Plywood over hardie for a subfloor?

[Mandefense]

Forum members,
I am a newbie and was refered to this forum by a member from another forum I use. I am really excited about this site and wished I would have found it sooner. I have a new addition with engineered joists spanning 16'oc and the length are 20'. (I could not use the deflecto but I have my specific joist weight info if needed.)I have 3/4 t&g subfloor. Now here is the problem I DID NOT research enough before I put down 1/4 " hardie on the subfloor thinsetted and screwed every 8". I want to use ditra for the tiles more than the hardie. Now schluter informed me it voids the warranty to put ditra over cement board. I thought cement board was stronger than plywood for a subfloor (I now know that is wrong) So do I rip up the hardie and put the plywood then ditra, or can I put plywood over the hardie then ditra. I really like the ditra product. Or am I crazy and should just tile over the cement board. I would like your thoughts please. Thanks again for any help. I searched the forum but couldn't find this problem in any threads, if there is another thread similar I am sorry and could you please direct me to it. Again thank-you for any help!
Matt

[cx]

Welcome, Matt.

Originally Posted by Matt

engineered joists spanning 16'oc and the length are 20'.

I'm gonna presume you mean your engineered joists are spanning 20 feet and are installed on 16-inch centers. Only way to determine the deflection of that joist system is to consult the manufacturer or his online published span tables.

But it's generally safe to assume the structure was engineered to at least the code-required L/360 and with a span that long almost as safe to presume it was designed to L/480.

Presuming further that you intend to use ceramic tile, your joist structure is adequate.

The 3/4" subflooring, properly installed, meets the minimum requirements for either Hardibacker or Ditra. Which you elect to use is up to you, but removing the Hardi at this point leaves you with damaged subflooring upon which to install your Ditra. I'd want another layer of plywood over such subflooring, but, then, I'd want another layer of plywood over any single-layer subfloor.

So. Presuming your Hardibacker has been installed per manufacturer's instructions, unless you're prepared to remove the Hardi, properly clean your subfloor of all the remaining thinset mortar from the Hardi installation, and install another layer of plywood, I'd just tile over what you have.

My opinion; worth price charged.