[angus242]

Got a bathroom floor replacement project. HO wants new tile - existing floor is cracked (DIY job from the '70's-80's) and can't tell (yet) what the underlayment is, but I suspect I'm going to find a plank subfloor once I get under the tile (probably tile on hardwood on plank).

I got in a debate with a guy I know who also dabbles in tile as to whether a layer of 1/2 inch hardie laid on thinset & screwed down to the planks would be a sufficient underlayment - or whether we also needed a layer of Ditra on top of the Hardie to uncouple the tile from any other movement from the floor (it's a 2'd storey bath in a 70 - 80 year old home).

I told him that if he was so set on Ditra, we could skip the hardie & just put down a layer of 3/4 inch exterior grade T&G ply and save a bundle.

Since neither of us are regular tilers - we thought we'd toss this out there for some expert advice.

THoughts?

Exterior plywood will always be better than CBU as plywood will actually strength the floor, CBU does not. CBU only acts as a bonding material for the tile. There is no difference between 1/4" vs 1/2" CBU on a floor unles you need the extra height.

If you are ultimately using Ditra, I'd go with the plywood.

You do not use T&G over an existing T&G subfloor. Install the second layer perpendicular to the subfloor & leave 1/8' gaps in the seams. You do not glue the 2nd layer down. TCNA does spec glue either.

If you want to up-sell, suggest using Kerdi-Band on the Ditra seams & up the wall for a 100% waterproof flooring installation. :thumbsup:

 

[angus242]

K, I'm jumpin' in on this one.

I also have a tile install to do, and I am also in the novice class.

2x10, 16"o.c @ 16' span, with 3/4' osb sub-floor. The rest of the house is getting 3/4 tng hardwood, so I would prefer to keep floor heights equal, but no call back on the tile is far more important.

So, my initial plan was thinset (mod)/Ditra/thinset/tile. It seems the Ditra XL will just about even me up, but would I be better off with thinset/Hardi/thinset(mod?)/Ditra/thinset/tile and just deal with the slight height change at transition?

Additionally, is there a particular thinset folks recommend? what about modifying agent?

Also i routinely glue additional layers of sub-floor, are there instances where this is bad?

Your subfloor is not strong enough to install even ceramic tile. There's an ongoing thread about OSB subfloors :blink:. I would rip up the OSB, sister in some 2x10's, add a layer of 3/4" T&G exterior ply and then you could Ditra. If you strengthen the joists, I think you could get away with the single layer of ply.

If you're concerned about matching floor height between the 2 materials, you could use Ditra XL.

While there's nothing wrong with using CBU under Ditra, the question is why? You're much better off with an additional layer of ply since that will actually stiffen the floor some. CBU will not.

As for thinset, I would use Kerabond + Keralastic over the ply and Kerabond over the Ditra. Back butter your tiles!

FYI, Mapei does not allow Kerabond over OSB :whistling

EDIT: Looks like Paul beat me to the punch! :notworthy

 

[angus242]

I just like using my foam gun with the poly adhesive, I put it on everything, including myself most of the time.

Like body waxing?

 

[angus242]

Precision Floor, Angus,

Sorry, my bad, that would be 3/4 ply, DOH!, it has a 1/4 under-layment over it, under the vinyl that is slated for removal.

Sorry Gents, recalculate?

Nothing changes. The unsupported span of 16' is what's hurting you.

Still say rip up the ply, sister, new 3/4" T&G.