SLC vs. backer board [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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09-11-2015, 11:33 AM
I am midway through a project. Old house, uneven floors, tiling a kitchen currently. We have ripped up the multiple old floors down to the subfloor, replaced all old boards and reenforced with 1/2" plywood. The house is on pillars so the floor was not level and we chose to pour SLC on most of it to level out.

1. Do we need cement backer board over the SLC or can we just lay the tile straight on the dry SLC (with mortat of course)?

2. Will a fresh layer of wet SLC adhere to an already dry layer of SLC?

3. Can you screw through SLC if we are putting backer board on top?

Thanks so much!!

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09-11-2015, 12:47 PM
Welcome, ccolt. Please change that permanent signature line to a first name for us to use. :)

We'd need to know more about your floor structure to give much good advice.

If you'll enter the required information on your floor joists into the Deflectometer in the dark blue bar above you can get an initial go/no-go reading on whether you qualify for a ceramic tile installation in that regard.

Then we'd need to know what you have for a subfloor and what you mean by "reenforced with 1/2" plywood."

1. We'd need to know just how the SLC was installed to know if you can tile over it directly, but for sure you cannot install a CBU over it.

2. That will depend upon the specific SLC you used. Most times the answer would be yes, but with specific requirements for the first pour and the application of a specific primer before the second pour.

3. See #1.

My opinion; worth price charged.

09-11-2015, 01:27 PM
OK will do CX. My name is caroline
I calculated my deflection and while I'm not entirely sure the numbers I put in were right, It came back at 348 which apparently isn't high enough for ceramic tiling. What are the risks that I take if I move forward with the tile installation?
What are my options?
The subfloor we have built up is about 1 1/4" thick, so providing quite a bit of support.

There is some "sistering" of beams which I noticed down there, so i the deflection rating might be slightly higher with that reenforcement.

In my research I hadn't read anything until today about checking the deflection of the floor. At this point I've done a lot of work to get here so I'm not sure I want to change the direction of my plan. But, I also don't want to be stubborn and waste money.

You said I should definitely not put backer board over the SLC right? Uh oh, that's what I was planning on doing today. I guess it's time for a trip to home depot to return that stuff. Is my best bet to buy more SLC and pour it over the entire area and then start with the tile laying once the SLC has dried?

09-11-2015, 01:28 PM
Oh by the way the initial layer of SLC was just poured over the latex sealed plywood. We did not put town metal lath first.

Also, wondering if now we should return all our CBU and go with Ditra as an underlayment? I know it's more expensive but it doesn't requiring screwing down like CBU. Is that the issue here? That we can't screw through CBU?

09-11-2015, 05:45 PM
If you were to tell us more about your floor structure, Caroline, it would certainly help.

There are two areas of deflection of concern in a wood framed floor to receive ceramic tile. One is the deflection of the joist structure, the other is the subfloor deflection between the joists. Both are important to a successful installation, and they are mostly independent of one another. To help you at all we need to have very specific information on both. Some photos of what you have, and what you started with, might be helpful. Use the paper-clip icon above the Reply dialog box to attach photos from storage on your computer.
Oh by the way the initial layer of SLC was just poured over the latex sealed plywood. We did not put town metal lath first.1. Was the "latex" sealer a product recommended by the SLC manufacturer?

2. What was the SLC you used? Brand name, make and model.

3. Did the the SLC manufacturer specify the use of metal or plastic lath when pouring over a wood framed floor?

4. Did the SLC manufacturer specify a minimum thickness for the product when poured over a wood framed floor? Did you pour that thickness?

09-11-2015, 07:17 PM
Sorry this is my first post so I didn't know what information would be pertinent.

House was built in 1960 on pillars over crawl space.
Joists are unknown wood in good condition, 5.5" deep, 24" apart, 8' long and 1.5" wide.
Subfloor is 3/4" plywood with another layer of 1/2" plywood glued and screwed on top.

Over time the floor has shifted so there is a high spot in the middle and it pretty much goes down from there on all sides (the pillar is under the high spot)

I think we have determined that the floor is just not stable enough for tile, and I'm ok with that now. If you disagree please let me know tonight, otherwise I will be laying down another layer of levelquik and then probably finding another flooring solution tomorrow.

Thanks for your time and recommendations.

09-11-2015, 09:09 PM
Caroline, even if you're planning to use some other floor covering, I really wish you'd at least visit Custom's website for that LevelQuick product. (

They do require a specific metal lath for use over a plywood subfloor and have specific thickness requirements. You'll wanna try to comply with all their recommendations to make that product work properly.

My opinion; worth price charged.

Tiger Mountain Tile Inc
09-11-2015, 09:53 PM
Hi Caroline and welcome to the forum. :wave:

Cx is right. The self leveler needs a primer and the lath for going over a wood floor. There's a very realistic danger that it will fail at that layer. If you install tile over it you could be throwing your money away.

You could try it and maybe nothing happens but we're just trying to help you have a good chance at a successful installation.

Let us know what you decide to do. :)