I'm going to expand a bit on my last post rather than editing it.
When you hire a pro, you expect he will follow a set of rules for his profession. In NA, and with tiling, that implies conformance to the TCNA guidelines.
TCNA does not endorse products or call specific products out in their guidelines, but either explains what is expected for a specific situation OR defers to the manufacturer for the installation of the product. So, in summary, you either follow TCNA guidelines, or the manufacturer's instructions, and if you do, you comply.
Now, what does the TCNA have to say about medium bed mortars?
"Medium bed mortars are designed to minimize slump and facilitate thicker bond coats, as compared with non-medium bed mortars. These characteristics make them useful for setting heavy tiles and/or tile with ungauged thicknesses and for setting tiles with at least one side greater than 15" where the final embedded thickness of the mortar will exceed 3/16" under the tile. They are intended to be used as bond coats 3/16" to 3/4" thick after the tile is embedded; they are designed as direct bond adhesives and (my emphasis) not intended to be used as truing or leveling underlying substrates or the work of others.
These mortars do not have unique ANSI or ISO standards to characterize them."
So, let's examine John Whipple's plan to pick one product over the other: in post #2, he calls out the use of a medium bed mortar to set tile over an uneven surface. This disagrees with both the TCNA definition and use of a medium bed mortar AND the manufacturers.
What does Laticrete say about the use of their Granite and Marble Medium Bed Mortar under their prep section (all are pretty much the same):
"All slabs must be plumb and true to within 1/4" (6 mm) in 10 ft (3 m). " (Note, this also conforms to the TCNA guidelines.)
What does Laticrete say about the use of their Strata_Mat:
"Floor must be leveled per TCNA/ANSI and/or specification requirements prior to the installation of Stata_Mat." (from page 3 of their installation instructions)
Nowhere does the manufacturer allow you to level the floor with the mortar, and specifies the floor should be flat before you install Strata_Mat - John's grade - FAIL
FWIW, you'll find the same instructions on the use of Ditra...John's grade - FAIL
John asserts Ditra-Set cannot be used as a medium bed mortar.
- The manufacturer calls it out as both a thinset and a medium bed mortar on their product description pages. Note in the mixing instructions, they have a fairly wide range of allowable water. Mixed at the low end, it acts like a medium bed mortar, mixed at the high end, it is more suitable as a thinset.
- Neither TCNA nor ANSI have a specification for medium bed mortar, so if a mortar meets the general guidelines (included above in the TCNA definition of a medium bed mortar), it is.
- Bostik gives examples of proper trowels for setting Kerdi and Ditra in their data sheet, but also says a larger trowel may be required to set some tile to provide proper coverage. They do not have a tile size limitation. They also do not list a maximum thickness of the mortar. Considering they call it out as a medium bed mortar, general guidelines would limit that to a max of 3/4". In one document, they suggest a 3/4" U-notch trowel to accommodate uneven (ungauged) tile to ensure proper coverage - that size trowel is WAY beyond what a thinset should be used with, supporting their claim to be a medium bed mortar when properly mixed.
So, to John Whipple's assertion that Ditra-Set cannot be used as a medium bed mortar - FALSE!
What should we expect from a pro:
- he knows and adheres to industry standards
- he has enough skill to meet those standards
- he uses products as they were designed to be used as defined by the manufacturer for the area of use
How does John Whipple stand up to these guidelines? At least in this thread, and his assessment, and reported use of the products, FAIL on at least two of those listed above. I believe he has enough skill to meet those standards, but he chooses not to.
Now, John will come back and say I'm not a pro, don't do it for a living, and nobody should listen to me because I'm a wanna-be hack. I'm not asking you to believe me, you can read the TCNA handbook and the manufacturer's installation instructions yourself. Then, ask yourself, should a pro, someone you are paying to do the job, use the materials as designed and as the TCNA says they should be used, or what he feels like it should be? When you take your car in for service, you expect them to use the right materials for the job, and to install them properly. Shouldn't you expect the same with your tile contractor?