Self-leveling cement below new tub

[MiamiCanes]

I'm going to be replacing my bathtub next weekend. The old bathtub is a 72x42 cultured marble tub. The new bathtub is the same size, but acrylic. Since I'm not going to really know the full scope of the job ahead of me until I get the old tub out (at which point I'll be without a bathtub until the new one is in place, because I only have one full bathroom), I'm planning for the worst and just taking for granted that the floor won't be level.

The question is, what kind of formwork do you use to keep the self-leveling cement from running beyond the footprint of the tub? Or... do I even WANT to keep it from running beyond the footprint of the tub? I'm guessing that in a perfect universe, the floor would already be relatively flat & SLC wouldn't run far anyway... but I've learned the hard way that perfection in building construction is a delusional fantasy, and any strategy that depends upon an existing floor actually being level, or a wall being square, or anything being within 1/8" of what it's theoretically supposed to be, is a recipe for expensive disaster ;-)

The best idea I came up with so far is to buy 1x2 furring strips, fasten them to the concrete slab with caulk around the footprint of the new tub's base (where it actually comes in contact with the floor), fill it with SLC, then pry up and remove the furring strips (hence the use of caulk) when it cures.

The thing is, can I assume that if the surface upon which the tub sits is "perfectly" flat, everything will be OK? Or do I have to assume that the tub's underside WON'T be perfectly flat, and it'll still be necessary to have a squishy layer of mortar under the new tub when I lay it in place to fill any voids that would otherwise exist? Likewise, how do acrylic tubs achieve sloping floor surfaces? Do they make the acrylic thicker at the rear so the underside of the tub can be perfectly flat and level, while the surface slopes properly towards the drain? Or do I myself have to give it a sloping floor surface to allow it to slope towards the drain without leaving a gap between the tub's underside and the floor it's supposed to be resting on?

BY the same token... does mortar chemically bond with cultured marble, fiberglass, or acrylic? If the original builder set my current (cultured marble) tub on a bed of wet mortar when he installed it, am I going to need crowbars (or worse) to pry the old tub away from it? Or will the old tub lift off like cookies on a teflon baking pan, and the worst thing I'll have to deal with is a rough surface that mainly just needs to be smoothed out for the new tub?

 

[Jadnashua]

SLC may not be your best choice for this. It's good stuff, though. If the old tub was set in mortar, you'll probably need to take it out. Is the floor on a slab or is it wood? If wood, it will probably come right out. If it is on a slab, it probably didn't bond well, and you can get it out as well. SOmewhat depends on what they use, if anything.

SOme tubs come with a foam base that accounts for the slope. If your new tub has this, then the slc may be viable. When I used it in my bathroom, I put some 2x4's across the doorways, tonailed into the frame. Then, I took some duct tape and put it on the floor with a fold up on the 2x4.

If the tub does not have a captive flat base, then I'd set it in mortar. This will support the bottom well, and provide a means to level it and get it at the desired overall height.

The tub probably won't stick to the mortar, but if you are worried, you could place a layer of plastic down before you set the tub in it. If it is a wood subfloor, you want to install either some plastic or felt on the floor to prevent the wood from sucking the moisture out of the mortar so it can cure properly.

 

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[Gary Swart]

I think mortar would be easier, but I used SLC under a 32X32 shower stall and it worked fine. Problem was the SLC found spaces to ooze through I used scraps of towels to caulk those spots and then when the SLC had set up, I cleaned up spots that had gotten through the caulk. The SLC did give the shower a very solid base, but with 20/20 hindsight, to do again I'd use mortar. It does not have to have 100% coverage. Do not use foam insulation.

 

[hj]

tub

You are making a fairly simple job into a project. Check the floor in front of the tub. If it is level then the tub will be level also. SLC is a "liquid" material, and you need a "plastic" one which will stay in place until the tub is set onto it, and then spread out because of the tub's pressure. SLC will flow to any low spots, or openings, and may not even reach the bottom of the tub, depending on how it is made.

 

[JohnfrWhipple]

Setting in your new tub

Plastic mortar sandwich.

What the boys above said. Bust out old cement, lay out some poly, top with mortar mix, top with poly and set your tub.

Make sure you have 2-3 bags of mortar mix, maybe 2-4 2"x4"'s and some strong backs to help you postion, readjust, and level that tub. Buy the longest level you can get to sit across the tub and have a 2'-3' one for the side.

You only get one chance - don't take the easy route of not pulling out the tub one more time to fine tune the grade.

 

[MiamiCanes]

Well, here's the thing: it's a 3-walled alcove, and I have access to it ONLY from the fourth side. In theory I could cut a hole through the drywall at the end opposite the drain, but the one end where having access would REALLY help -- the drain end -- backs up against a cement block exterior wall that's separated from the neighbor's cement block exterior wall by a sheet of Tyvek.

That's why I figured my best bet is to get the floor independently level, so I can then wrestle the tub into position and (hopefully) take as an article of faith that having a flat surface for it to rest on will ensure that the tub itself is level.

Another idea I had was to build the formwork from 1x2 furring strips glued down with caulk, then trowel a dam of stiffer mortar around the inside perimeter of it before pouring the self-leveling cement.

 

[Jadnashua]

I still think I'd just plop some mortar down, then smush the tub down into it while looking at the levels...perfectly level and no big hassle.

This brings up another problem...if the tub has an apron and you can't access things at the drain end (maybe from below?), how do you expect to get it connected to the drain? It can be done, but it's often not a DIY'ers job.

You didn't indicate what's on the bottom of the tub...that may push it toward one option or the other.