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I think the floor was not prepped correctly. The tub creaks a lot and there is a great deal of vertical play when standing and moving around.

The tub does appear to be level, so I don't think that's the problem.

It doesn't seem to be such an issue when it's full of water and I'm sitting down- I suppose the weight is more evenly distributed.

   |w|  o  |
   |a|     |
 ->|l| tub |
   |l|_____|
   |_______|

I was thinking of:

  • Cutting a hole in the in the wall (where the little arrow is)
  • Pouring some self-leveling cement into the frame underneath the tub

It's a fiberglass tub and there is not a big gap between the bottom of the tub and the floor, so I don't really know if much cement will flow underneath the way I want.

Would a different product work better for this? Maybe expanding foam? or something else?

I can't remove the tub because there is tile lapped over it in a way I can't remove without damaging everything.

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What exactly is wrong with the tub? Is it not supported enough? Or is it not level? Does it move and you stand in it? –  auujay Jan 27 '12 at 19:36
    
Oops somebody "helpfully" changed my title. I'll update the question to have more information. –  willoller Jan 28 '12 at 0:15
    
If the tub wasn't set level in the first place, pouring leveling compound under it isn't going to fix it. Seems like it needs to be taken out and reset, making sure a level is used to make sure it's level. Also, put a big ol' blob of quickset on the floor then set the tub on top of it. If you hired someone to install it, looks like they need to come back out. –  oscilatingcretin Feb 9 '12 at 2:09
    
I agree, but I think the tub itself is level (at least a level on the top of the tub reads between the lines). –  willoller Feb 9 '12 at 5:33
    
Maybe the tub is level, but the floor is not. I assume it's possible that one of the corners of the tub are not touching the floor. When you get in it, your weight is enough to cause movement, but not enough to set the tub into place if it is supported by the wall. It really sounds like the tub needs to come out and the floor checked for levelness. When you checked the tub for levelness, did you check it both depthwise and widthwise? –  oscilatingcretin Feb 9 '12 at 12:57
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6 Answers

up vote 3 down vote accepted
+50

If you look at the underside of the tub (through your proposed hole) you should be able to tell if it was meant to sit on the subfloor or not. Some kind of wooden frame will be partly embedded into the fiberglass to help support it. If there are "skids" that should touch the subfloor but do not you can use shims to support them.

I had a shower enclosure that was not well supported underneath. It eventually cracked and leaked on the ceiling below. When I removed it, it was obvious that the enclosure was sitting on the floor in the front and hanging from the wall in the back. The reinforcements on the bottom of the enclosure were not touching the floor. I don't know if they were meant to or not.

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This is what I'll try - thanks! –  willoller Feb 11 '12 at 1:40
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Reading his issue, it sounds as though there wasn't a mortar bed laid down when the tub was installed so now when he stands in the shower he is afraid it will crack due to lack of support. I agree that a leveler would probably not be effective, and he should use a regular type of mortar mix, like quikcrete.

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Thanks - yes, no mortar was put down, much to my chagrin. –  willoller Nov 4 '12 at 5:32
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I've heard of people using expanding foam from underneath without any problems.

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Please explain how you would use it. –  Niall C. Oct 25 '13 at 13:51
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Another option to support the bottom is sand and gravel -- I wouldn't use self-leveling cement. (Dry mortar is one thing, it's mostly sand, but SLC is a different thing entirely.)

You're right that it needs to be supported somehow, though. Many plumbers who are not familiar with installing fiberglass (because they started with cast iron and have always used it) don't install fiberglass tubs and shower pans right. And sadly, many cheaper installers just don't care.

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WHAT I ENDED UP DOING:

For completeness: Both the floor and the tub are level. The tub has a honeycomb structure underneath that is supposed to be supported by quickset. There is a little "foot" at each of the 4 corners which lift the honeycomb about 1.5in off of the floor.

So, to me, this looks like an installation problem. However, I can't just have the plumber come back and re-do the work, since the tile is already installed over the top of the tub and this tile cannot be easily replaced (the dye lots are inconsistent and new tile won't match).

What I ended up doing was cutting a hole in the drywall just above the plate, about 4 inches high and 5 feet wide. Then, I used 2 kinds of yellow expanding foam: one for filling "large spaces" which expanded a lot. I sprayed this first as far back as I could get the nozzle under the tub until I had emptied 2 cans (one can got stuck and is forever entombed).

I sprayed a third and fourth can for "small spaces" which also had a faster cure time and smaller expansion. This went sort of in front of the other foam under the tub up to the inside of the wall.

After curing, I put the drywall back, taped, painted. Looks fine.

As for the tub: The squeak is totally gone. The shifting is gone. The floor of the tub is quiet and feels solid. I recommend this solution for anyone with this problem.

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Aaaand since nobody else seems to have mentioned it - I have always had this "bedding" solution for tubs explained as being supposed to be plaster. Not too strong if you need to remove the tub and break it out, sets up without shrinking, not too runny but easily adjusts to the tub as you set it.

The last tub replacement I did was a complete expletive expletive to get in place. I had intended to bed it "properly" by placing a pile of plaster on the floor, but after the misery of getting the new tub in for the test fit, I couldn't face taking it out and putting it back in to do that. I wasn't all that sure I could actually get it back in before the plaster would set, for one thing. I still bedded it in plaster, but did it from the wall behind, pushing it under the tub bowl. Worked perfectly.

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