When I step into my tub, it sinks down a bit from the weight. It does not sink down a tremendous amount, but it is enough that it jars the caulking and creates a little crack in the caulking. Then, of course, it leaks a bit through the crack when there is any splash.

Is it normal for the tub to sink a bit? Or does it mean it was badly installed? If badly installed, how would I better support it?

If it is normal for there to be a little sinking, any idea how to avoid the crack in the caulking?

When you tub was installed it should have been surported by a 2x4 board nailed to the wall on the back side of the tub, it is possible that it was not installed properly or that over time the wood may have shrunk with age. Sometimes on the end of the tub where the drain is they leave an access hole to get into the faucet and drain of the tub. If you have one of these holes it is possible that you might wedge between the tub and floor enough to stop the tub from moving. If this is not possible my other solution would involve cutting into the wall either at the end of the tub where the access hole should be or maybe into the opposite side of the wall that is along the back of the tub. If there happen to be a closet on that wall you could cut in without much damage being exposed. If you cut into the wall you have a couple of choices one being that you could shim up under the supporting 2x4 to make the tub steady again or you could spray the foam under the tub and it would expand making a firm base under the tub. If you don't understand let me know and I will try to help if I can.

Thanks for the response. I do have an access hole on the front side (faucet side) of the tub. If I go in through there, where to I put the shim or the foam? What kind of foam? Thanks again.

You would put the shims under the tub between the tub and the floor that it is sitting on... if you used foam it woulld be just under the tub and it would fill the space between the tub and the floor... the foar is an insulating foam theat expands as it is used, On of the hardware stores would know what you wanted. At first I would use the wooden shims, put then is as many places as you can reach from the access hole. You may have to have someone in the tub trying to make it move as you are tightening the shims under the tub. Good luck

Actually, what you are describing comes from a poor installation - the installer is supposed to puddle mortar and place the tub on that, adjusting it until it is level. If it is a fairly new installation, demand that the installer return and put in a new tub - properly, this time - at no cost to you.

And I'm sory letmetellu, but foam (and BTW, the one you're talking about is called "Great Stuff" by Dow) is NOT a good idea: foam is not very dense. A good part of foam is air, which will compress under the weight of the person in the tub, so the tub will still not be supported properly.

Sorry for the bad news,
Moishe

Serg,
Letmetellu was bang on until he suggested foam as a support for the tub floor. PalmMP3,(Moshie) was absolutely correct when he advised against it. Foam consists of air bubbles and in time they will compress under weight.
Your best bet would be to make a morter or cement mix and puddle it under the tub near the center and around the drain area. Do not have someone in the tub when you do this and do not put any weight in the tub until the until the mix sets up. Good luck, Tom

Recognizing the limited access I have through the existing access hole, do u think the idea of wood shims rather than cement would work? If not, other ideas I could implement with limted access?

Wood shims will be fine. Use 2 X8" blocks and shim up under the blocks if necessary. Good luck. Tom

Gentlemen, before I did anything, I asked a plumber to take a look. Apparently, I have a cast iron tub. The plumber tells me that, when installed, the tub should have been bracketed to the wall on the backside (i.e. the long side opposite the long side where the shower curtain would hang and you would get into the tub; in other words, the far side if I am standing in front of the tub facing it with the drain to my left) for stability with two brackets, one on the left side of that backside (i.e. near the drain end of the tub) and one on the right side of that backside (i.e. near the back of the tub if I am looking at it from inside the tub with my back toward the drain). He claims that the brackets must not have been properly installed and, as a result, my tub moves between 1/8 and 1/4 of an inch up and down. He didn't say anything about mortar or anything like that. Do you agree with how he describes the proper installation of the tub? Is mortar, as you suggest, not used for cast iron tubs? Or perhaps not used in this part of the country (Houston, Texas)? Do you believe it could still be fixed with wooden shims?

Sorry, not cast iron, enameled steel.

Hey Serg,
Cast iron tub huh? This type of complaint usually comes from a plastic or fiberglass tub that hasn't been properly bedded at instalation. Bottom line! Pay no attention to us and listen to your plumber. This can be remedied by 2 X 4s cut and secured to the studs on the backside. Good luck, Tom