Unstable tub floor

[Serg]

I just bought a new house and have found that, apparently, the tub was not very well installed. I smelled something strange in the closet that is on the backside of the faucets/drain and, when I opened up the access door, discovered a build-up of mold. I have pinpointed the problem to be instability of the tub, which was causing cracks in the caulking and letting water through. For the time being, I have recaulked with silicone caulk, but I would like to fix the instability.

The tub is for a three-wall enclosure. With the little access I have, I have shimmed under the the side of the tub with no wall (i.e. between the foundation and the apron). This has given better stability to the overall structure; however, the floor of the tub is still unstable. When I step into the tub, the floor of the tub gives a little, which creates instability. From reading other posts, I understand that the installer should have put mortar under the tub to prevent this type of instability. He didn't. Given that the tub is fully installed now and I have very little access, is there anything that I can do to support the floor without completely pulling out the tub and starting over (and, ideally, without cutting a lot of sheetrock around the tub)?

[tbplumbloco]

You can try some foam insulation,comes in a spray can,you must fill the tub with water because the insulation expands as it dries.

[Serg]

Do you know where I buy that stuff? What does filling the tub with water do?

[tbplumbloco]

Foam insulation is called "great stuff",the big box store,the stuff expands filling the tub with water will weigh the tub down,I know from experience if you do not fill the tub w/water the tub could move because of the foam insulation will expand and move the tub.

[jadnashua]

They make at least a couple of different configurations of the foam. One remains flexible, the other is brittle. The flexible one is also low-expansion and denser. I'd go with that version. Can't say from experience...just my gut feelings.

[Norcal]

The mortar base isn't intended to add stability to the tub, that job is done by proper wooden framework and screws thru the flanges.

The mortar is basically to support the bottom of a fiberglass/plastic tub from flexing and eventually cracking.

I don't think using foam insulation is going to fix your problem...that expanding insulation is pretty weak stuff as far as structural integrity is concerned.

[Serg]

Flex is some of the problem, so maybe it will at least help. Any other ideas?

[geniescience]

....that expanding insulation is pretty weak stuff as far as structural integrity is concerned...
Can we discuss this?

Spread out over a tub base, the weight is easy to carry. Pressing a finger into foam will not give a clear indication of what foam can do.

[hj]

If stepping into the bottom of the tub flexs the joint where the tub meets the wall or surround, then you have more of a problem than just poor support under the floor. Filling the void under the tub may cure some of the symptoms, but the real problem, namely a poor initial installation, will remain.

[Serg]

Ugh. So no way to fix other than pulling the tub out and starting over?

[jadnashua]

Take a flat piece of foam, then take a thin piece of plywood, say 1/4". Stand on it, it will feel quite firm. The base of the tub will spread the load out, and it is point loads with foam that are a problem, not distributed loads.

If you don't have ledger boards to support the edges, that is another problem altogether...That needs to be fixed, too. If you do take it out, you can then lay the thing in a mortar bed, and it will be nice and firm.

[Serg]

I suspect they didnt put in the ledger boards on all the edges (e.g. on the edge where the faucets are). Is there any way to "fix" that (or at least reinforce it) without taking the tub out?