Short answer: use the thinset your membrane or tile maker names. Unmodified thinset is a cement powder you mix with water. Modified thinset already has polymers in the bag to help it grip and flex a little. Neither one is right for every job: Schluter, for example, makes SET (unmodified) and ALL-SET (modified) for its DITRA and KERDI membranes.
Unmodified thinset is mostly cement. Laticrete 317 is "a bagged cementitious powder... mixed with either water or latex admixture". PremiumPlus lists "High cement content".
Modified thinset has the polymer already in the bag. VersaBond is a "high bond strength, all-purpose, polymer-modified mortar". Mapei Ker 121 is "polymer-enriched for deformability". In plain words: it can flex a little.
There is also a middle path: an unmodified mortar mixed with a liquid additive instead of water. PremiumPlus meets ANSI A118.1 on its own, and A118.4 and A118.11 "when mixed with Thin-set Mortar Admix". Laticrete 317 is fortified with 3701 Mortar Admix or 333.
Use the mortar your membrane's installation handbook names. On FloorBox, the Schluter handbooks (DITRA, DITRA-HEAT and Shower System, in English and French) are under Documents on the Schluter SET page.
So modified thinset can go on some membranes: Schluter makes one of each. Follow your own membrane's list. The PremiumPlus page also says: "High cement content for use with uncoupling mats and membranes."
New to membranes? See What Is an Uncoupling Membrane (Like Schluter DITRA), and Do I Need One Under Tile? and our Uncoupling & Heated Floor Membranes category. For showers, see Is Cement Board Waterproof, or Do I Need a Membrane in My Shower?
Check the mortar page for your job. Here is what our product pages say:
| Job | What the FloorBox page says | Source |
|---|---|---|
| Porcelain, mosaic, glass or stone tile | 317 "must be fortified with 333... or 3701 Mortar Admix" | Laticrete 317 page |
| Outdoor | 317 must be fortified for exterior use. VersaBond is "for interior and exterior floors and walls". SET lists Outdoor Usage: No | 317, VersaBond and SET pages |
| Heavy traffic areas | 317 must be fortified | 317 page |
| Exterior grade plywood or dense tile | PremiumPlus: use Thin-Set Mortar Admix | PremiumPlus page |
| Large and heavy tile | SET and ALL-SET both list "large and heavy tile" | Schluter SET and ALL-SET pages |
| Schluter membranes | SET (unmodified) or ALL-SET (modified) | Schluter SET and ALL-SET pages |
Browse Mortars and open the Documents tab for the technical data sheet.
Not sure which one your membrane or tile calls for? Contact us.
Can I use modified thinset over Schluter DITRA?
Schluter makes both: SET is unmodified and ALL-SET is modified, and the ALL-SET page says it is "engineered for use both under and over all DITRA and KERDI products". Follow the DITRA handbook under Documents on the SET page.
Can I turn unmodified thinset into modified?
Some, yes, with the maker's additive. PremiumPlus meets ANSI A118.4 when mixed with Thin-Set Mortar Admix, and Laticrete 317 is fortified with 333 or 3701 Mortar Admix.
Which thinset for porcelain tile?
Check the mortar page. Laticrete says 317 unmodified must be fortified for porcelain. Schluter SET and ALL-SET both list porcelain.
Can I use thinset outside?
Only one rated for it. VersaBond is for interior and exterior floors and walls; Schluter SET lists Outdoor Usage: No.
What does ANSI A118.1 mean on the bag?
It is the standard listed by unmodified mortars on FloorBox, such as Schluter SET and PremiumPlus. Modified ones list A118.4 or A118.15.
How much area does a 50 lb bag cover?
It depends on the product: PremiumPlus lists 90 to 110 sq ft and VersaBond 60 to 95 sq ft.
