside of the tub. Just finished my leak test and all good there too so I'm pretty happy. If only I didn't have so much more to do


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