To Ditra or not to Ditra...

[HimediA]

Hi All

I placed a 'tiler wanted' ad recently and have had 4 people out to look at the job, so thanks for the great response everyone!

All 4 guys seem to know what they're talking about. The issue I have that 2 suggest that Ditra matting should be laid underneath my tiles, 2 say that in my case it's not necessary.

I am less concerned about the extra cost than I am about the extra depth it will add to the finished floor. I won't go into details as to why this will be an issue but trust me, that extra 3 or 4 mm will make all the difference, and I'm really (really really:smilewinkgrin🙂 keen to avoid it.

My subfloor is an old building (so all well settled). timber joists (that I have doubled up joists in couple of areas where approx 1.8m between brick supporting piers) over which I've screwed down 8x2 ft 22mm T&G green floor chipboard (old floorboards removed first). After decs are all complete I'll be laying 8x4 ft sheets of 12mm WBP ply at right angles to floorboarding screwed down at 100-150mm centres and well unibonding over to seal. The floor already has absolutely minimal 'bounce' and that will only be improved with the ply.

The room is a kitchen approx 7.5m x 5.5m. The tiles are 600 x 400 x 15mm Moleanos Limestone that will be laid brickbond down the length of the room. Plan to allow to lay then 1 coat sealer, then grout (say 4mm) then 3 or 4 further coats of LTP Mattstone sealer. Adhesive I've purchased is Larsen Flexible standard set white. Grout Larsen Flexible narrow joint limestone colour.

I have had stone floor laid over various timber subfloors before and never had any issues with cracking. Generally they have been Travertine though and not limestone.

So, question is, to Ditra or not to Ditra?

And yes, I appreciate the 'better safe than sorry' argument but as I say, that extra depth gives me real headaches.

Thoughts/recommendations/warnings/assurances appreciated...

Cheers
Paul

 

[Dan]

What's the width and length of the room?
Edit: found it ignore that.

I'd have said the floor is big enough to warrant using a decoupling membrane (Dural not Schluter mind) to be fair.

Though without looking at it and testing for deflection nobody could guarantee either way.

 

[HimediA]

I would say 'read the post Dan' but as you are taking the time and trouble to kindly respond to my plea for help that would be really mean wouldn't it? :smilewinkgrin:

Room is 7.5m lengthways, 5.5m across (approx).

And thanks again!

 

[Stewart]

I would always use a decoupler with stone tiles. If the floor is as ridged as you say, why not use 6mm backer boards then the decoupler, then the tiles. If you use 12mm ply you will effectively void any guarantees from the adhesive manufacturer.

 

[HimediA]

Thanks Stewart. Other than the fact that I have 13 sheets of 12mm WBP sitting in my barn ready and waiting (so not quite the answer I was hoping for) that is Plan b then...

Any more thoughts gratefully received...

 

[johnryan]

that's a big floor to tile, and given the fact that it's going to be half bond too, i'd definitely put down a decoupler. i'd say your taking too big a big chance if you don't. though as mentioned before, without seeing it, i could be wrong. seems like a big risk though.

 

[HimediA]

Thanks for the reply John, appreciated.

 

[johnryan]

no problem, glad to help. though don't take my word as gospel though, the tilers you had to see your job are in a far better position to comment than i am. another point i'd make is perhaps going to the dealer who supplied the stone, and ask them. they can normally give good advice.

 

[HimediA]

Thanks John.

I have asked the dealers but have yet to receive their reply. And as for taking advice of tilers, 2 say yes, 2 say no need. It's an expensive one to get wrong...

 

[doug boardley]

same as Stewart to my way of thinking.

 

[johnryan]

your not wrong about that. it's tough call to make, but the way i see it, is that if the manufacturers of stone products recommend something, and most tilers i know say the same, i'd be wary of trying anything different. hope it all goes well for you anyway.

 

[The Legend; Phil Hobson RIP]

I would go with Stewart and Doug

 

[Stewart]

Just out of curiosity....... what did your 4 tilers say about the 12mm ply?

 

[HimediA]

What's the width and length of the room?
Edit: found it ignore that.

I'd have said the floor is big enough to warrant using a decoupling membrane (Dural not Schluter mind) to be fair.

Though without looking at it and testing for deflection nobody could guarantee either way.

Thanks Dan. What would a typical deflection test comprise?

And is the use of a membrane due to length of room and lateral movement (expansion/contraction I guess?), or is the robustness of the sub-floor and vertical deflection the issue?

If the latter, I confess I don't see how a flexible membrane will really help (although that's my engineer's training and not stone flooring expertise so I stand to be corrected!)

If the former ie. horizontal movement as opposed to vertical, is that really likely to be a risk given no underfloor heating?

And all you experts please understand I'm not doubting your expertise, it's just my darned questioning engineer's mind....

 

[HimediA]

Just out of curiosity....... what did your 4 tilers say about the 12mm ply?

Two said would have preferred to see 15mmm but 12mm should be OK
All said should be fixed down at max 150 centres.
Three said make sure it's sealed (one with tile primer one said SBR fine, one advised PVA fine)
None came up with Stewarts suggestion of thinner backer board & Ditra.
It's a minefield out here gents...:mad2:

 

[Gazzer]

The main point is assessing the deflection that is there.
I personally would go on how it felt as i walked across the floor. There was a old story we used to hear that if a pint glass of water was filled to the brim and then you walked around the glass 1m away. If any water ran down the side of the glass the floor had too much deflection........I also heard that if it didnt , that you should walk heavier !!!!

 

[HimediA]

'The old ones are the best'... but that is at least a demonstrable test! I'm now anticipating the look on my wife's face when she sees me walking around a glass of water on the floor. She'll think I've listened to her at last and it's some AA mantra thing...
Now I wonder, will my 16 stone bulk be sufficient or should I strap a few 600 x 400 slabs of the appropriate flooring limestone to my back...:lol:

 

[HimediA]

What's the width and length of the room?
Edit: found it ignore that.
I'd have said the floor is big enough to warrant using a decoupling membrane (Dural not Schluter mind) to be fair.
Though without looking at it and testing for deflection nobody could guarantee either way.

Now Dan you're trying to confuse me... :yikes: what is the difference? performance? price? other?

 

[Stewart]

Schluter and Dural both perform the same. As a recent convert from Schluter to Dural, I've found the only difference is price, with Dural coming in much cheaper.

 

[johnryan]

i think the ditra matting transfers the downward stresses to whatever is below it, and reduces the stress on the tiles somehow. it's not just lateral movement it counteracts, but vertical too. at least that's how i thought it worked.

 

[HimediA]

The main point is assessing the deflection that is there.

If deflection is the issue that is something I can assess and make a judgement on (pint glass or no pint glass.)

But if that's the case why do people say that with a room my size (7/5m x 5.5m) I should consider using matting? What difference does the size make, assuming supporting brick piers are in every 1.5m or so under most floors surely the vertical deflection is going to be similar whether 2m, 20m or 200m?

 

[HimediA]

i think the ditra matting transfers the downward stresses to whatever is below it, and reduces the stress on the tiles somehow. it's not just lateral movement it counteracts, but vertical too. at least that's how i thought it worked.

Thanks John, but the engineer in me can't really see how a flexible 3mm membrane can really add much in the way of vertical shear strength. If the floor's solid then it's solid (well, within reason, you know what I mean), if it's really not solid then no thin plastic membrane in the world is going to add any significant deflection-preventing strength.

Of course I could just be a numpty.

 

[doug boardley]

dural or ditra is not there to take up the deflection, it's installed to combat lateral stresses.

 

[Aston]

hi himedia,

1, the delection (up and down movement) needs to be sorted. get this wrong and the height you've saved will seem irrelevant to the extra outlay of a newly tiled floor!!
2. the de-coupling membrane is to help prevent lateral movement (side to side) unfortunatey, all buildings move due to expansion and contraction through out the year, so if the floor is solid (no up and down movement) then id still put down a de-coupling membrane to counter balance the expansion and contraction.

the larger the area, the move need for a decoupling membrane.....

 

[HimediA]

dural or ditra is not there to take up the deflection, it's installed to combat lateral stresses.

Thanks Doug, that I can understand. But on a long-standing floor that has been significantly strengthened and really does feel darned solid, and with no underfloor heating or any significant underfloor moisture levels that are likely to bring about any lateral movement, I'm guessing that matting ain't really going to do a lot, except add to my bill and floor height...

Of course those cynics out there may now think I've just ignored all the advice and have gone with the answer I liked the sound of and could make fit. That really isn't the case. Promise.

If anyone out there can tell me why matting is going to make any significant difference to my floor then I'll lay it.

 

[rk ceramics]

You could use a AD1 additive to make the adhesive more flexible so no Ditra. cheers Richard

 

[HimediA]

Thanks AstonTiling, your 'everything moves throughout the year' point does make sense.

 

[Gazzer]

Probably a quicker way would be to chat with our Sponsor Dural Welcome

My point about deflection is just one issue. If the floor were free from deflection then I would still want a backer baord laid at least...I dont like tiling to wood...it will move. Seeing as you are laying stone then I would want a decoupler but the floor has to be solid. The size of the floor is quite large and as such will have more movent both ways than a similar constructed floor of a smaller size.
You may want to consider movement joints too....all that is discussed by clicking the Dural links on the side of the page here.

 

[Gazzer]

You could use a AD1 additive to make the adhesive more flexible so no Ditra. cheers Richard

Not if the adhesive is already polymer modified !!

 

[HimediA]

By that I'm assuming you mean labelled as a flexible adhesive (such as the Larsen one I have)?

 

[Gazzer]

Yes correct. You shouldnt add any additives to already modfied adhesives or grouts.