Ballsed up my ensuite...options

[Handyman]

Hi

I laid 18mm Ext grade plywood over 8x2" joists and screwed at 200mm centres - the room is 3.1 X 1.7mtrs on the 1st floor and all seemed pretty solid when down.

I've now laid the porcelain tiles on flex adhesive and used a flex grout but once the grout was set and walking on the tiles I could hear cracking and after a week the grout has let go at certain areas ( not sure I can correlate the cracking to board joints) and my options appear to be :

Rip out all the old grout and replace with an epoxy (how much flexing can this take?)

Take down ceiling in lounge and add 4x2 " noggins to strengthen high traffic area in bathroom above

Add extra 8x2" joists to double up existing

Or do all 3 items?

Thinking about it some more - in the lounge the span of the joists is actually 4.1 mtrs so I should have expected some bounce on the boards an reinforced.

[macten]

It's hard to know without seeing first hand how much bounce the floor has. You don't want to tile until the floor is solid, not pretty solid..
Also what brand of Addy and Grout did you use?
Some DIY stuff isn't really fit for purpose.
I would remove it all and reinforce the joists and add extra noggins if needed. Then overlay with 18mm WBP ply and to guarantee against failure use a top brand of 2 part flex addy and a flex grout.