View Full Version : Acrylic Tub Installation

07-29-2008, 10:06 PM

I'm in the process of remodeling our bathroom and purchased a Kohler soaker tub. As it is on the second floor and in a tight space I went with an acrylic tub for weight considerations (and the wife liked it more). Everything is sitting level and no problems getting it in, however, I'm unsure about the whole support issue under the tub (seeing as it's acrylic). Some things I've read said you don't need to put anything under the tub (between it and the floor), other things have said sand, spray foam, mortar, thinset, etc. The directions called for cement or mortar.

I had a pail of leftover premixed thinset from when I did the subfloor, so I spread that out underneath the tub. I miscalculated how much I'd need and where to place it, so only the back half of the tub appears to be sitting in it (from what I can see from the access panel for the plumbing). I was contemplating what else to use or how I'd fill the remaining space, if necessary. I'm now concerned however as I read in one of these posts that you don't want to use thinset.

Do I have to worry that I made a terrible mistake by using thinset, and do I now have to worry about it somehow causing a problem when we go to use our new bathtub?

Thanks in advance for any help/advice anyone can offer!

07-30-2008, 04:40 AM
I usually recommend sandmix (premixed bag of sand and cement - add water and mix). Putting a mortarbed underneath the tub is wise and relatively easy and inexpensive. What's the tub bottom design? Waffleboard or skids or...? Thinset could work, but only if you had very low clearance as thinsets tend to be designed for maximum thicknesses of 1/4" for each application. If you need thicker support, then medium bed mortar and thick bed mortars are needed. Sandmix it.

If you can remove the tub to install a mortarbed and reinstall, that would be best. If not, you may need to get creative with possibly using a grout bag to inject mortar. Post some pix if you can and various people here can make good suggestions. Greg

07-31-2008, 09:43 AM
Hello Greg,

Thanks for the advice! Unfortunately I can't move the tub now (it's stuck rock solid with the thinset), but I have access to the underside where the plumbing is. There is about 1.5 - 2" of clearance under the tub, depending on location. The thinset is built up about 1.5". I just globbed it all down at once (yes, stupid on my part I know now), but it seems to have hardened and stayed thick. When you say designed for lower thicknesses, does that mean design as far as support or strength is concerned, or just that it is more difficult getting it to dry/set, stay thick (not run I guess?)?

That's a great idea with the grout bag! I may have to try that. I originally attempted to do a mortar bed, but the "quick setting" mix I used became too hard too fast and was not easy enough to work with (trying to lay it out and get under the tub was a problem with all the clumps which fast became cement rocks). Any recommendations on brand?

As for design, it has some thin boards that look like they are to provide minimal support and some small feet (I guess these are the skids?), all covered with the same fiberglass spray as the tub. At work, otherwise I'd try to get some pictures up.

07-31-2008, 10:16 AM
Cement, like you'd make a sidewalk out of, won't work. A simple mix of mostly sand with portland cement (often called sand mix) would work and can be purchased pre-mixed - only add water. For a shower floor, it's about 4-6 parts sand to one part (by volume) portland cement mixed only wet enough to hold together when you squeeze a clump in your hand (not dripping wet). The stuff is like wet beach sand if mixed properly and will smush. It's the same thing they make shower pans from. Good compressive strength, but a lousy wear surface (but you aren't walking on it like a sidewalk, so that's okay). You could mix some of this and stuff it in, but you'd likely end up with some voids...you'd have to be careful.

A modified thinset has laytex in it and is a little flexible. A dryset thinset mortar does not and is hard. Neither one is designed to be installed more than about 1/4" thick and won't have the strength or survival (under tile) it would if installed properly. Given enough time to cure and dry, it'll probably be okay under your tub. Very thick thinset tends to crush if you get a point load, which would be difficult under a tub. Keep in mind that having the thinset sticking to the acrylic tub which will expand and contract at different rates means it will put some stresses on the tub. A sand mix wouldn't stick much at all, just support, since it has so much sand in it the cement can't do much more than hold the mix together, let alone stick to anything.

08-01-2008, 04:34 AM
If you can reach a few different spots, it would only help even to add a few "mounds" of support. Both Sakrete and Quikrete brands have premixed sandmix bags available from most hardware stores. If it was an inexpensive tub, I'd probably start anew and "bed" a new tub for long life...Greg