# Thinset under Hardi backer



## Speedball (Nov 2, 2008)

I'm not an expert but why would a person put thinset 'under' hardibacker?


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## jerryh3 (Dec 10, 2007)

Speedball said:


> I'm not an expert but why would a person put thinset 'under' hardibacker?


Because you're supposed to. Screw the hardibacker down while the thinset is wet, then get off of it and let it dry overnight. Most 1/4" hardibacker I've seen has a grid and screw pattern on the "up" side.


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## troubleseeker (Sep 25, 2006)

Speedball said:


> I'm not an expert but why would a person put thinset 'under' hardibacker?


It fills the minor low spots in the substrate, thus eliminating the potential for an area that could flex under load, and when the backer is fully bonded to the substrate, it is much more solid. Of course this step is skipped more often than it is done, even though all cement backer manufacturers state to do this in their instructions. 
Generally the rough side is up for thinset applications, and the smooth side is up for mastic applications. This is found either on the label stuck to each board, or sometimes printed directly on the product.


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## JJC (Nov 8, 2005)

Speedball,
Because it is a requirement of the hardibacker manufacturer. Actually all cement board manufacturers state this in their literature.
Divinepoodles,
You want the thinset to be wet because the board needs to bed into it and it helps take up any variations in the floor. then mechanically fasten the board while it is wet. One side of the board has indents to help people locate the required spacing between fasteners.


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## Divinepoodles (Oct 29, 2008)

Thank you all for the help. It all makes sense now.


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## JazMan (Feb 17, 2007)

Speedball,

That's a fair question coming from you, a regular homeowner DIY'er. Problem is there are many tile installer "Hacks" that ask the same question. Those guys are ripping their customers off with poor workmanship.

Jaz


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## Speedball (Nov 2, 2008)

Appreciate the good replies and no, I'm not in the business.:thumbsup:


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## drewhart (Jul 13, 2008)

instead of backer board, we put 1/2" plywood down for 5 x 9 bathroom floor. we put a million screws in the plywood to minimize any movement. is this bad? i'm not in the business either. it feels solid.


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## Speedball (Nov 2, 2008)

Drewhart, 
What was just under the 1/2" plywood?


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## drewhart (Jul 13, 2008)

nothing. just kidding! 3/4" sheathing. six inch wide boards. older, we nailed them down as secure as could first.


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## JazMan (Feb 17, 2007)

Drew,

Plywood as the underlayment in a bathroom is not a very good idea. It's not a good method anywhere, but it can work in dry floor installations. Tile doesn't make the floor waterproof you know?

Jaz


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## drewhart (Jul 13, 2008)

is backer board waterproof? in new constuction i have rarely seen anything other than plywood used. my parents house has four bathrooms. the house is fifteen years old and there has never been a problem.


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## poppameth (Oct 2, 2008)

The concrete based backer boards are not waterproof, but they are dimensionally stable when it comes to moisture. Wood is not. It will expand and contract as moisture content changes. Now you could put a membrane like Ditra over the plywood and it will make it much more waterproof and uncouple the tile installation from the wooden subfloor.


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## Speedball (Nov 2, 2008)

drewhart said:


> 3/4" sheathing. six inch wide boards. older, we nailed them down as secure as could first.


I'm getting ready to do the exact same thing!.... only maybe screw the boards down. Then plywood....but if something is better I need to find it.


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## drewhart (Jul 13, 2008)

i have since read that if you use plywood it should be exterior grade.


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## JJC (Nov 8, 2005)

True it should be Exterior Glue Plywood.(EGP) All this does is help hold the laminated layers together while the moisture from the thinset expands the wood and then when the thinset dries the wood layer shrinks back to size. Think about the stress put on the layers of wood and thinset as they go in one direction and then return to hopefully the original position.


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## bjbatlanta (Jul 16, 2008)

Read the tile mfgr's. instructions for proper backing. I'll bet not one lists plywood as an acceptable substrate for tile.........you'll void any warranty if you don't follow the recommendations.


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## jerryh3 (Dec 10, 2007)

bjbatlanta said:


> Read the tile mfgr's. instructions for proper backing. I'll bet not one lists plywood as an acceptable substrate for tile.........you'll void any warranty if you don't follow the recommendations.


Most support exterior grade, but interior or OSB is not recommended.


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## bjbatlanta (Jul 16, 2008)

I stand corrected, I would have thought cement board was the standard........that's what I get for thinking.


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## 26yrsinflooring (Jul 1, 2008)

*Use Ditra!*

All this hassle.....

*Come on you guys just use Ditra*!

One little tidbit of info that might interest you is:If you do not damp mop the hardibacker prior to the tile install and the floor fails:
*You have no warranty! *

And....... they can tell if you a have done it the right way!

Betcha ya did'nt know that!
Don't feel bad most people don't know.

Use Ditra! It solves all these issue.


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## Speedball (Nov 2, 2008)

I checked out the Ditra website and it is an interesting product. A little video showing it may be a very easy product to work with.


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## detroitMi (Oct 18, 2008)

"I'm wondering if you have to let the thinset dry before you screw down the Hardi backer? And is there a right and wrong side to the Hardi backer?" Answer to your question: don't let it dry,yes there is a wrong and right side,you cann tell which side goes... common sense


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## Divinepoodles (Oct 29, 2008)

detroitMi said:


> "I'm wondering if you have to let the thinset dry before you screw down the Hardi backer? And is there a right and wrong side to the Hardi backer?" Answer to your question: don't let it dry,yes there is a wrong and right side,you cann tell which side goes... common sense


Maybe you CAN tell which side goes...but why would someone ask if they already knew? :bangin:


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