I'm an experienced tiler with a problem job lined up. I have a fairly flexible floor 3mx3m which has 25mm ply on joists, and I'm wanting to use a 500x500mm ceramic or porcelain tile.

Of course I'm worried about a tile of that size cracking from the movement of the floor. I usually source my adhesives from Topps, which ones should I use?

Welcome..

You need to address the flexing before any tiling can commence... if the floor is 25mm ply and flex's then it could be wide joists centres or insufficient joist depth and the joists are flexing and they will need strengthening.

How is the ply fixed down?

The ply is fixed at 150mm centres. The floor has a problem caused by the removal of a fireplace 30years ago, the builders put a subframe of joists into the existing floor where the fireplace had been. I am able to work from the underside to strengthen the floor, but not to the extent of replacing the joists.

As long as you get the flexing sorted then it will be fine.. This might involve sitting new joists in beside the old ones.

Here's a dodgy sketch of the joist pattern with the area added, you can see why it causes the floor to bounce a bit but this a common problem with converted houses in this area.

In order to add a new joist to span the room I would have to cut the joist that connects the added subsection and then hang that off the new joist. I'm concerned that would make the problem worse.

Ok, After seeing what you have then you could put a new header joist across to support the main frame..

Yes, just had a look at it. Not the easiest of jobs but achievable.

Cut the joists back to allow the header to go in from left to right and the use joist hangers to support the main floor.. A bit like they do on a return on a staircase..