Large format stone on UFH timber suspended floor- a job for Ditra matting?

[robbyraven]

Hi all,

I'm doing a barn conversion at the moment and the ground floor requires 50 sq.m of indian mint sandstone. The floor to be tiled consists of about two-thirds timber suspended floor with wet UFH under a 22mm moisture resistant chipboard deck, and the other third is a floating chipboard deck over an existing concrete slab, also with UFH. Both floors are very sound but obviously have a little flex, like any floor of this type. The stone is variable thickness from 20mm to 40mm. A friend of mine already has this stone on his suspended floor, and he used dabs of adhesive to reduce the amount required- however I am told this is unsuitable for UFH because the air gaps would stop the heat getting through from the sub-floor, which sounds reasonable. So, am I looking at using a (very expensive) thick bed of flexible adhesive? The Schluter Ditra matting has been recommended to me as the ultimate solution, but is bloody expensive over 50sq.m!

Can anyone confirm or deny that a solid bed of flexible adhesive would be the best solution? It's really gonna blow the budget if I have to use it, but if it's the only solution then I guess I have no choice.

All comments/feedback gratefully received, I could really use some advice on this!

Many thanks,

Rob

 

[Dan]

You can't 'dot and dab' stone, it's not wise at all. And yes... the underfloor heating does require solid objects to transfer heat through, air pockets are a really good insulation, that's why we use trapped air to insulate double glazing. Pourable thick bed flexible adhesive are the route I would be at least looking in to.

Ditra matting is good but you would still need to use a solid bed of adhesive on top of it, so you will still need the flexible adhesive.

If you're looking to save costs on the job, I wouldn't go for the stone.

Anybody else have a view?

 

[Mudster]

Ditra matting will increase the amount of adhesive you need to use, as not only will you now have to slid bed a large area, you'll also have to fill the corregations of the matting.

From the spec you've given me I wouldn't fit that floor, you can't fit stone to a floating floor and expect it to stay intact for any reasonable period of time, it simply isn't stable enough.

The suspended floor could be reinforced using Aquapanel glued and srewed, but the floating floor would need to be removed and screeded to mathc the suspended flooring once reinforced. There is no other option.

 

[robbyraven]

Thanks Admin and Mudster for your replies. I made a mistake in my original post: I'm quite happy going for the solid bed of flexible adhesive, it was only the Ditra matting I was shying away from on a cost basis.

Admin, you say 'I'd at least use a pourable flexible thick-bed adhesive', which would be fine with me. Do you think there would be much advantage in using the Ditra matting as well? Asking around, it seems that Ditra is not used all that often- under what circumstances is it deemed entirely necessary?

Mudster, you say you'd never fit stone to a floating floor- does this mean you wouldn't fit any kind of tiles to such a floor? If not, can you tell me why/what would happen if I did? The reason we went for a floating floor instead of a screed was to preserve headroom- this floor is in a lean-to, so the ceiling at one end is 4m high, but on the other side it's only just high enough for a doorway. It 'seems' pretty solid, but I know that might not mean much. How would you recommend I finish this floor? It's the kitchen, so we really want stone/tiles rather than wood or carpet.

Thank you for the advice so far, look forward to some more useful pearls of wisdom!

Rob

 

[Mudster]

Ditra matting is extensively used in the USA and generally misused here as far as I can tell. It isn't a reinforcement in itself, so the floor would need reinforcing before you used it, 12mm ply or similar. Which in my mind negates the use of it altogether, I've yet to see the full benefit of this product.

I'd generally advise people not to fit tiles on a floating floor, it's not a question of it it will fail, just when.

How is the floor currently constrcuted? I assume you'll have a concrete base on which insulation is fitted - usually 50mm+ then a layer of T&G floorboards.

I'd remove all of this then get it screeded.

 

[robbyraven]

Would the suspended floor really benefit from a 12mm ply reinforcement? This floor is brand new, 2x8 joists at 400mm centres with a 3.6m span (above building reg requirements) with 22mm T&G moisture resistant chipboard, glued and screwed. I can see how ply would benefit an old floorboarded floor, but would it help here?

The floating floor is 22mm T&G chipboard glued together over 50mm grooved polystyrene containing under floor heating pipes, over 40mm celotex to bring it up to building regs insulation requirement, all over an existing concrete slab.

How about this as a plan:

Ditra matting over the floating floor, followed by solid-bed single part flexible adhesive, followed by the stone

Over the suspended floor, thick solid-bed single part flexible tile adhesive directly onto chipboard.

Would this cause major problems, and if so, what would they be?

I'm told I could use 2 part flexible adhesive and dispose of the Ditra, but with irregular thickness tiles, the 2 part adhesive would cost much more than the Ditra matt and single part adhesive combined. Does this sound right? I'm frankly amazed at how expensive 2 part flexi adhesive is.

Thanks again,

Rob

 

[Mudster]

A substrate for natural stone needs to be 50 % more rigid than it does for porcelain or ceramics.

the 12mm ply would be fine for ceramics, but not enough for stone. Generally I'd recommend aquapanel be glued and screwed over the existing suspended floor to remove localised movement.

There are no circumstances under which I would fit natural stone on a floating floor. This needs to be screeded, no alternative.

If a floor is moving to an extent that it needs a two part flexible adhesive, this is a sign it is not suitable for fitting a natural product.

To add to this, what it would cost to strip and screed the floating area would be saved in substrate preparation and overexpensive and unnecessary products (Ditra Matting and 2 part adhesive). You are then guaranteed no failure of your new flooring.

 

[Dan]

I didn't see the floating floor bit!!! You need to fix that... really.

But for underfloor heating you need the flexible adhesive, for height issues with two different floor heights and for filling in ditra and covering underfloor heating mats or cables i'd opt for a pourable thickbed flexible adhesive. I think. . . *thinks* yeah, i'm sure I would.

 

[Smiths]

Simple, get rid of the floating floor. I refuse to tile over it because it will eventually fail.

 

[robbyraven]

Right, I'm getting mixed information here. We can't get rid of the floating floor and screed insteads because the headroom at the low-end of the lean-to won't accomodate it. The floating floor is over grooved insulation panels which contain underfloor heating pipes. I've had two tilers come to quote, and both said they would be happy to do it, providing we use ditra matting and single part flex adhesive. I know one of these tilers personally, and he is no cowboy, and the other one comes highly recommended by my friend who sells stone flooring, so he should be pretty reliable too.

So, on one hand I am told that ditra matting, flex adhesive and flex grout can be used reliably to lay stone over floating floors, but here I am told it can't be done under any circumstances. Can anyone tell me whether my tilers aren't to be trusted, or if perhaps the posters here are being slightly over-cautious?

Don't get me wrong, I do appreciate all comments. I do really want stone in these rooms, and as screeding is not an option, it is much easier for me to hang on to the possibility of achieving it with Ditra etc. than to give up and go for some other floor covering less suited to a kitchen, like wood or carpet.