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Unread 01-02-2012, 01:30 PM   #1
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Tiling across the joint between two slabs...

Hello. I'm preparing to continue the installation of 18" porcelain tiles that I started installing four years ago in the original portion of my one-story concrete slab home (I know, I know, slow down). I'll be running them through more of the original part of the home into a room that was added on about 26 years ago.

If it matters, my home is in San Diego, is 31 years old, and the single-room addition was built in 1985 by a previous owner. The addition's slab is about 18' x 18'. My home is built on decomposed granite (the "soil" is very hard and not at all like clay or mud). When I dug trenches for sprinkler lines, I found it much easier to use a rotary hammer or pick axe in addition to a shovel.

I would like the tile to be a continuous installation. My main concern is the joint between the original slab and the slab for the addition. I really want to tile right over it like it isn't there, but I am, of course, concerned about cracking.

Please click the thumbnails of the pictures below to see the area where the slabs meet. As you can see, half of the joint has been smoothed over with some sort of leveling compound, and the other has a sort of groove or channel that is about 2" wide and 2" deep. I removed a portion of the wall that covered the channel, which exposed it. The "smoothed over" area will have to be ground down as it was not done well and is kind of like a minor speed bump between the slabs.

My tile guy (who I've used before on small, uncomplicated jobs) wants to grind down the smoothed over area and fill the channel/groove with thinset. He then wants to put a thicker coat of Redgard down, which he says will make it possible to tile right over the joint like it isn't there. He's talking about grinding down enough to leave a thicker layer of Redgard.

I'm worried about cracking, but he says with Redgard, and given how long the slabs have been there, and given that the leveling patch on the smoothed-over portion of the joint has not cracked, it should be fine. (The previous owner had carpet in the addition and vinyl flooring on the slab on the other side.)

Does this sound likely to work? Do you think that the tiles that span the joint will eventually crack?

Note: I can't plan to leave a grout line where the joint is because I'm building off tile from another part of the house. To leave a grout line (and maybe that wouldn't do any good, anyway), I'd have to put cut across all of the tiles, which would look awkward and bad.

As a kind of side question, do any of you ever put duct tape over cracks as a sort of isolation membrane before tiling? I have a couple of small cracks that do not span the entire slab, and he mentioned doing that. That sounds crazy to me (it seems to me the thinset wouldn't adhere to the tape), but I've seen mention of it here. So, duct tape on cracks?

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Last edited by jgleason; 01-03-2012 at 11:01 AM. Reason: please use the paperclip icon and attach images. Don't use the img tag.
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Unread 01-02-2012, 02:02 PM   #2
half a mill Phil
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Hi Lee, I may be shot down in flames for this, as I am not familiar with US building regs. But in the UK we would use a decoupling membrane eg: schluter ditra mat, and install a preformed expansion joint where the two substrates meet. Sorry if I am over stepping my status as a new member
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Unread 01-02-2012, 02:46 PM   #3
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You should use something like redgaurd, rather than duct tape over your small cracks; even better would be to use a membrane or uncoupling, as suggested by Phil, over the whole thing. Either way you will need to honor the joint between the slabs all the way through the installation, meaning no, you should not just tile over the joint.

Should you choose not to do things correctly you may never have a problem, but your risk of a problem would be substantial.

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Unread 01-02-2012, 02:55 PM   #4
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Hi Phil--I don't doubt that that would be a better way to do it, but it just isn't possible. This tile continues from a front room where I've already laid it, and I didn't install a membrane there. so there would be a noticeable high spot wherever I started to install a membrane. There is no door or transition between the rooms. From the front room through to the addition is, in a sense, just one large room. And I just don't think I want to get into doing a membrane no matter what.

Unless somebody had a better idea that can work given that I can't install a membrane, I think I'm pretty much limited to these three options:

(1) Doing the Redgard as described in my original post and just running the tile right over the joint like it isn't there with the hope and expectation that it won't crack.

(2) Doing the Redgard and cutting a grout line along the length of the joint (if that would even help).

(3) Doing the Redgard and installing some sort of border over the joint that will end up dividing the two rooms.
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Unread 01-02-2012, 03:06 PM   #5
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Option 1 is cracked tile waiting to happen.

Option 2 is cracked grout waiting to happen.

Option 3 is cracked border tiles waiting to happen.

Option 4: Create a grout line over the joint, fill with a urethane sealant. Or, use one of Schluter's expansion joint profiles. Either way, you honor the joint and allow for the inevitable movement that will occur there.
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Unread 01-02-2012, 03:09 PM   #6
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Hi Paul--You posted while I was typing my reply to Phil. In it, I explained why I don't think I can use a membrane (and I don't really want to get into that, either.

I've always thought that I would have to do something along where that joint is to recognize the likelihood that the slabs might move and cause cracks. Then my tile guy said I really don't have to. That didn't sound right to me, but then he pointed out that the leveling patch has never cracked. It's been there for 25 years with no cracking, including one very noticeable earthquake a couple of years ago.

I know that a strong enough earthquake would almost certainly cause any tiles spanning that joint to crack. Of course, some level of earthquake will bring the entire house down.

One idea my tile guy came up with was to continue the tile all the way up to within two or three inches of the joint. Then he'd cut some sort of border out of the same tile, and then lay the same tile on the other side of the border in a different pattern. I'm not wild about that. I'd really rather it be the same all the way through. Of course, I much less wild about the idea of it cracking a few years down the line and having to deal with it then.

I guess it comes down to how likely any tile spanning the joint would be to crack, and what my alternatives are.
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Unread 01-02-2012, 03:20 PM   #7
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I'm trying to steer you to industry recognized methods. There are reasons for them being written the way they are. In the end, it's all about your own risk tolerance. It's your house and your money, so you can do whatever you please. There is no tile police.
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Unread 01-02-2012, 03:25 PM   #8
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I'm a newbie here, just learning so this is more of a question than a suggestion....

If the OP has a stack of spare tiles what would be the major issue with tiling over the crack, after using Redgard? If the tiles do crack would it be an absolute nightmare to install replacement tiles? Obviously if the cracking occurs within months of them being installed it's not reasonable to keep trying to replace them, but if there are no issues for, say, 5 years, and then a couple crack couldn't he just replace those at that point and potentially be fine for 5 more years? Elegant? No. Practical? Maybe. Am I way off base here? The fact that the leveling compound is uncracked after so many years makes me think the chances of the tile cracking within months of installation is probably small...
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Unread 01-02-2012, 03:33 PM   #9
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I added a slab to my kitchen 20 years ago and tiled over it with zero problems. I tied the new slab to the existing with rebar set in Reziweld 6" into the existing at 18" OC. I tiled over the joint and after 20 years no cracked tile or grout. My house sits on adobe near SF Bay in an earthquake zone. I was as dumb then as I am now and didn't know to use a membrane. If your new slab was properly constructed and adequately tied to the existing slab, and you're house sits on stable rocky soil, you're ahead of the game. The mods here are conservative...for a good reason. Your money, your house, your decision to risk it.

One strategy that I didn't see mentioned is to run a cup grinder along the length of the seam, taking off a slight amount of material a foot or so on either side of the seam which will allow a deeper layer of Redguard without creating buildup.

Last edited by Bodie Powers; 01-02-2012 at 03:38 PM.
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Unread 01-02-2012, 03:55 PM   #10
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Hi bbcamp--Your first post came in while I was typing my earlier replies, so I just now read both of your posts.

I appreciate you trying to steer me toward approved standards. And I appreciate your opinion of my three options. It's pretty much what I thought it would be. What you say seems completely logical.

Before my tile guy came, I was really leaning toward laying the tile continuously across the joint building off the pattern in the front room, but I had planned to cut a grout line across the tiles in the middle of the joint. It wouldn't look great, but it would hopefully direct any cracking to that line and just crack the grout (which seems a lot more manageable than cracked tile).

But then my tile guy seemed pretty insistent that we could tile right across it, and he pointed out how the leveling compound hadn't cracked over the past 25 years. I guess than means if tile had been laid over it, it wouldn't have cracked over that same period.

I know what you are saying is right. My best option is to leave a grout line and urethane it or use an expansion joint. I guess I will either do that now or I will tile right across joint, and, if it cracks, replace some tiles and do the grout line then.
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Unread 01-02-2012, 04:07 PM   #11
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Hi Bodie--Since he's going to be grinding down the existing leveling patch, my tile guy said he would grind down further for a thicker layer of Redgard.

I don't know how the slabs were tied together. Based on all the other garbage work the guy did on the house, I'm guessing he might have cut corners here, too. I don't know how basic tying the slabs together with rebar is. If it is the kind of thing people always do with out question, he might well have done it.

The soil here is very, very stable. There are about 200 homes here that were all built 31 years ago by the same builder. I don't think anyone in the neighborhood has had a slab crack, and I don't see any driveway cracks.

If I just tile over it, I know it will fail someday if for no other reason than as the result of an earthquake. But it could be decades before an earthquake like that hits, and, when one does, it may do enough damage that that joint cracking won't matter.

All of these posts help as they are giving me the varied opinions I wanted. Yours in particular helps because you did the same thing successfully in a less-stable area (of course, your slab may be attached much better than mine). The conservative posts help confirm what I suspected: If I tile across, I am indeed taking some risk.
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