Laying Wonderboard on Slab? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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09-22-2001, 05:32 PM
Hi All!

Thank you for all the help you have given us in the past.

We are going to go with the saltillo tile. We need to raise it up to match the height of the hardwood floor. Now the question. What is the best way to put wonderboard on the concrete slab? Height is a problem. We want to add as little height as possible. Right now it should be a perfect match.

Any suggestions on how to attach it?


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09-22-2001, 06:18 PM
Wonderboard, or any other cement backer board is not designed/intended for use on a concrete slab. Why are you considering this application?

09-22-2001, 06:38 PM
We need to raise the tile up to the same level as the hard wood floor. We didn't want to use plywood because of all the problems we have heard of trying to put tile of a wooden surface.


09-22-2001, 06:49 PM
OK, as I understand this, you will be installing Saltillo tile directly on a concrete slab that meets a hardwood floor, which I assume is also on slab. Saltillo generally requires a "thicker" setting bed than a "flat-backed" ceramic tile. You could use a larger notched trowel to apply a 1/2" - 3/4" setting bed, which in addition to providing better coverage on the back of the Saltillo tiles, would also allow you to attempt to match the existing height of the hardwood floor. You would probably need a Medium Bed mortar, rather than a Thinset mortar to accomplish this.

Another possibility would be the use of a self leveling underlayment to bring the slab height up before starting the tile installation. How long is the area of transition from wood - tile where the height difference is a concern?

09-22-2001, 07:01 PM
We have two areas. One is 30" and the other is just over 6 feet. The hardwood floor is finished at 1 5/8" higher then the slab.

09-22-2001, 07:20 PM
The thickness of the Saltillo combined with a setting bed spread with a 3/4" notched trowel would minimize the height difference, but probably not eliminate it. This basically boils down to a judgement call on your end and what you can ultimately "live with".

I don't know what the total square footage is of the area you will be installing the Saltillo in, but I do know the cost of a self leveling underlayment can get "pricey" fairly quickly. You have several options. There will be some type of transition molding between the tile and hardwood, IE. a T-mold or some other trim seperating the wood/tile. You could potentially use a trim piece called a "reducer" at those two locations to smooth the transition from wood/tile. Many of the home improvement stores (Home Depot, Lowes) carry stock pieces that can accomplish this.

If your dead-set on a "same level" installation, you will ultimately have to raise the level of the slab by a self leveling product, or a mud bed screeded to the appropriate height. If your not up to a "mud job" yourself, you might be able to hire a tile contractor locally just for that aspect of the job. Hope this helps a little.

John Bridge
09-22-2001, 07:28 PM
Hi Barbs,

Apparently, the hardwood floor is on one-by (three-quarter inch) sleepers. The logical thing to do is raise the floor with a mud bed (mud screed). The mortar is bonded to the slab and smoothed. Don't know if you're into that. I went over it pretty thoroughly in my book.

There is no approved way of attaching cement backer board to a concrete slab. The idea has been slung around here a little, but I'd hate for you to be the guinea pig.

The other thing that comes to mind is the use of transition pieces or thresholds made from the same wood as the floor. 3/4 in. is not a lot. It's done all the time.

09-22-2001, 07:39 PM
Yes, you have been very helpful. Thank you very much!

We are laying about 450 feet of tile.

Hi John.

The hardwood floor is put down on 3/4 plywood. There is a membrane under he plywood and a vapor barrier over the plywood. Then the hardwood. Is it the attachment of the wonderborad to the slab that is the problem?


09-22-2001, 07:42 PM
We have two areas. One is 30" and the other is just over 6 feet. The hardwood floor is finished at 1 5/8" higher then the slab.

John Bridge
09-22-2001, 07:57 PM
It's just that cement backer board (CBU) was never intended for that purpose, so there really are no guidelines. If I were going to do it I would cement it down with a high-dollar thin set, something like Versa Bond, for instance. It should work. I don't see why it won't. But once again, I just can't recommend it, because I've never done it and don't know anyone who has.


You can put down the Saltillo with a lesser product. Custom's Master Blend is one that I use. You should not use the very lowest grade for anything. You've got to pay about ten bucks a sack for a decent thin set. Or, you have to use latex admix.

09-22-2001, 08:11 PM

Cement and cement fiber boards were designed to provide a tile setting surface superior to wood/drywall without the skill/time necessary to accomplish a mud job. Unlike a mud job, these CBU's are totally dependent on the existing structure to which they are applied to provide the support necessary for a tile installation. They contribute little or nothing to the structural integrity of the installation.

All installation instructions require the use of mechanical fasteners (nails/screws) in addition to any leveling/laminating material used between the CBU and existing structure. Mechanically fastening to a concrete slab would be a costly/time consuming effort that would potentially damage the boards and would not have the endorsement of any manufacturer that I'm aware of.

09-23-2001, 11:04 AM
what about using fat mud. might be a little difficult for a first timer! sounds like you need about 5/8 - 3/4 " of build up.3/4" is pushing the limit of medium bed thinset!

09-23-2001, 10:01 PM
Thanks for all the advice. You guys are awesome!! Much as I'd like to make tile laying history we've nixed the Wonderboard idea (seemed like a good thought....we've used it before laying tile on wood foundation). We'll just set the tiles as per Johns' instructions, and do some creative work on the transitions. My woodworker husband is looking forward to the challenge.
LDavis suggested using a medium bed mortar rather than a Thinset mortar. Any preferences as to brand name? We'd rather pay more now and have it last. Besides, I don't think anyone could talk me into doing this remodel again!
In humble gratitude,

Rob Z
09-23-2001, 10:34 PM
Hi Barbs

I you have access to H____ D____, try the Custom brand Marble and Ganite mortar. It is a medium bed mortar, and costs (I think) about $25 a bag. You'll need a 1/2 x 1/2 " trowel or a 3/4" U notch trowel. Try, if H____ D_____ doesn't have any.


John Bridge
09-24-2001, 04:03 PM
Hold it. Wait a minute. I don't want to go up against my friend Rob, but he, being in Yankee Land, doesn't have much experience with Saltillo. It's just that the stuff consumes massive amounts of thin set to install, and medium bed will burn a huge hole in the budget. In a normal Saltillo installation you can expect about 30-35 sq. ft. coverage per sack. L. Davis is not in Yankee Land, but I don't think he realized how much that medium bed stuff costs. Hell, it's damn near as much as the backer board!

Use the Master Blend (gray) from Home Depot. Prime the concrete with latex. You can buy a gallon and spread it around with a sponge, and let it dry.

The Master Blend costs about $9 per sack and will work just fine. When you get ready to do the tile you'll need to ask a few more questions.

09-24-2001, 04:15 PM
Hi John

What does the Latex do?

John Bridge
09-24-2001, 04:57 PM
It helps the thinset grip the concrete. What you are trying to do is beat the system. As you know, thinset (being concrete made from portland cement) forms a "mechanical bond." It oozes into the pores of the slab and grabs on by simply getting hard. The latex forms a "chemical bond" that you would otherwise not have. It's sort of like mixing latex in your thinset, but not quite.

I'll probably get run in over this one, but I've been doing it for years, and I didn't invent it. I know others who do it also.

Bud Cline
09-24-2001, 06:34 PM
Your not alone John, I've been using latex concrete bonder for decades. The damned stuff works, and will allow you to feather garden variety sand mix if you paint the bonder on the surface and also add a little in the mix.

09-24-2001, 06:52 PM
I use bonding agents too.Especially if concrete is a bit too smooth for my liking.The Saltillo tiles are so absorbant on the back that you get a good mechanical bond with any thinset anyway.However,I always use the same $17 thinset for almost everything.But with Saltillo you don't have to use such an expensive mortar to get a good grip on the tile.

Medium bed mortar is probably better technically and scientifically if you were to get really analytical over it.Unfortunately,the stuff is too expensive and rare.

Occasionally I use a white marble and granite mortar that claims to be a medium bed mortar but it is a fine powder and way too sticky.Would think that if it claims to be a medium mortar that it would have a coarser aggregate.I only use this for polished marble and granite on walls.

09-24-2001, 07:22 PM
I guess what I was getting at was...does the latex do anything if it isn't mixed with portland(slurry)?...I mean latex will melt if exposed to water by it's self...doesn't it?

09-24-2001, 07:31 PM

I agree the medium-bed mortars are expensive, I've started using them exclusively on all the larger (16" and larger) floor tile jobs I've done recently. I generally use a minimum 1/4 x 1/2 notch trowel with these tiles and have been very impressed with the medium bed mortar on these jobs. The tiles are easier to level and don't seem to have the sag/shift problems I seem to fight when using standard thinset with these larger tiles.

According to the "thinset technos" the strength of thinset falls off rapidly in thicker cross-sections. I'm sure you have set more Saltillo than I have over the years and if you've been satisfied with the results and those methods, thats good enough for me.

I still don't understand why the medium-bed mortars are so much more expensive. As I understand it, the primary difference is larger aggregate. Again, I wonder why larger sand grains cost more than smaller sand grains?

09-24-2001, 08:10 PM

(This is Barb's Husband, though I don't now how long if I don't get this stuff down!)

You have been most helpful!!! Thin set, meduim set, fat set... next I'll hear about the L.A. Jet Set. Seriously, you have answered more questions than I would have even thought of asking. And then have even more questions. Just one more question for the moment:

We have gotten the pre-sealed Satillio tile and am wondering if I can throw them (very gently) in a tray and pre-soak them for a few hours before setting them down. I have heard that you can and can't soak pre-glazed.

Thank you again for all the help that you have given us,


09-24-2001, 08:22 PM
Good question.

Presoaking is a good and time-tested method.However,I've always just run a really wet sponge on the backs individually directly prior to setting the Saltillo tile.

You're trying to prevent the absorptive back from soaking up the moisture out of the mortar(which will weaken the mortar;lack of moisture).The longer the moisture stays in masonry products,the stronger the mortar will get.

Bud Cline
09-24-2001, 08:58 PM
Hey John, what's a "pre-sealed" saltillo???

What are they sealed with and can you safely soak them suckers?

John Bridge
09-25-2001, 04:59 PM
I've set the pre-sealed only once, and I didn't soak them. I was afraid the excessive moisture would be trapped by the sealer . . . and I still am. I would not soak the pre-sealed. I would keep the thin set mix a little on the wet side and go for it.

I hesitate to further confuse things here by going into the properties of the medium bed mortars. In fact, I won't.

Get some help, Curt. It's a big job.

09-25-2001, 08:36 PM
Yes, John, it is a big job......But WE CAN DO IT!! One of the big problems with the internet is the wealth of information that can inform, but also frequently confuses. It's so wonderful to hear about all the different opinions, and we've learned alot just from reading the discources on this board. Eventually, though, one has to concentrate on the basics, obtained from all sorts of sources (including your book), which in our case amounts to:
1) Latexing the slab
2) NOT soaking the tile
3) Using the Grey Thin Set from my favorite store
4) Laying the tiles carefully so as to insure coverage
5) Worrying about the transitions when we install the hardwood
6) Have fun!
We're going to have a go at'll be fun! We'll send you pictures when it's done.

John Bridge
09-26-2001, 03:42 PM
Barbs, Curt,

Send us pictures during the course of action. And ask any additional questions that might come up.

The only thing that wasn't mentioned is "back buttering." Spread your thinset on the floor and place the tile down all the way. Now pick the tile up and butter additional thin set in the back of the tile where needed. You'll see "where needed." You can also do a little adjusting for height with thin set, but do it prudently. The tiles are rustic. Leave 'em that way.

Rob Z
09-26-2001, 09:39 PM
Barbs, Curt

Have fun!