Another HardiBacker/Thinset Install Question [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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09-06-2008, 08:12 PM
I see that most recommend placing a moisture barrier such as felt roofing paper under the HariBacker and placing a layer of thinset as each board is installed. Does it matter if the thinset is modified or unmodified? I actually see a reference to both when looking thru their installation info.


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Tool Guy - Kg
09-06-2008, 08:17 PM
Hi Jeff,

You're apparently referring to installing Hardibacker on the floor. A moisture barrier isn't used under a floor....only on the walls of a tub or shower.

To install the Hardi on the floor, you can use either modified or unmodified under it. :)

09-06-2008, 08:19 PM
I've never heard of anyone recommending a vapor barrier under hardibacker. It probably wouldn't hurt anything if every thing else was done correctly, but I don't see that it would serve any purpose either.

Unmodified under, modified on top.

09-06-2008, 08:22 PM
A moisture barrier isn't used under a floorI've never heard of one installed under Hardi or any CBU.

did a handy man in town cast this urban legend on you ?

09-06-2008, 10:42 PM
The funny thing about moistre barrier behind cbu on walls is that when you call Hardie and ask them they say "we don't require moistre barrier behind it you have to check you local building codes" :) They really don't care!

09-07-2008, 07:16 AM
No, but the TCNA recommends it,Hardi i guess isn't worried about your studs,wiring.mold, and such, just their board.

09-07-2008, 08:20 AM
Like Stoner says, the TCNA has the moisture barrier requirement in the Handbook, not for floors but for showers that is.

If you've got a copy, turn to page 62, Method B415-07 for backer board showers. Then additionally, per ANSI A108.02, the barrier should be 15# felt or 4 mil+ poly sheeting.

09-07-2008, 11:18 AM
What about thinset under hardibacker? if so, modified or unmodified?


09-07-2008, 12:13 PM
Both are ok for under cbu. I always use modified that way i don't have to buy two diffrent kinds. undmodified is chipper thought about $8-10 when modified could be from $18-$60.

09-07-2008, 12:18 PM
that's why I use the unmodified, since I use a coil nail gun to anchor the CBU, I'm only using the thinset to fill the voids.

Dave Taylor
09-07-2008, 12:27 PM
The prevailing wisdom here-a-bouts says to use unmodified thinset beneath Hardi CBU. Reason being..... thinset between a wood floor and Hardi is there to fill possible voids between the two materials.... and unmodified thinset fills this purpose while not adhering the CBU strongly to the wood substrate..... thus enhancing decoupling of any movement between the two materials.... thus aiding in insulating a tile covering from floor movement.

If I recall, however, current installation instructions may call for modified thinset beneath the CBU.... or unmodified thinset. With Hardi (in my experience) it depends on when you read their installation instructions..... they seem to change frequently over time.

I say……..
Use unmodified thinset under the CBU.... and use whatever thinset (modified or unmodified) the tile manufacturer recommends for setting their tile.

Exactly what kind, size and brand of tile are you planning to use?

Oops.... I'm slow in responding.

Like Brian..... I use unmodified thinset under the CBU and...... I use Backer-On screws to hold the Hardi down.

09-07-2008, 07:56 PM
Just a quick question related to this...

Is it ok to use 1/4" Hardibacker on floors, or is the 1/2" preferable in terms of providing more stability to the tile bed?

09-07-2008, 08:27 PM
a cementboard adds no structural support to a floor.
that is made up in the strength of the plywood.
the 1/4" & 1/2" thickness is mainly for height issues.

you need to make sure your floor passes the Deflection test ( and if it does you can use 1/4, 1/2 or even Ditra.

Tool Guy - Kg
09-07-2008, 08:27 PM
Welcome, Elfin.

That's a common question around here. Cement board thickness doesn't matter for the floor. The structural support is coming from the plywood under the Hardi. Use whichever thickness helps you achieve the most level transition onto the adjacent floor. Generally, a tile install is taller than the surrounding floors, so 1/4" Hardibacker is more often desireable. :)

What about thinset under hardibacker? if so, modified or unmodified?Either is fine with Hardibacker. Here's their installation guide ( :)

09-07-2008, 08:29 PM
slowpoke :tongue:

Tool Guy - Kg
09-07-2008, 08:32 PM
I got links and answered 2 people. :tongue: :tongue:

09-07-2008, 08:37 PM
Thanks, you two... that's what I thought, but it just seems soooo thin, ya know? :)

Tool Guy - Kg
09-07-2008, 08:39 PM
As long as you concentrate on the proper plywood below, you are good to go. ifs, ands, or butts.

...well, 'cept for Edna''s kinna hard to keep away from that. :uhh:

Dave Taylor
09-08-2008, 05:20 AM

Tool Guy - Kg
09-08-2008, 09:20 PM
...and don't let that guy jump on your tiled floor! :eek:

09-14-2008, 09:49 AM
The reason I asked about the type of thinset, I used modified, I have not yet laid the tile. the backer has been down about four weeks and it was put down with roofing nails along with thinset. When I walk on the backer, I seem to "hear" movement although my floor meets the deflection requirements. I have not placed thinset on the seams yet. Maybe the noise is just the backer shifting and rubbing the roofing nails. I'm a little hesitant about putting the tile down at this point.

Any thoughts to the "movement" that I appear to be hearing?


09-14-2008, 10:04 AM
Maybe the noise is just the backer shifting and rubbing the roofing nails.There should be no noise and no movement at all in that CBU installation, Jeff. None. Not the tiniest bit.

Did you do the installation, or someone else? Do you know that the CBU was installed while the thinset was still wet?

My opinion; worth price charged.