# Overkill to use both Hardie and Ditra?



## Panzer5 (Oct 21, 2008)

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?


----------



## CookeCarpentry (Feb 26, 2009)

Hardi and Ditra is not overkill - just not necessary.

What size are the floor joists? Span?

If height is not an issue, overlaying existing subfloor with 3/4" t&g plywood, screwed (to glue or not is still debated, though Schluter will tell you no glue).

Then modified thinset to adhere Ditra, then unmodified for tile.


----------



## ROVACON (Apr 19, 2010)

I say do what you have to so that you don't have to go back :thumbsup:. i was always taught that you should have a minimum of 1'' of subfloor to great a greater stiffness. I don't think it is over kill. However, I do glue and screw the additional layer before putting on the DITRA. have not had any problems to date. Hope this helps


----------



## PrecisionFloors (Jan 24, 2006)

CookeCarpentry said:


> Hardi and Ditra is not overkill - just not necessary.
> 
> What size are the floor joists? Span?
> 
> ...


This would be my recipe as well. :thumbsup: Joist construction, size, and span still needed before anything else though.


----------



## opiethetileman (Apr 7, 2005)

just like the bathroom I just finshed the Ho did not want a threshold in the doorway. Yikes i wanted a T cap she voted me off the island on it. So they used 3/4 sub and I used 1/2 hardi with 20x20 tile with a 3/8 trowel to get the floors flush. I made it up front and clear that grout has a 99% chance of cracking on the doorway. Wood expands nothing you can do about that.

also ditra has 2 sizes xl which is thicker for single layer floors. I like 1/4 hardi on floor for height and that way you can use a thicker trowel if needed. i do like hardi better on the floor than durock... But durock also has durock on a roll as well to go over wood subfloors. Its a very thin layer like kerdi.

I would aslo use a good thinset if you go over hardi or durock. Ditra requires unmodified over thier product.

Also if you use hardi backer make sure you thinset it and nail it down. Also when tile over over it make sure you take a damp sponge and wipe off dust and prep the surface correct and you will have a secure install ..


----------



## moorewarner (May 29, 2009)

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?


----------



## PrecisionFloors (Jan 24, 2006)

moorewarner said:


> K, I'm jumpin' in on this one.
> 
> I also have a tile install to do, and I am also in the novice class.
> 
> ...


This translates to a deflection of L / 329. Does not meet deflection requirements for ceramic tile. Since you already have some structural work to be done, I would get rid of the OSB and run two layers of plywood, modified thinset, Ditra, dry set - tile. 

A highly recommended thinset would be Mapei Kerabond with Keralastic additive to set the Ditra with and then plain Kerabond to set tile with.


----------



## WarnerConstInc. (Jan 30, 2008)

Just get a bunch of liquid nails.


----------



## angus242 (Oct 20, 2007)

Panzer5 said:


> 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).
> 
> ...


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:


----------



## WarnerConstInc. (Jan 30, 2008)

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


----------



## PrecisionFloors (Jan 24, 2006)

WarnerConstInc. said:


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


Dude, that's why you're supposed to wait till after work to smoke :jester:


----------



## angus242 (Oct 20, 2007)

moorewarner said:


> K, I'm jumpin' in on this one.
> 
> I also have a tile install to do, and I am also in the novice class.
> 
> ...


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 (Oct 20, 2007)

WarnerConstInc. said:


> 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?


----------



## WarnerConstInc. (Jan 30, 2008)

PrecisionFloors said:


> Dude, that's why you're supposed to wait till after work to smoke :jester:


Oh, that stuff tastes funny too and may be the cause of my constipation.:laughing:


----------



## moorewarner (May 29, 2009)

angus242 said:


> 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.
> 
> ...


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?


----------



## angus242 (Oct 20, 2007)

moorewarner said:


> 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.


----------



## moorewarner (May 29, 2009)

WarnerConstInc. said:


> Just get a bunch of liquid nails.


That reminds me of a "rehab" a friend who rehabs took me too, that had been "rehabbed" by a friend of his.

He wanted to show it to me and get my opinion on it, as his friend was trying to sell it to him in exchange for a debt.

His buddy had had a bunch of crack-head-hacks in there doing the "repairs". They sprayed texture on everything,

the walls,

the ceiling,

the trim,

the floor,

the fixtures,

the windows,

e.v.e.r.y.t.h.i.n.g. 

I told him, RUN FORREST!!


----------



## PrecisionFloors (Jan 24, 2006)

WarnerConstInc. said:


> Oh, that stuff tastes funny too and may be the cause of my constipation.:laughing:


:whistling:laughing:


----------



## GLADD (Jun 19, 2021)

moorewarner said:


> K, I'm jumpin' in on this one.
> 
> I also have a tile install to do, and I am also in the novice class.
> 
> ...


Pretty old thread now, but came across this in search of proper installation of subfloor in use with ditra. And want to add some source information from the manufacturer for any people like me.

Like many things follow the manufacture installation requirements. In fact, if this was a true pro job there would be specifications that state this.

here’s a link to the installation document for ditra and ditra lx and the subfloor requirements -https://s3.amazonaws.com/scclegacy/media/ditra-handbook_eng.pdf?v=201507140600

you’ll void your warranty if you don’t follow these requirements.

For applications using ditra there are maximum structural requirements for subfloors. Ditra functions as a uncoupling membrane which helps with deflection and tile movement effectively allowing it to work without a second layer of underlayment. It essentially is the second layer of structure required in traditional built up.

based off the information you provided it seems like page 4 is the correct application. And you would not need additional underlayment for a proper installation.

Also, ditraXL is designed to create a flush condition with hardwood without the need for a second layer of underlayment

also consult the manufacturer handbook for thinset. Again you’ll void your warranty otherwise.

additionally, you could take a look at the “tile council of North America” handbook for current and best practices for installing tile.


----------

