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efjellanger 04-03-2010 12:31 PM

starting bathroom rebuild
4 Attachment(s)
Hi all, I've been lurking here for a while, trying to soak up what I can. This forum puts an impressive amount of know-how in one place.

I am finally starting this bathroom remodel that has been needed for a long time. I tore out what was there before and now I've got a nearly clean slate, and I have a few questions about what I found underneath all the cheap crap out of which the bathroom was constructed.

I'm on the ground floor of my condominium, which is nice because with concrete slab I don't have to worry about structural concerns with tile.

The old shower was FRP over drywall, with some kind of burly plastic base. When I pulled that out, I found a big hole with the shower trap in it- about 14" square and a foot deep, and this gap was just bridged by the old shower base. So I need to fill this in and put in the drain. Should I fill the bottom with dirt and put cement over it, or do the whole thing in cement?

My original thought with the floor was to just set the tile onto the concrete slab, but now I've read that if the slab cracks your tile will crack too and I want to do it "right". The building is 20 years old and the slab has some tiny tiny cracks in it. What's the best way to do this? Is tiling directly on the slab safe? Should I put down a layer of RedGuard? I have read a little about Ditra but it seems like it may be more than is necessary in this instance.

There's a chunk missing from the slab up against one wall, it seems like it's simple to fill that in with cement, but maybe there is some gotcha I'm not thinking of?

I am also not sure what to do about the toilet mount, the bolts are pretty rusty and the area is kind of rough. I guess it's probably just a matter of getting new bolts, but I have never done it before and any tips are appreciated.

I've had several people suggest to me that I lay down some radiant floor heating but I am not sure it's worth it- this is an entry-level home no matter how you slice it, and being surrounded by other units it stays pretty warm. On the other hand I guess tile is cold on your feet. What are all your thoughts on that?

Thanks in advance for your help and patience! I am sure this is just the tip of the iceberg of my cluelessness.

Kman 04-03-2010 01:12 PM

On your shower, first you need to determine what type of waterproofing method you're going to use, then you can move forward from there. It's very possible that the trap is set to high for a Kerdi drain.

On your floor, you definitely want to use some type of crack-suppressant, whether it be a paint-on membrane like Redgard or Laticrete 9235, or you could use a thinset that works the same way, like Laticrete 125. While Ditra is not technically considered a crack-suppressant, it might possibly keep cracks in the slab from cracking the tile.

On that little chunk out of the floor, it's hard to tell exactly how far it comes from the wall, but if it's 1/2" or less, I would worry about it since the trim will cover it and you won't be putting any pressure on the tile that close to the wall anyway. Maybe you could measure it or post a pic from above that will give us a better idea.

efjellanger 04-03-2010 01:57 PM

Thanks Kevin.

My intent has been to use Kerdi on the shower, it sounds like a really good way to go. I was thinking about getting the premade pan, and cutting it down- the niche for the shower is about 35x41 inches (with no curb built yet). Seemed simpler than pouring my own.

What are the height restrictions for the Kerdi drain? The top of the PVC is about flush with the slab but the pipe rises about 5" above the outflow pipe. Could I hacksaw off a bit if I need to?

The area of the chunk that is exposed is 5x1 inches.

Houston Remodeler 04-03-2010 02:07 PM

As far as the square hole at the j trap - I would fill it with sand to about 3 inches of the top. Pack that down well, then cover that with cement. The idea is if you ever have to get at that drain you just break off the top layer and dig.

On the drain - as long as the drain flange sets about 1" over the slab you should be fine. That 1' allows you to fill in below the flange with greater ease.

efjellanger 04-09-2010 10:55 PM

Okay, from doing some reading, I'm wondering if this patch needs to be tied into the adjacent slab? Or is it small enough that it's not a big worry? The idea of it shifting under the shower is... unsettling.

I also got the idea that a piece of plastic between the sand and the cement is desirable. And I got the tip of using a 4" ABS coupler to leave space for the drain. So far so good.

I've seen it advised to use both thinset and real brick mortar to build a curb out of bricks... I take it this is just a matter of personal preference?

Is it okay to just fill the divot in with cement?

Steve C 04-10-2010 07:39 AM


Originally Posted by Eric
I was thinking about getting the premade pan, and cutting it down- the niche for the shower is about 35x41 inches

Hi Eric. I'd recommend learning how to build your own pan. I think the Kerdi tray's great, but you need to be able to cut an equal amount from each side of a square tray. If you don't, the perimeter of the tray will be unlevel and uneven due to the slope built into the tray.

I believe there's a John Bridge publication, and a bunch of folks who've done it a bunch of times, around here somewhere that will help you with your pan.

efjellanger 04-10-2010 09:50 AM

Thanks Steve. I think I'm leaning toward building my own pan now. I was afraid of making the slope properly but now that I've learned some more about it, it doesn't seem so intimidating.

I'm still wondering about the answers to my other questions, if anybody can help I'd really appreciate it. I think I'm about to head to the hardware store...

dhagin 04-10-2010 11:00 AM

Hi Eric,

With that hole, I don't think I'd worry too much about getting some rebar in there, though you could stick a few short pieces of #3 in there if you'd like. Looks like there's a small ledge around the bottom edge of the concrete? I'd leave that exposed for the new concrete to set on and call it good. I'd use crushed gravel and pack it in there up to the bottom of the 'ledge', then concrete up from there.

...but before all that, what condition is the drainage piping in? Structurally sound? Loose? Any holes? Check this link in the Liberry for Kerdi Drain rough-in info... you got room to get the new drain in there without reworking the piping?


It's always recommended to install plastic under concrete slabs on grade, so stick a piece in there if you can. Be advised that unless it's currently under the rest of the slab, installed correctly, and overlapped properly at your patch, the benefit may be greatly reduced.

For the cracks;
Are the sides of the cracks the same height or different? If different, how much? How wide are the cracks? How long?

For the missing chunk;
How wide? How close to wall? also, answer the crack questions above...

For the toilet, clean around the flange well and you'll see some slots to remove and replace the old bolts with new. Just take them out for now and replace when you install the toilet, they'll just be in the way until then. Is the flange intact? or broken and loose? :)

efjellanger 04-10-2010 11:22 AM

Thanks Dana. The hole in the slab is pretty uneven, but it does generally taper narrower at the bottom, and parts of it have the kind of ledge you're talking about. There's also already plastic between the slab and the dirt, which is a good thing. The drain pipe looks to be in perfectly good condition to me.

For the patch, I saw it suggested elsewhere to paint the edges of the existing cement with thinset, is this important or will new cement ever bond to old?

I had read the Kerdi drain rough-in instructions before but I think I understand a bit better now. If I intend for the drain to sit one inch above the slab (which will allow ample room for the mud bed), it means that the pipe should end 1 3/4" below the slab? I have plenty of room in there right now and I think if I use the 4" coupler method I will have plenty of room to cut the pipe (with the drill bit) later.

The cracks are really just fractures, I tried to take a picture of one (above) but it really doesn't show well. No vertical displacement. Not wide enough to get a fingernail in, except in one spot where I can just start to dig a nail in. But there are a few of them, they are long (several feet) and they criss-cross the room, so I tend to think some kind of suppressant is needed.

The missing chunk is shown in the third picture, it's about 5" by 1" exposed and it goes underneath the wall.

The toilet flange is intact and seems in good shape apart from the bolts.

Thank you for the help!

dhagin 04-10-2010 11:47 AM


Originally Posted by Eric
For the patch, I saw it suggested elsewhere to paint the edges of the existing cement with thinset, is this important or will new cement ever bond to old?

That's a cold joint where the new concrete meets the old. The new will shrink and pull away, slightly, from the old so I wouldn't worry about painting the edges... Probably wouldn't hurt tho.

I'd use a crack isolation membrane over the cracks, minimum, or over the whole floor, best. I typically cover entire floors with membranes, but if it's stopped moving you could just cover the cracks. Either way, follow the manufacturers install instructs for whatever membrane you choose. I like NobleSeal CIS, link on right side of page --->

For the missing chunk, just clean the loose stuff out and fill with concrete. It's so close to the wall it won't be a problem. Cover with membrane if you'd like after allowing proper cure of the fill.

efjellanger 04-14-2010 02:33 PM

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I have filled the hole!! I believe my next step will be to paint the floor in RedGard. So many questions.

I have observed that water doesn't really absorb into my slab. The slab is pretty glossy and smooth. I read in another thread that if this is the case you should scarify the slab before applying thinset. If I'm RedGarding it, how does this change things? The RedGard application guide doesn't seem to mention anything like this. And I will be applying thinset to the RedGard, not the slab directly. Uh... right?

I am also wondering, should I RedGard everything, including underneath where the shower pan is going? Of course I do not want the pan to crack if my patch shifts a little bit.

Should I install the curb first, or should I install it on top of the RedGard layer?

Thank you, as always.

cx 04-14-2010 02:58 PM

Not at all sure why you'd want RedGard there, Eric. But while we're waiting and talking, it's a good eye-dee to cover that floor with poly so it stays damp as it cures.

Is that a piece of 4" pipe or a 4" pipe coupling you have there for a form? If pipe, you gotta git it outa there before the concrete sets up too much to remove it.

You're now committed to building a mud floor for the Kerdi shower?

If you did that patch correctly, you shouldn't have any trouble with it "shifting" at all. What sorta mix did you use for the patch? You dowel into the old concrete at all? You did/did not paint the old edges with some thinset mortar before pouring the patch?

efjellanger 04-14-2010 03:42 PM

I'll remember to keep it covered next time- that photo was taken a few days ago, it's set-up now.

It's a piece of coupling, so I should have the room I need with it still in there.

I now intend to build a mud floor. I suppose it's still POSSIBLE to use the Kerdi tray but I don't think I'm going to do so.

The patch material was about 1:2 cement to sand. No dowels. No thinset.

cx 04-14-2010 03:49 PM

I'd suggest you wet it thoroughly and cover it with poly anyway. Still a long way from being cured.

And with that mix and that placement method, I think you can anticipate a good deal more shrinking and cracking than if you'd used something more akin to concrete for your patch.

How long do you anticipate leaving it alone before you need to build your shower floor?

efjellanger 04-14-2010 04:25 PM

I see. I do not know much about cement, so this is a learning experience. I don't have any strict timeline I need to follow. How long do you recommend?

Would you be worried enough about this to say that it should come out and I should try again? Materials and labor are pretty cheap at this point in the process.

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