It's been so long since I used mastic I'd forgot they still sell it. An advantage thinset has over mastic is where any amount of inconsistency,unlevel or imprefection exist's. Not many floors are perfect in all those. Besides, I always wind up with dried mastic every where but behind my ears.

Thanks for the advice Sophie. It's a small bathroom so I want to try to do it myself. Worse case I'll have to get someone to redo it.

I will definitely have to get the medium bedding mortar made for large tiles and use that to set them. Does it matter what I use between the plywood subfloor and cement board when attaching it to the subfloor. I was told to use the thinset/mastic under the cement board as well. Thanks again.

Under the floor tile you need waterproofing. This can be a painted-on product or a waterproof membrane.

From lowest layer: plywood subfloor, thinset, hardibacker screwed down into the wet thinset, thinset, tile.

No mastic.

Casey: waterproofing floors is optional.

mfran: I'd highly recommend using Ditra instead of Hardi. It's way easier to install (cut with a utility knife), and better technology.

The very first thing you do is verify deflection and how it relates to chosen tile material's rating. You must look at the whole thing from structure> Joist> underlayment> subfloor before even talking about cement board and crack isolation. You are asking such novice questions id be careful and do A LOT of homework. DIY is great and you may pull it off but get the correct process and information involved so you dont waste your time and materials. These forums are loaded with bad information more added daily.

Thanks for all the advice and tips. I've been doing plenty of research off the manufacturers websites. Lots of good information I feel I can trust. Not so much about what you read on the internet or get told in the store. Had two "tile and flooring" employees tell me exact opposite things at the same store in the same day!

There you GO. MFG., TCNA, ANSI, I.R.C. Its all you need other than hands on time honed skills.

"I've been doing plenty of research off the manufacturers websites." Yet you have selected Hardie board and you are here asking how to install it. See my point?

Hiring even an expensive Pro now will be cheaper than the rot and mold remediation issues guaranteed to be in your future. You would be out of your depth on a kitchen backsplash. Much less a mission critical shower.

Why would anyone not want waterproofing under their bathroom floor tile?

@sombreuil

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