Are you wondering about modified vs unmodified thinset mortars? Perhaps, you are thinking of installing a Ditra-Heat floor or a Schluter Kerdi Shower System?
The internet is full of information and even misinformation on this very subject. In fact, this debate has stoked many of the conversations and threads on the tile and building forums for years.
Therefore, if you’re wondering where this argument currently stands then please read on. There have been some developments in recent years and new information to digest.
Similarly, be sure to check out the main post that I wrote which is an ultimate guide to thinset and helps you decide which one you need for your projects.
Then, head over to this post when it comes time to actually start mixing thinset mortar as well as separate instructions for mixing small batches of thinset mortar.
First, a quick refresher:
Why does modified vs unmodified mortar matter?
The debate matters because of the way the different products cure. Modified mortars need air to dry and unmodified’s don’t. That is to say, the longer an unmodified mortar holds onto its moisture the better.
What is Unmodified Mortar?
Unmodified mortar is defined by its ingredients: sand, cement, and lime. Further, there are none of the modifiers that have become a staple of more modern mortars. 
This type of thinset is also what Schluter Systems has specified for its membranes like Kerdi, Ditra, Ditra XL, Ditra-Heat, and Ditra-Heat-Duo.
The reasons Schluter likes it for their products are answered below.
What is Modified Mortar?
As time has gone on, manufacturers have tinkered with their mortars in an attempt to come out with the latest and greatest product.
Consequently, what they’ve been doing is adding modifiers to their mortars to enhance their strength, flexibility, and open times. It started back in the ’50s by adding liquid rubber to the cement mixtures. [4,5]
Nowadays, the terminology has gone from latex-modified to what it is today: polymer-modified mortars.
Does Schluter want you to use modified or unmodified?
Schluter claims that modified mortars won’t cure properly if they are unable to dry out. Why wouldn’t they dry out? Consider these things:
- Porcelain tiles, which are basically impervious, are getting more and more popular
- The tiles are getting bigger and bigger
- They are being installed over a plastic uncoupling sheet membrane like Ditra
So, you’re installing a big, impervious tile over a plastic sheet membrane with a mortar in between that needs air to dry. How does that work? Schluter argues that it pretty much doesn’t. 
Can you use unmodified thinset on porcelain tile?
Well, this is where things get interesting. See, one of the reasons that mortar manufacturers add polymer modifiers is to strengthen the mortar and help it to ‘grab’ onto the back of a very dense and hard-to-grip porcelain tile.
Further, the companies that make mortars typically WANT you to use modified mortar with the big, dense porcelain tiles.
This is where the conflict comes into play:
- Schluter wants you to use unmodified mortars over their products like Ditra and Kerdi
- Thinset manufacturers want you to use modified mortars when installing porcelain tile
So if you want to install porcelain tile over Ditra, what do you do?
To Each their own
What’s happened is that the different manufacturers are setting their own rules and innovating their own products. For example:
One company got the great idea to market to the Schluter faithful and came out with a product called Ditra-set. This was (is) an unmodified LFT mortar that can be used to install Schluter products.
However, this wasn’t a complete solution as it wasn’t always easy to get a hold of and wasn’t necessarily sold everywhere Schluter Ditra and Kerdi are sold.
Kerdi and Ditra Competitors
Now mortar manufacturers have come out with their own uncoupling membranes and shower systems. For instance, Laticrete has its Hydroban line of waterproofing products and there are now several uncoupling membrane brands on the market.
Furthermore, these companies specify that their modified mortars are to be used with their membranes and it solves the conflict from their end.
Schluter now manufactures their own thinset mortars
Schluter went the other way and started making their own line of thinset mortars and, yes, two of the three are polymer modified.
- Schluter Set (unmodified and LFT)
- Schluter All-Set (modified and LFT)
- Schluter Fast-Set (modified, LFT, and rapid setting)
Further, all are approved for Schluter products with the exception that the unmodified SET not be used to install Ditra over plywood subfloor.
How can they come out with a modified mortar, you ask? According to Schluter, these “setting materials are specifically formulated for use with” their products. 
Now, Schluter will still guarantee the use of unmodified thinset mortars meeting ANSI 118.1 with their products. However, they only offer their lifetime system warranty when using their brand of Schluter mortars. 
Best thinset to use with Schluter Kerdi and Ditra
The table below references some of the thinset mortars that comply with Schluter’s installation instructions. I’ve highlighted my preferred choices in bold.
|Table created by
subfloor & OSB
|Tile 15 inches
|Mapei Kerabond T*
|Custom Premium Plus*
|Schluter SET *^
|Schluter ALL-SET ^
|Schluter FAST-SET ^
Hopefully, this post was able to add some clarification to the unmodified vs modified thinset mortar debate and where things stand in relation to Schluter’s products. For more information, be sure to see my comprehensive post on thinset, the terminology, and which thinset to use with which tile.
Frequently Asked Questions (FAQ)
No. Unmodified thinset meeting ANSI A118.1 only is not approved to bond to plywood. Instead, look for a thinset mortar that meets ANSI A118.11 or an additive to mix with the unmodified that will bring it up to this standard. [6,7]
Although it breaks Schluter’s installation instructions, nothing bad happens when you use modified mortar to install Kerdi. You MAY need to allow for additional drying time.
Yes and no. In fact, Schluter’s instructions say to use an unmodified mortar meeting ANSI A118.1 when installing their products over concrete.
However, Schluter requires a modified thinset mortar meeting ANSI A118.11 when installing Ditra, Ditra XL, Ditra-Heat, or Ditra-Heat-Duo over a plywood or OSB subfloor. 
Schluter SET is an unmodified mortar meeting ANSI A118.1. However, ALL-SET is a modified mortar that meets or exceeds ANSI A118.4T, ANSI A118.11, and A118.15T 
Yes. In fact, Using ALL-SET to both install Schluter products and to tile over those products may qualify for Schluter’s lifetime system warranty. See Schluter’s website for details.
This post will answer your questions about which trowel size to use.
When mixing thinset, Schluter recommends adding up to two additional quarts of water per full bag of AllSet.
You definitely do not want to use premixed tile mastic to install Kerdi or Ditra membranes.
- Bostik Ditra-Set data sheet pdf
- Custom Building Products Premium Plus product page
- Laticrete 272 product page
- Laticrete 317 product page
- Mapei Dryset mortars category page
- Download page for Schluter mortar data sheets, Kerdi and Ditra, Ditra XL, Ditra-Heat, and Ditra-Heat Duo Handbook pdfs