Laying terracotta tiles on various sub-bases

[revo]

I have resently added a new extention to the back of my property and in doing so taken out most of the original back wall. The original part of the property has a suspended timber floor. I replaced some of the joists and added some blocks for support as well as some noggins. I then screwed the old 6" wide soft wood floor boards back down. But not before adding a wet underfloor heating system. I have been told that I should add another layer of ply boards on top of the floor boards, but the heating manufactures have told me that I shoud not have more than 25mm of timber floor. This being that timber is a good insulator and the heating will not get through more than the 25mm. The old boards I have reused are 22mm, they're not tongue and groove and some have small gaps between. I've added a 3.5mm layer of ply, that I've stapled down and gone mad with the staples. In the new extention I've put in the underfloor heating system and poured concrete. The two floors are now level with each other. The concrete has been down now for over 3 months and the heating went on a week ago. No signs of any cracking or movement. The timber floor seems quite solid. I want to lay terracota tiles throughout. Do I need to use a membrane like dura mat? Is there anything else I should consider doing before laying the tiles? There is also a very slight fall in the floor, could I attempt to level this out as I bed the tiles down or just go with it?

 

[One Day]

You should really take up the 3.5mm ply AND the original boards and replace the lot with good quality 25mm wbp ply.
Seal the reverse and sides of the ply using a quality SBR like BAL's, and for belt and braces, yes - use an uncoupling membrane.
You will also need a movement joint where the two different substrates meet.

Personally, this is the only way I would do this job.
I would never tile onto 3.5mm ply. Far too much risk of movement caused by the heat variations, staples will do nothing to stabilise it either.

Probably not what you want to hear after all your efforts preparing the floor! Sorry!

 

[revo]

Your right not quite what i wanted to hear. I'm starting to run out of money, so replacing the sub-floor and using terracotta tiles with all that goes into sealing them is getting pricey! I get it that you don't like the idea of the 3.5mm ply, what if i removed this and then put a membrane over? Or if i replaced the boards with the type you say to use and didn't use the membrane at all? Would either of these be so wrong? And are you saying it doesn't matter what type of tile i use the sub-base should be prepared the same way?