saxmaan,

An unmodified thinset (Masterblend) is fine between the Hardi and the subfloor.

Tara,

The reason for the layer of thinset between the cement board and the subfloor is to fill the voids/gaps between the two. It'll keep the cement board from moving up and down. That long term flexing can lead to problems down the road.

Shame, shame, for building disposable housing.

Mongo

It's unfortunate that this attitude is so prevalent, and not just among DIYers, either.

And then people wonder why their floors don't last.

It's just IMPOSSIBLE to make a tile floor last more than a couple of years!

I actually had a GC say this to me about two years ago. I came in to do a tile floor, where he'd already put down the cement board, and of course, there wasn't a drop of thinset between the CBU and subfloor. When I mentioned this as a problem, that was his reply.

No wonder.

I heard a reply that modified thinset should not be used because if you tear the floor out, it will be hard to get the backerboard up. Is this true. If I use modified, is that technically a better beefy cement board installation? What about Versabond...is that kind of between unmodified and modified?

This might be a strange question...but how do you gut a new installed bathroom floor? The old mud jobs with the wire mesh and tar paper actually tear out with relative ease once you get underneath and get sections to break off. I am thinking, with a new installation, you have thinset and cement board with tons of screws, and then more thinset and tiles. And then you have thinset all over the plywood versus the tar paper and mesh that keeps the plywood nice and clean?

Is it recommended to put tar paper under a cement board installation????

In my limited tiling experience, I followed the instructions exactly. How could one not realize that the thinset helps prevent the Hardibacker from flexing downward and the screws help prevent it from flexing upward?

I heard a reply that modified thinset should not be used because if you tear the floor out, it will be hard to get the backerboard up. Is this true. If I use modified, is that technically a better beefy cement board installation?

It depends on who you talk to as to whether or not it should be used. It's a big debate in the industry. The manufacturers say to use modified, and TCNA who is completely unbaised, says that you're better off using UNmodified thinset for all CBU. Personally, I'll listen to TCNA before I listen to a thing a manufacturer has to say, because TCNA has nothing but the welfare of the industry at heart when they do their r&d. They're sort of the Consumer's Reports of the tile trade. Also, it makes sense. Why use a modified thinset, when the LAST thing you want to do is bond one layer to the other?

What about Versabond...is that kind of between unmodified and modified?

Actually, it's a midrange modified thinset, about equivalent to Mapei's Ultraflex II, Laticrete's 253, or Hydroment's Single Flex.

Is it recommended to put tar paper under a cement board installation????

The ONLY time tar paper should be used under tile is either as a vapor barrier BEHIND the cement board for wall installation around a tub or shower, or as what's called a "cleavage membrane underneath a thickbed mud installation. NEVER under or over cement board on a floor.

How could one not realize that the thinset helps prevent the Hardibacker from flexing downward and the screws help prevent it from flexing upward?

Good question.

IF YOU CARE ABOUT YOUR COUNTRY CARE ABOUT ITS STRUCTURES...people give they're lives for the sake of this country...it's a disgrace to not spend a few miserable bucks for the sake of it.

To assure less trouble upon future removal I have specified best 3/4 " plywood screwed down with lifetime coated screws..with a 5/8" plywood over that..THEN thinset/hardie board. And best honed carrera mosaic marble tile on that..then seal with best stuff.
Even this job is INFERIOR to a tile set of 60-100 years ago.
In a minor country like Lebenon everything..cielings..floors and all are poured CONCRETE.
Don't say America is the "Best Country on Earth"..PROVE IT.

I'm in the process of laying Hardibacker cement board over 3/4" plywood in my kitchen. The manufacturer says to use mortar or modified thinset with 1/4" trowel. Has anyone used mortar between wood subfloor and the Hardiboard?

I have only ever screwed down hardiboard using the required pattern and using 1 1/4" deck screws. I am currently doing a remodel on a bathroom in my own home, that doesnt need it, we aer just changing the dymanic. it took me hours to remove the hardibaord and tile because it was so well secured. I did the orginial myself and have been a lisenced contractor since 1998 doing high end custom homes. The premise that mortar between the subfloor and hardiboard to reduce movement would make sense in an older home with floors that werent level. Floors that are level, with an 8" OC screw pattern will be flat to the floor with zero movement, guaranteed.. While removing the floor, nearly every screw looked brand new so make sure you have a good grout sealer that you apply yearly. This will also help keep your groutlines clean. My biggest concern with mortar under the backerboard is the damage you will cause to the subfloor when the homeowners tastes change in 15 years and want it all removed, just ask my wife. Even if the backerboard breaks its bond with the subfloor mortar you will at the very least need to sand it all back to level. I will add a strip of waterproofing material in front of the tub for at least 6" as the grout line against the tub tends to 'crack' or break its bond against the tub allowing water penetration at that point. But, after 16 year of being installed and having a son that took showers so hot water ran down the walls, almost zero issues even near the tub. The screws here were still intact as well, the subloor itself only looked slightly stained for about 2" in front of the tub. Also, the hardiboard i installed was not the waterproof type they add now which will be a nice addiiton to the process.