Best subfloor advice - Tile Advice

[Milkbottle]

My client is wanting a loft floor tiled with 600x600 polished porcelain tiles.
Hes building a new frame for the whole floor (40m2).
Hes wanting to put 18mm ply on top for me to tile onto.
I'm trying to advise him to use backer boards.....but it seems the client knows best😁
Usually I would walk away but its a good job.
What would the forum recommend?

 

[Cherie]

Ditra?

 

[Cherie]

Well, almost done with getting up tar/mastic/asbestos. Purple power is working the best. Have ,5 inch dri plywood t&g to go onto t&g subfloor. Not doing leveler, I don't think.amazingly floor is level. Floor is not flat. Will see after sanding. After ply is Ditra. I am also going halfway up the wall and papering back wall painting above tile. Have to replace some drywall. Broke BY
leaning against it to get leverage. Do I have to do anything other than get all the glue off drywall, skim coat

then red guard?

 

[Cherie]

Well, almost done with getting up tar/mastic/asbestos. Purple power is working the best. Have ,5 inch dri plywood t&g to go onto t&g subfloor. Not doing leveler, I don't think.amazingly floor is level. Floor is not flat. Will see after sanding. After ply is Ditra. I am also going halfway up the wall and papering back wall painting above tile. Have to replace some drywall. Broke BY
leaning against it to get leverage. Do I have to do anything other than get all the glue off drywall, skim coat View attachment 123375 then red guard?

Hi guys had a few health issues interfere, hubby not me, but made progress. I only have the floor to do. We are putting in a walk-in shower in another room for husband then I can remove this shower and do the floor. Put it back in and done.