# How to fix "flex" in the bottom of a bathtub



## iowa70 (Nov 30, 2009)

Hi,

I'm new to this forum and I've looked through and tried to find someone else with a similar problem but with no luck. I have an acrylic/fiberglass bathtub which the bottom of the tub flexes when I get in it. I'm worried that this flexing will cause a crack in the tub. I have styrofoam installed as a base underneath the tub. I can see this b/c I have opened up a piece of drywall in my closet so I have access to 2 sides of the bathtub (doing repairs to adjacent bathroom). I was wondering if I should take the styrofoam out and put mortar underneath the tub to have more of a solid base? Would this fix the problem? Any other suggestions other than putting mortar in? I've also thought about making a "box" out of 2x4's and a some plywood and shimming underneath the box to support the bottom of the tub. Thoughts?? Ideas??? Any help is greatly appreciated.

Thanks!


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## DangerMouse (Jul 17, 2008)

Hi and welcome to the forum.

if it were me, i'd do it with mortar since i had access to both sides!

DM


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## iowa70 (Nov 30, 2009)

That's the direction I was leaning also. I was wondering how that would work out though. Should I fill the tub full of water first so that it's weighted down when I put the motar in? Or since it's already nailed to the strips and there's tile around the sides of the tub I shouldn't worry about it? Also, will the mortar shrink when it dries as to cause a small gap b/t the mortar and the tub and thus possibly cause rubbing against the bottom of the tub when someone steps in it?

Thanks for you thoughts.


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## DangerMouse (Jul 17, 2008)

if you filled it with water first, when you emptied it, it'd rise up! THEN you'd have a problem, i'm thinking...

DM


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## DangerMouse (Jul 17, 2008)

oh, and the stuff they make for tubs shouldn't shrink.

DM


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## iowa70 (Nov 30, 2009)

Ok, so leave it empty and put the mortar in. You mentioned there's a certain type for tubs? What type is that specifically? Also, any other tips/tricks that you have would be helpful. Thanks!


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## Thurman (Feb 9, 2009)

O.K., one more time, and I don't mind helping, if it will. First-you have tackled the biggest problem: can you access any side of tub? You state that you can see under the tub from two (2) sides, and that's very good. As "DangerMouse" stated, yes there are special compounds for this purpose. I used to use un-sanded grout until I found out my local plumbing supply house carried this "special" mortar. There are a number of different ways to do this, all with the same result (hopefully). New tub applications are certainly different than your project, so I'll skip the new tub part. I have used two different methods in my business. IF you can easily access the bottom of the tub, which it sounds as if you can: do remove the styrofoam under the tub. Are there any wooden "slats" under the tub? IF so, they may be glued onto the bottom of the tub from the factory, I've seen this, leave them be if they are there. Get everything ready and in order before you start this task. You will have only one shot at doing it--Period! I like to mix this "settling compound" a little thinner than suggested as I will be pouring it into something, rather than as if a new tub installation. I like to use Heavy-Duty Contractors garbage bags for this. Place one bag into another IF the slats are not present, IF the slats are present, then place one bag between each slat, lying flat, and the opening towards you, as far out as possible. I do fill the tub one-half full of water to simulate someone sitting or standing in it, seems to work for me. When you have the bag(s) under the tub, and are ready with the mortar, start pouring the mortar into the bag(s) until they are full. Remember me saying "as far out as possible"? This is where you will use the neck of the bag as a funnel, when an amount is in the bag that looks as if it is full under the tub, push out all the air you can, twist and tie off the neck of the bag to prevent leakage. Prop the neck up if possible. I have also used "Great Foam" to do this. Buy, maybe four cans of GF, and some of the semi-transparent tubing that will just fit onto the spray nozzle of the GF, about three feet of tubing. Use the same procedure as for the mortar, but don't use mortar. Attach the tubing to the spray nozzle of the GF, maybe put some good ole' duct tape on it to hold it on better. Tape the tubing to a scrap piece of small wood, old broom stick, etc., so you can "aim" it better. Place the tubing into the bag under the tub, give it a really good shot of GF, and watch it swell. Great Foam has not (so far) swelled too much for me doing this. The excess comes out into the neck of the bag and can be cut off when done. I know this was a long post, but I hope this helps, David


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## DangerMouse (Jul 17, 2008)

see? that's why i like this place.... Thanks Dave!
and here i was just going to tell him to slop some Quikrete under there and call it a day! lol (J/K)

DM


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## Gary in WA (Mar 11, 2009)

Sounds good, Thurman! But if you remove the foam pieces from under the tub, then fill 1/2 with water = -200#, wouldn't that put a lot of stress on the fasteners/sides/bottom of the tub without the support there now?
Be safe, Gary


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## COLDIRON (Mar 15, 2009)

*Foam*

Buy a few cans of expanding foam fill the void under the tub with it let it expand and setup.

Once its sets up you won't have any give in it anymore. I did it with my shower pan it was the best fix ever.

Note: did you know they lift houses with foam to level them.

Very strong.


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## iowa70 (Nov 30, 2009)

Thurman said:


> O.K., one more time, and I don't mind helping, if it will. First-you have tackled the biggest problem: can you access any side of tub? You state that you can see under the tub from two (2) sides, and that's very good. As "DangerMouse" stated, yes there are special compounds for this purpose. I used to use un-sanded grout until I found out my local plumbing supply house carried this "special" mortar. There are a number of different ways to do this, all with the same result (hopefully). New tub applications are certainly different than your project, so I'll skip the new tub part. I have used two different methods in my business. IF you can easily access the bottom of the tub, which it sounds as if you can: do remove the styrofoam under the tub. Are there any wooden "slats" under the tub? IF so, they may be glued onto the bottom of the tub from the factory, I've seen this, leave them be if they are there. Get everything ready and in order before you start this task. You will have only one shot at doing it--Period! I like to mix this "settling compound" a little thinner than suggested as I will be pouring it into something, rather than as if a new tub installation. I like to use Heavy-Duty Contractors garbage bags for this. Place one bag into another IF the slats are not present, IF the slats are present, then place one bag between each slat, lying flat, and the opening towards you, as far out as possible. I do fill the tub one-half full of water to simulate someone sitting or standing in it, seems to work for me. When you have the bag(s) under the tub, and are ready with the mortar, start pouring the mortar into the bag(s) until they are full. Remember me saying "as far out as possible"? This is where you will use the neck of the bag as a funnel, when an amount is in the bag that looks as if it is full under the tub, push out all the air you can, twist and tie off the neck of the bag to prevent leakage. Prop the neck up if possible. I have also used "Great Foam" to do this. Buy, maybe four cans of GF, and some of the semi-transparent tubing that will just fit onto the spray nozzle of the GF, about three feet of tubing. Use the same procedure as for the mortar, but don't use mortar. Attach the tubing to the spray nozzle of the GF, maybe put some good ole' duct tape on it to hold it on better. Tape the tubing to a scrap piece of small wood, old broom stick, etc., so you can "aim" it better. Place the tubing into the bag under the tub, give it a really good shot of GF, and watch it swell. Great Foam has not (so far) swelled too much for me doing this. The excess comes out into the neck of the bag and can be cut off when done. I know this was a long post, but I hope this helps, David


Thanks Therman for the great advice. I think I'm going to try using the mortar. Also how close should I put the bag to the drain? Thanks again!


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## COLDIRON (Mar 15, 2009)

*Post #10*

Use post #10 it's the same procedure as #11 without all the complicated instructions. 

Use some common sense when doing it can't explain that on here.


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## shan1 (Jul 31, 2018)

Hi Thurman, I am also having this issue, I am trying to find that product great foam you recommended but am not finding that exactly, is made by a company Great Stuff? And if so where do you find and is there a certain version?


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## CaptTom (Dec 31, 2017)

Anyone have long-term experience with foam (only) under a tub?

Seems to me it would eventually compress, just like the Styrofoam the OP mentioned. But I've never tried it, just curious.


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## Nealtw (Jun 22, 2017)

In all the house we have built I have seen one installed with a mortar base the rest just have nothing and they do fine. And that is all the brand names and the cheap one from China.


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