Part Floorboard, Part Screed - Tiling

[parkview094]

We have recently enlarged our kitchen space by knocking through to an old outside WC/coal cupboard.
The 'old' kitchen floor is floorboards (1 1/2" T&G board). The 'New' part of the kitchen is concrete screed. There is also an old cement hearth.
Mrs P would like a new tiled floor to the kitchen.
I've spoken to 4 different tilers and each has had their own (conflicting) view on how to prepare the floor for tiling.
The floor is broadly level at the moment and the floorboards are very firm.
One tiler suggested overboarding the whole lot with 18mm ply, but by the time we've added UFH, decoupling, tiles - we'd me looking at raising the floor by almost 2" which makes a bit of a mess of the back door.

So..

I came across NoMorePly which, at only 6mm, seems to offer the same benefits at 18mm ply. This reduction in thickness would help us a lot.
I've spoken with the NMP folk and they've advised their boards need to be laid on a cement adhesive over the screed areas and screwed over the wooden bits.

With this approach, how would you ever get the boards level? Surely the boards over the floorboards would be 3mm lower (thickness of cement adhesive) than those over the screed? Does this actually matter in practice - can that difference just be lost with the adhesive layers between the decoupling mat?

Has anyone had any experience with NMP - is this worth a go?

Many thanks in advance.

 

[vinn]

Its not clear to me what you have actually settled on - why not, for a start, post pics of the floor area and some dimensions?

Does the old WC floor have a membrane below it - is this floor what you call the "new part of the kitchen"?

The hearth might be a source of possible rising damp? Does the chimney breast still exist?

Does the T&G floor have air bricks and through ventilation?

FWIW: 3mm is easy to make up in adhesive.

 

[parkview094]

Floor plan and Photo of the current floor:

Indeed - the old WC (screed area) has a bitchumen layer between the old quarry tiles and the screed.
There is an old slate DPC on the hearth. There is no evidence of any historic rising damp thankfully around the hearth. The Chimney breast does indeed still exist.
There are air-bricks beneath the T&G floorboards with good ventilation. The joist nearest the rear wall had rotted at one end and has been replaced hence the ply area of the floor. I'm reasonably convinced this was caused by a combination of a previously blocked air-brick which I've now re-instated, and ground levels being too high on the outside which is on the list to lower.

The 4 tilers opinions have been:

1. Ply over the timber area. Self levelling over the screed/concrete to match the levels. Then ditra, then tiles.
2. "Dont waste your money overboarding the floorboards since they're already as solid as they could be." Just use flexible adhesive and ditra over the whole lot.
3. 18mm ply over the whole lot - screwed into the wood and screwed/plugged over the concrete.
4. "18mm ply is old school, not the correct way of doing things any more and may rot." Use Marmox board over the whole lot.

Dad's opinion is Liquid screed over the whole lot. Also avoid UFH beacuse of the potential thermal difference over wood/concrete.

Very confused!

 

[parkview094]

..quick note to add.. the reason the builder (who took the wall down for us) didn't take up the floorboards and replace the whole floor with ply was because, in his opinion, the 1 1/2" floorboards were so solid, there would be no merit in doing so. (His original quote had included replacing the old floor completely with ply, but he removed this from the bill, so it wasn't just a cost cutting measure)

 

[vinn]

OP,
You have so many joins in the floor and the remains of the dividing wall - (with celotex presumably the edge insulation for a concrete slab?) - that inserting movement joints is not practical.

Perhaps the simplest thing to do would be to follow Tiler number 2's recommendation regarding installing a flexi, cementitious adhesive & Ditra or similar over the whole floor.

Elec. UFH could be laid in the tile adhesive bed but it depends on how much height you have to play with (the adhesive would have to be thickened up) - to keep the FFL low then eliminate the UFH.

FWIW:
Make sure that there is a 8mm - 10mm expansion gap between the tile edge and any abutments.
Break plaster contact about 40mm to 50mm above the floor.
Do you need three doors - doors rip an awful lot of space from unit runs?

 

[chappers]

if your timber floor really is rock solid, then I would go with option 2, using an S2 flex adhesive over the decoupling membrane

 

[vinn]

This user name chappers is something else - he claims to never read my posts but no sooner have I come up with a suggestion than he often pops up with a same suggestion, sometimes he follows me word perfect.
He doesn't even have to google but just follows my advice to the OP whether its right or wrong.

Its not at all flattering its strange, he's maybe attempting a weird way of gaining a bit of self esteem?