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-   -   Fill a sunken tub and convert to shower - Advice appreciated! (https://www.johnbridge.com/vbulletin/showthread.php?t=129534)

MLBZ521 07-24-2020 01:59 AM

Fill a sunken tub and convert to shower - Advice appreciated!
So...I've been lurking researching on this forum and others for some time now... I've found some similar threads, but I feel I should probably try and get advice for my particular situation.

We have (had) a sunken tub in our master bath and want to fill it and have a walk-in shower. I'm in the Phoenix, AZ area and it's a single level slab home. The hole to fill is roughly 61" L x 31" W x 11" D (thought he depth varies in different spots).

Currently, we're planning to do a curb-less entry and also considering a linear drain. The drain is already in the center of the floor, so it doesn't need to be moved, and I would think a longer drain in the center would give a larger "drain plain" (at least in my head it makes sense, heh).

What I've read is to:
  • backfill tub up to 4" - 6" from floor height
  • add rebar to tie into existing foundation
  • then cement the rest of the way

My concerns and questions:
  • I've read to potentially use rock and/or 3/4 minus material for the back fill as that would be easier than dirt alone. (Wouldn't need to be compacted as much?)
  • How far up should I backfill and how far from the floor level should I stop the cement? Assuming I would then build a pan on top of that, I obviously don't want the cement the same level as the floor with the plan to not have a curb. (Would how the pan is built determine this measurement?)
  • Should I build a "box" around the drain and fill that separately? (Seen this in a few things I've looked at.)

I'm attempting to take this one step at a time and thoroughly research as I go. But where I'm at, I know if I don't take in consideration my next step, that I may not end up where I want to be.

Mainly, (I think) I just need some direction on the measurements on the back fill and then I can start looking into my next step. But if I'm missing something and/or I should take something else into consideration that I haven't yet, please let me know.

Old tub/shower has been completely demoed out and is ready to be filled in pretty much. Plumbing has been moved, so that's good to go as well I believe.

Any advice or wisdom is greatly appreciated!


cx 07-24-2020 10:12 AM

Welcome, Zack. :)

Couple clarifications first. You're planning to use a direct bonded waterproofing membrane to create your shower receptor, right? And presumably on your walls, too?

By "cement," I'm guessing you mean to backfill your existing void and use concrete to complete the filling of your existing void.

I've poured many concrete foundations with a dropped area for the shower. We (customer and myself) have generally opted for a step-down entry for those, but sometimes did fill them for a zero entry set-up.

To do either of those after the fact is a bit more difficult as you don't have the option of bending your reinforcing steel to create a continuous 4" pour in the dropped area the same as the foundation field. For what I think you're trying to do, I think you could finesse the doweling and reinforcing to achieve a satisfactory floor. Less than perfect, but probably adequate.

With some sort of bonding flange drain I'd think you'd need less than 2" drop in your concrete patch to allow for a proper sloped mud bed to be placed for your shower floor. If your existing concrete is a minimum of 4" thick (fairly common requirement), you may be able to accomplish that without having to break out any more concrete around the existing opening.

As for fill, some course rock first would be OK and then your 3/4"-down base material should be fine. The base material is good not because it doesn't need compaction, but because it packs so well. I'd want to leave enough room on top to allow for a full 4 inches of concrete in the existing hole, leaving enough room on top for your sloped mud floor.

Depending upon the drain you elect to use you may be able to set your P-trap and just create enough of a leave-out above it to allow for setting your bonding flange drain. You'll want the drain in hand to determine both what you actually need for a leave-out and the actual depth below your existing concrete grade you need for your application. Not sure why you would want a linear drain if your plumbing is in the center of the shower floor, but that's up to you. A simple centered bonding flange drain would seem to be the simpler and easier choice given that location.

Fairly simple application in concept, but you may find it a little more complicated in actual accomplishment.

My opinion; worth price charged.

MLBZ521 07-24-2020 11:58 AM

2 Attachment(s)
Thanks for the reply CX.

Let me get back to you on the waterproofing plans.

Correct, yes, backfill and concrete over to fill the void. Attaching a picture of how it currently sits and what I was thinking as far as the linear drain and the slope of the floor. This is just what I was thinking, it may not be practical.

It sounds like what I was beginning to think, I need to decide on my drain and that setup before I can execute the backfill step.

cx 07-24-2020 12:37 PM

Interesting. Are the sides of that hole structural, Zack? As in as thick as the slab and reinforced, perhaps?

I'm still not seeing the advantage of a linear drain in that application.

PC7060 07-24-2020 01:32 PM


I've read to potentially use rock and/or 3/4 minus material for the back fill as that would be easier than dirt alone. (Wouldn't need to be compacted as much?)
3/4 stone (aka #57) is considered self compacting and would be preferable to using soil as the fill.

Re the linear drain, I have to agree with CX. Linear drains are typically installed to the sides of the showers. Installing in the middle would be a unnecessary expense and would look out of place (IMO).

Good luck!

jadnashua 07-24-2020 03:25 PM

Depending on how agreeable your inspector is, they tend to want a 2" minimum drop from the outside of the shower to the drain. You can do that with a curb, or the slope.

Most tubs came with a 1.5" drain line...a shower, by code, requires at least a 2" line, so you may need to double-check what you have. Many places want at least a 2" line when it's buried below the slab, so you might be okay already, but may need a new P-trap if they made that bigger on the outlet.

cx 07-24-2020 04:07 PM

Jim, when folks in the NTCA's Technical Committee looked into that 2" above the drain shower requirement, they found that the plumbing industry people and the code compliance inspection people actually believed that to apply only to the height of the top (finished top, not waterproofing layer) of the curb in a shower that had a curb. If there was no curb, they had no requirement at all. Makes absolutely no sense to me, nor did it to the other people on that committee, but that was the bottom line at the time. That's a few years back, but I've not heard of any change since then.

And we've kicked around the 2" drain line for showers here a number of times and the code does not appear to require a 2" drain line unless the actually fixtures in the shower require more than an 1 1/2" drain as best I can read the code and a shower with a single shower head would not require a larger drain.

That said, my plumber and I always installed 2" drain lines as a minimum everywhere when roughing in foundations.

MLBZ521 07-25-2020 09:54 AM

CX: Yes, I believe the hole is as thick as the slab. I think it may tapper away slightly at the bottom though (or that could just be how it broke out).

Regarding the linear drain: It wasn't solely function, but form as well. I'm currently reconsidering the drain though.

All: I'll verify the drain size regardless; I have read that and I've been meaning to check it.

Regarding the waterproofing, what would be the recommendations from your guys perspective for my application? I've seen various options in my research so far, but I'm open to suggestions and techniques.


cx 07-25-2020 11:19 AM

The only really workable option for what you've got in mind would be a direct bonded waterproofing membrane of some sort. For my money it would be only one of the sheet-type membranes.

If the sides of that depression are thick enough to be structural, adequate doweling into the sides at the proper depth to fall in the center of your new floor pour could make things a good bit easier.

I wasn't suggesting that you could not use a linear drain in that manner, only suggesting that it would add a bit of complication to the forming and tiling of your shower floor. Certainly can be made to work if that style drain is part of your aesthetic concept. Or, more importantly, Mrs. Zack's concept. :)

My opinion; worth price charged.

MLBZ521 07-25-2020 12:36 PM

That's what I was thinking, some type of sheet membrane. Do you have any suggestions that I should look at specifically?

I'll see if I can "explore" the thickness of the slab around the tub before I drill into them. If they are, about how far out would you recommend and the spacing between them? Should I do it from all four sides as well? I also have some diamond lath material that I could use as well if that would be beneficial.

I understand what you mean and I appreciate the the input on the drain (and everything else). The more information I have allows me to make a better informed decision in the end.

Lazarus 07-25-2020 01:17 PM

Schluter's "Kerdi" has been around for many years and is pretty hard to beat. There are some that might not consider it the "Gold Standard," but they are in the minority. There are quite a few good other sheet membranes out there as well. :scratch:

cx 07-25-2020 01:59 PM

1 Attachment(s)
I would be in what Laz thinks to be a minority, Zack. Schluter's Kerdi is a good and useful product and much touted on this particular forum, but there are others out there with better perm ratings and one in particular that is thinner and easier to handle in my opinion. That would be the former USG Durock Shower System membrane, no longer available from USG since they were bought out a while back. If you can still find some of that membrane, that would be my first recommendation. I'm told Contractors Direct bought a large quantity of the material before it left the market and may still have some available.

There are also a couple membranes from the Noble Company that are competitors and, again, have lower perm ratings than Kerdi. I don't personally favor them as they are somewhat stiffer, but one of them, ValueSeal, is the same thickness as Kerdi and the stiffness is something that some folks consider a positive feature.

There are other choices out there, too, but I've not personally used any of those others.

I would dowel from all four sides with #3 bars long enough to overlap in the center and have them spaced no more than 12" on center. That's the schedule I've always used in the field of my foundation slabs and I just tend to do my patches similarly. Like here if you can see it:

Attachment 214539

My opinion; worth price charged.

MLBZ521 07-25-2020 02:51 PM

CX: Are you bars continuous or overlapping? (They look continuous to me.) How far from the sides (corners) did you start? Looks roughly like six inches?

I'll look into the former USG membrane and see what I can turn up.

I was just looking more into Kerdi as yes, I see that mentioned, documented, and used often. Since I was just looking at it (and other systems may be similar or I'd have a similar question regarding them), for the Kerdi flange/drain systems, do you have to use their drain with their flange, or can you use any* (*I'll use "any" loosely here) drain with it?

cx 07-25-2020 03:13 PM

Those bars are overlapped, Zack. Just doesn't show very well in the photo. I stick them three inches or so into the concrete sides, which I've usually drilled at a slight angle, then bend them to horizontal and wire-tie them together. I just start wherever seems like the right place. That technical enough for you? :)

Always best to use the bonding flange from the same manufacturer as your membrane, but I know from experience that the Schluter and USG drains are interchangeable and I would suspect that other manufacturers' similar bonding flange drains (any that use thinset mortar as a bonding agent) would also work as well. As always, see my warranty information below.

Certainly nothing at all wrong with using Kerdi for your project, 'specially if it's more readily available to you. I'm hoping the USG membrane comes back on the market soon. We keep up with the owner of the system and he keeps telling us it will be back. If not, I'll be using Kerdi for my next shower project, too.

My opinion; worth price charged.

MLBZ521 07-25-2020 04:51 PM

Yeah, that's great info, thanks.

Regarding the drain/flange specifically: we were looking at some of those "invisible" drains (that you can lay the floor tile in), but I wasn't sure if I could use those with the Kerdi or (insert other manufacturer's) flange.

I would assume so, but better safe that sorry. I see some Kerdi "kits" (e.g. one, two) that include the flange, drain, etc. and also has a flange without the drain "kit" (e.g. one).

Ok, one more question for now I think...

Planning to use cement board for the shower walls. Should I bond the sheets with mesh tape and then cover that with a waterproof membrane (e.g. Kerdi/Noble/etc). Can I do that? Should I do that? Is that overkill? (Again, I'd rather be safe that sorry -- this entire project started because of a water leak, though it wasn't from the shower itself, so I'm being overly cautious.)

Also asking the above question to determine how much of the (insert manufactures') waterproof membrane I should be looking to purchase.

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