Modified Thinset – A Brief History

by Roger

Laticrete 3701What is the difference between modified and unmodified thinsets?

Which unmodified thinset is better?

What makes a modified thinset modified?

Why do you drink so much beer?

These are questions I get asked a lot, along with ‘why is my dog on fire’ (because you used the incorrect product for a specific installation).

Unmodified thinset is simply a thinset which does not have any latex polymers or other products added to it. It is essentially portland cement, silica (sand) and lime. Recipes vary, but those are the basics.

History (pay attention – there may be a quiz…)

To understand modified and unmodified you should understand why modified exists. Way back in the 1940’s Henry M. Rothberg was a chemical engineer. Back then the standard installation procedure for floor tile was the full bed method. This was a 2″-3″ deck of portland cement and sand upon which tile was installed. The need for the thickness is at the heart of the development of modified thinsets.

It needed to be that thick in order to retain enough moisture for the cement to fully cure.

Concrete cures through a process called hydration. The cement chemically reacts with water and grows interlocking crystals – these interlocking crystals are what gives cement it’s hardness. These crystals grow as long as they are exposed to moisture (water). Once the water is gone the crystals slow and stop. So…

The longer the cement is exposed to moisture the longer the crystals grow. The longer the crystals grow the more they interlock. The more they interlock the stronger the concrete.

It’s all about moisture retention in the mix. Enter Latex (or rubber). Rubber was added to concrete mixes in the early 20’s to repair and solidify sea walls, and later added to brick mason’s mortar to make brick installations stronger. Adding rubber or latex to cement mixes helped the mix retain water for longer periods of time.

Mr. Rothberg realized that the common tile installation methods at the time had limitations. He set out to find a way to solve this. He began experimenting with the natural latex which was being used in sea walls and brick work but soon realized the these products had limited working time and were difficult to store for any amount of time before degrading.

He then set out to develop a synthetic form of latex which would be easily stored for longer periods and had an extended working time in order to be feasible for tile installation purposes. After developing and testing more than 300 different chemical compositions of synthetic latex rubber he finally found one that met his criteria.

It was introduced to the market as ‘Laticrete’. It was a liquid latex polymer which was added to concrete mixes to make them stronger and give them some flexibility.

That’s right, my favorite company actually has a story. :D The name is a pseudo-compound word formed from ‘latex’ and ‘concrete’. This was the name of the synthetic polymer Mr. Rothberg created for use in tile installation products in order to retain moisture in the mix and allow it to cure stronger and not be as brittle.

In the 1960’s the (then) Tile Council of America developed a powdered thinset with dry polymers which were activated by adding water. Soon afterward it was used by nearly everybody for nearly every installation. This actually led to a lot of problems, mainly due to misunderstanding of the product and it’s limitations – it was being used for everything with unrealistic expectations.

The latest modified thinsets have come a long way from the original TCA types and are now tested to minimum standards in an attempt to keep expectations realistic. Powdered or liquid polymers added to regular thinsets help the mix retain water for a more durable end result, as well as adding flexibility, bonding power, or any number of specialty capabilities needed for the numerous installation requirements.

Any thinset that has either a powdered or liquid latex polymer added to it is considered a modified thinset. Any thinset that does not contain these is an unmodified thinset.

This post began as a description and information on unmodified thinsets and which are better. I realized very quickly that this was not a subject that can be easily explained in one blog post. It can – but you’d get bored. So my next post will deal with that topic now that you know why modified thinsets exist and what they do.


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I laid 6×24 tiles on a concrete floor using AcrylPro from Custom. After 96 hours (label says 24-72 hrs dry time), it was still not dry. I pried the tiles up (seriously, I was able to pry from one end of a 24″ tile and it came up without breaking), and except for a tiny bit at the edges of the room, the adhesive was just as wet as when it came out of the bucket. Obviously, AcrylPro was not the right product for this job (though nothing on the product labeling indicated it was not appropriate). So what should I have used? Is there a guide for which thinset to use for which project? (I should also mention that there is thoroughly hardened dark brown–almost black–glue from original 1979 vinyl tile installation. It is not coming up without grinding the floor.)


George Sulllivan

When installing Old world Veneer stone what type of mortar or modified thinset mortar is suggested ?



Building brick curb base in basement over asbestos tile. Any advice on adhering brick to asbestos? Also, is metal lath needed over brick?
Thank you.


Terence Murphy

Will modified thinset adhere to a concrete floor that has vinyl tile glue on it. Does it need to be stripped? How? Or coated with something like plaster weld or latex


DJ Private

Thank you for the great educational article.
We have to consider the Saltillo tiles over the concrete covered with Tuflex (Urethane waterproofing membrain.
Is there any setting product ( thin or medium set) can be considered?
Mechanical or chemical removal of the waterproofing is really not desirable.
The other options would seem to be adding a thin bed of lath reinforced deck mud or Dirta and then going with simple unmodified for Saltillo.
Any advice is hugely appreciated!



Hi Dj,

The waterproofing HAS to be removed. It is an impervious coating, that’s what makes it waterproof. Unfortunately since it is not a tile substrate specific waterproofing that means that it will also prevent any type of thinset from bonding to it. You can go over it with a deck mud layer, but it needs to be a minimum of 3/4″ if reinforced with lath.


Bruce Zeigler

I would like to know What type of modified thinset should I use over ditra to put 12 x 24 tiles in a bathroom.



Hi Bruce,

Schluter requires that you use NON-modified thinset over ditra to maintain the warranty. If you want to use modified mapei ultraflex 2, laticrete 253 or Customs versabond all work well.


Bruce Zeigler

Thank you for your fast response!


Bruce Zeigler

You are right, the modified thinset goes under the ditra on the plywood sub-floor. You use the laticrete 253 under the ditra on plywood correct? Thanks for your help!





Joe the Stack

About two weeks ago I taped some backerboard joints on a wall using backerboard tape and thinset. Today I was cleaning the area with a wet rag, and I noticed thinset was coming off onto the rag. I kept wiping, and I had to rinse the rag 5 or 6 times before it stopped coming out dark grey, and I could see that some of the thinset on the joints was being removed. Other joints that I had taped the same way several months earlier did not react to the wet rag. Is this because the cement in the thinset is not fully cured after only two weeks, or has my bag of thinset exceeded its shelf life, or maybe was there too much or too little water in the mixed thinset? Or is it that thinset on bare backerboard left exposed doesn’t retain quite enough water to fully cure? If I had tiled the next day, I never would have noticed this, and maybe it wouldn’t have mattered because the thinset in the taped joints would be covered by the thinset adhesive layer and the tile, which would be a much moister environment compared with exposed to the air and thus would cure properly. I don’t mind buying a new bag of thinset if that is the answer, but all of the above is only speculation on my part. Actually I have done some reading on tilesetting, thinset, and concrete, but I couldn’t find the answer to this anywhere. Your thoughts? Thanks!



Hi Joe,

My guess would be that, since it was exposed, the backer sucked moisture out of the thinset. Since it does this from the back of the layer (against the backer) the front of the layer – what you are wiping – simply did not get enough moisture to cure fully. If you had tiled immediately afterward, or soaked the backer down before it probably would not have done that. The good news is that the cement that is still on the front of the layer can still utilized moisture from what you are going to install over it, it isn’t dormant. You’ll be just fine tiling over that.


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