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: