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

[iowa70]

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!

[Basement_Lurker]

Setting the tub or shower base in a proper mortar bed during installation is the only proper way to install fiberglass units. You may be able to pack in a somewhat dry mortar mix after the fact since you have access to the underside of the tub, but I doubt it will be very effective as you won't mould the mortar to give proper support. And if your base doesn't have plywood underneath, you will probably create high spots which could make things worse over time.

[Thatguy]

Figure what places in the tub bottom are most in need of support. Put shims there. Just reducing the flexing will prolong tub life, assuming fiberglass is subject to fatigue failures just as metal is.

[iowa70]

Setting the tub or shower base in a proper mortar bed during installation is the only proper way to install fiberglass units. You may be able to pack in a somewhat dry mortar mix after the fact since you have access to the underside of the tub, but I doubt it will be very effective as you won't mould the mortar to give proper support. And if your base doesn't have plywood underneath, you will probably create high spots which could make things worse over time.

Thanks for your response. So putting the mortar around the tub after it's installed wouldn't be than having the flexing caused by the styrofoam? I guess is it better to try to fix the flexing problem now by using mortar or wait until it cracks and just replace the entire tub?

[iowa70]

Figure what places in the tub bottom are most in need of support. Put shims there. Just reducing the flexing will prolong tub life, assuming fiberglass is subject to fatigue failures just as metal is.

There's about 3-4" of styrofoam underneath the tub and this styrofoam is already shimmed. I've gone around and pushed the shims in farther but the flexing is still pretty bad.

[Basement_Lurker]

The best thing you can do is replace the tub and set the new one in a proper full mortar bed. The tub you have now has already been flexing for some time now and has weakened somewhat.

But obviously you aren't interested in doing that, so you should just stick to shimming the foam as best as you can to give the base more support. I really don't recommend you removing the foam in favour of trying to stuff mortar underneath the base....you won't be able to do it adequately enough to provide proper support.

[jnyost]

Fill the tub about 1/2 or 3/4 of the way full and use expanding foam underneath the tub. Don't get the windows/door version, just get the regular expanding foam. It works well. That's what they used on my new Jacuzzi tub. They say they stopped using mortar a long time ago.

[jadnashua]

Foam will literally glue the tub to the floor and will be a major pain to remove. Also, if you don't get the stuff that remains flexible, it can crumble if you get enough point load to compress it (you might not), or expansion/contraction degrades it (which you will likely get).

IMHO, the right way to do this is with mortar, and it is MUCH easier done when setting the tub than after. It's strong enough to not crumble, if done right, and has great compressive strength (and it's cheap!).

Depending on access, you may be able to push some deck mud underneath and solve your problem. Deck much is much like wet beach sand - it will compact, does not run, and is cheap. You'd probably want to start with the tub full and try to pack it fairly dense, otherwise it will not have much strength. Pack too tight, and you'll bow the bottom, too loose, and it may crumble. Generally, it won't stick to things. You'd also want to try to place a sheet of plastic or felt paper on the floor to minimize the subflooring from wicking out the water which weakens the cement. It will be a hassle.

[jnyost]

My contractors actually state the opposite which is why I posted it as an option. I've not done it myself, just helped them do it to my tub. They have pulled several tubs that were insulated in and didn't have any problem. My post was a reflection of their experiences and thoughts.

The insulation is not so hard that it will crumble, it's just more firm. The window/door stuff is soft and would provide very little support. The standard stuff is strong enough to provide support but yet it still gives. You can squirt some on the ground and squeeze it after it sets. You'll see it's not rock hard, just very firm.

[jnyost]

Excerpt from my jacuzzi tub installation manual for supporting the tub.

minimal expansion structural foam having a density of a minimum of 5 lbs./cubic ft

[miamicanes]

I'm interested in this thread, too. I set my new tub (acrylic, with built-in OSB bottom resting on two fiberglass shells) in mortar when I installed it last Sunday. It's "99%" solid, but there are a couple of spots where I've heard it creak a tiny bit while walking around inside of it... mainly, the spot above the tub's center point, which is NOT directly supported by the tub's factory-supplied supports (one is ~6" wide and sits below the front of the tub between the middle and where the drain hole is, and the other is ~24" wide and sits below the flat part of the tub that's furthest from the drain).

Are acrylic tubs truly ever supposed to be "completely" rock-solid, without the slightest hint of a creak anywhere, ever? Or is an occasional creak an inherent trait that you're never really going to eliminate 100%? Mortar would be pretty hard to shove under the unsupported middle span between the fiberglass supports, but foam would be pretty easy to inject under there.

Just to emphasize, it's generally not "creaky", but while walking around inside of it installing the valves & stuff on the wall, I've averaged a brief, muted creak or two per minute of active motion.

[hj]

Get a can, or cans, of low expansion foam. Put a long piece of plastic tubing on the end of the nozzle, then slide the tubing under the tub and start shooting the foam in as you pull the tube out. It will glue the tub to the floor, but that will only be a minor problem when, and if, the tub is removed someday. You also do not want a "flexible" foam because that would just allow the tub to keep flexing. Low expansion foam will fill the space and expand sideways instead of trying to lift the tub, which it couldn't do anyway once it is locked in position.

[miamicanes]

Well, phase one of the foam-filling is complete. I began by injecting "large gap" Great Stuff foam into the ~1cm void that Laurel Mountain inexplicably left between most of the front support and the OSB bottom.

The good news: the creaking sounds mostly went away when I step over the newly-filled area.

The bad news: it's hard to explain, but I kept feeling like I was about to fall down a little while ago when I took a shower and stood over the newly-reinforced front part of the tub while washing my hair. The tub floor there still deflects when I stand on it... it just deflects silently and more slowly. I kept getting this slow, constant "sinking feeling" that was actually pretty disturbing and disorienting when I had my eyes closed while rinsing the shampoo out.

I'm not really sure why it felt so "weird". All I can think of is that before, when it made creaking noises every now and then, it kept me alert and expecting to feel motion for a moment after it happened. Now that it's silent, and the motion is dampened and slowed down, it's more insidious and startling.

Was the "large opening" foam the right one, or should I have used the "normal" or "fire" version of "great stuff"? I pulled a blob of the version I used from the garbage can, and noticed that it was fairly compressible. Are the other versions firmer when cured?

Also, is there any straightforward way to get a longer tube? When I filled the first gap tonight, I actually had to use duct tape and TAPE the dispenser in the "flow" position, because I couldn't actually reach it to operate it while filling the gap. If I can somehow attach another foot or two of something like clear aquarium tubing, it would make it a LOT easier to aim and dispense.

[PlumbPowerHouse]

As a plumber, we run into this a lot. We do new construction as well and on every new rough-in we put mortar beneath the tubs. It helps to support and gets rid of the flex. Since the tub is already installed, it will be harder to fix, but it's still possible. I suggest trying the mortar if you can do it. Wedges would be the next idea.

[hj]

Just buy a longer piece of the same size tubing that came with the foam canister.