Underlayment for tiling

Hi Kazza, it sounds to me like the problem is underneath your marine ply which has been insufficiently prepared. I would suggest taking the wood up too and adding "noggins" in between the joists..... Don't panic this sounds like a massive job but to the right tiler with the right tools its nothing major and is just time consuming, however it is an ESSENTIAL part of our trade. The mesh type underlayment you mention I'm guessing is "Ditra Matting"?? Although Ditra is a fantastic product it is not a plaster or remedy for all problems and it will do absolutely nothing to remove "bounce" in your substrate, the only way this can be done is by my explanation above. Ditra was not designed to go on wood, it was designed to go on screed and act as a decoupling membrane between the screed and the tile, it would deflect any cracks that appear over time in the screed from reaching the tiles and making them crack under the stress. If you prepare your substrate properly and remove the "bounce" you can then re-use the marine ply (as long as it is of adequate thickness minimum 18mm) you can then use a suitable fibre reinforced latex that is designed especially for going over wood to help with the "levelling" of the floor. Spend a bit of extra time (roughly 1 day) and a bit of extra money (whatever you tiler charges on a day rate plus materials) getting your substrate prepared properly and you will in the long wrong run save a fortune having to re-do it all again, if done correctly it should last you a LIFETIME without any problems. Hope that helps a little Kind regards Toby