Which ANSI thinset class to use by substrate and exposure (A118.1 / .4 / .15 / .3 / mastic)
Substrate and exposure pick the class. The classic trap: over a sheet membrane use UNMODIFIED — modified mortar can’t cure sealed between two impervious layers. Mastic is interior-dry small wall tile only.
What this diagram shows
A selection guide matching the job to the ANSI mortar class. Over a Schluter Ditra or Kerdi sheet membrane, use A118.1 unmodified, because a polymer-modified mortar cannot dry or cure when it is sealed between two impervious layers. Over cement board or plywood, use A118.4 modified general-purpose mortar. For large-format tile, gauged porcelain panels, exterior, or wet areas, use A118.15 or an LFT mortar for the highest bond and non-slump performance. For pools, chemical exposure, glass, or green marble, use A118.3 epoxy. A136.1 organic mastic is only for interior dry small wall tile — never floors, wet areas, or tile larger than 12 inches.
Mortar / Thinset Calculator
Pick the right ANSI mortar (A118.1/.4/.11/.15/.3 or A136.1 mastic) for substrate and tile, then get bag count by trowel notch. NTCA, TCNA, ANSI A108.
Related diagrams
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