ThinSet Help: Modified vs. Flexbond vs. Fortified [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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Zach from Oregon
02-11-2015, 01:48 PM

I have another unique tile setting project and I need help picking the correct thin-set mortar to use for my job (see below). My questions are- what type of thin-set do I use UNDER the backerboard (modified?) and what type of thin-set should I use ON TOP of the backerboard knowing that I am using 1/4 inch instead of 1/2 inch because I can not raise my floor too high. I was advised to use flexbond on top of backerboard to provide a little more 'flex', but flexbond is twice the price as other thin-set types. :confused:

My Job:
I setting tile in my kitchen. My current kitchen floor is
sheet vinyl that is glued to particle board. The particle board is is
in good shape and firmly attached to my subfloor. I assume they
installed a layer of particle board on top of the sub floor in order to
have a smoother surface on which to install the sheet vinyl.

I want to leave the vinyl installed if I can for two reasons 1) If I
tear it up it could pull up some of the particle board with it and 2)
There is a very good chance there is asbestos in the sheet vinyl.

I have been advised that it is fine to tile directly on top of the existing vinyl. if using 1/4 backerboard. If I use 1/2 backerboard there is a good chance my dishwasher and fridge will no longer fit under their respective cabinets.

Thank you for your time and advice,

Zach (from Oregon)

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02-11-2015, 02:33 PM
Hello, Zach. Welcome to the forum. :)

Do you want the bad news first or the really bad news first? :D

Sorry to say the vinyl has to go, and even more importantly the particle board underneath it has to go as well. It's just not stable enough for tile to go over it. It swells very easily with exposure to water, and will often end up several times its original thickness.

How long has the vinyl been down? How old is the house?

Zach from Oregon
02-11-2015, 03:19 PM
The house was built in the 60's, and the sheet vinyl has been around for a while. I don't think it is original to the house, but it is old enough to have asbestos.

Would applying a coat of a water proofing agent like Red Guard on top of the vinyl help at all?:cry:

02-11-2015, 03:29 PM
Not really, and even if you did, you'd be puncturing it many times with the screws used to secure the cement board.

Particle board was widely used in the 70's as a flat underlayment for carpet and vinyl, and back then we didn't know that it was as crappy as it is. I've torn out a dumpster load of it, several times over, and in every single instance it's had some measure of moisture get to it, causing it to swell up. Your new tile won't be able to tolerate that.

This is your floor, and you may choose to tile over it as is, but I wouldn't do it on a dare. Trust me, get rid of the particle board. It'll only cause you problems.

02-11-2015, 04:06 PM
As to the type of thinset to use, most, not all, cbu manufacturers call for an unmodified thinset underneath the cbu. It is there primarily to provide 100% support, the fasteners are what really hold it in place so there's no real bonding required. But some of the cbu manufacturers DO want a modified.

As to setting the tile, over cbu, it also depends, but most tile manufacturers require a modified. There are several levels of modified indicated by the standard that they abide by: (A118.1 is unmodified) A118.4, A118.11, and A118.15, with the successively higher number indicating a more robust set of capabilities. IN many circumstances, you do not need more than A118.4. Because the higher the number, the more complex the additives, they cost more.

If you're trying to minimize the buildup, you might consider something like Strata_Mat or Ditra instead of cbu in the first place. When using Ditra, the manufacturer calls for a premium (not a price leader) version of A118.1 mortar on top, and potentially a modified underneath (depends on the substrate).

Modified, flexbond, fortified, all are modified thinset mortars with a trade name thrown in to confuse people. The real differentiator is what standard do they pass. Each higher number standard passes all tests for those lower numbered ones, and more.

Zach from Oregon
02-11-2015, 04:48 PM
Thank you Kevin and Jim for the feedback.

Ripping out the vinyl and particle is not an option (at least at this point in time). Any recommendations on what can be safely installed on top of the existing vinyl? I am guessing my options are limited to more vinyl? I saw an option with thick vinyl tiles that you could spread grout in between. Even so, I assume I will still run into problems with particle board and moisture bubbles at some point?

Remodeling projects are oh so fun! :jack:

02-11-2015, 06:35 PM
As was said, particle board is not a very nice substrate - it blows up like a sponge when it gets wet. But, it's your house. IF I were to consider this (and I probably would not!), when installing over vinyl I'd follow the Ditra installation instructions starting on page 9 but, if you read the restrictions, your subfloor must adhere to one of their approved, tested installations, and particle board is NOT in there! I'd then go to the added effort to waterproof the seams and up the walls a few inches in hopes that that would keep any moisture out of the particle board. This would NOT get you a warranty, as it's not an approved method, but it's your house, your choice.

Another layer of vinyl or maybe linoleum (greener) would work. I've not had any experience with the vinyl tiles so don't know how reliable they are long-term.

02-12-2015, 09:08 AM
It sounds like concern that the floor covering may contain asbestos is driving the boat on getting a solid substrate in place and proceeding with the project. If that's the case, you might consider having it tested to confirm or alleviate your suspicions. To do so, carefully razor cut a quarter size piece from an inconspicuous spot (under fridge or other appliance), place it in a ziploc bag and take it to local lab for testing. Most labs charge about $30-$50 (depending on turnaround time) to test the sample. Peace of mind can be a good thing. Obviously, proceed with appropriate caution. Also, I'm not sure but it strikes me that driving screws through the linoleum floor covering might not be a good idea if you don't know whether it contains asbestos.

02-12-2015, 09:17 AM
Even if the existing material is friable and even if it does contain asbestos, driving screws through it to fasten some layer above it is not gonna release any of the material into the local atmosphere. I wouldn't worry about that at all, Chris.