# thinset under backerboard.



## Big Dave (Feb 23, 2006)

Just for my information, how much thinset do you need under backerboard? Or better what size notched trowel should a person use?

Dave.


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## Glasshousebltr (Feb 9, 2004)

1/4" square notch.

Bob


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## Big Dave (Feb 23, 2006)

Thanks Bob, that's all I wanted to know. I've been told 3/8 square notch and I've been told 1/8 v notch but now I know the facts.

Very much appreciative, Dave.


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## R&D Tile (Apr 5, 2005)

And use un-modified thinset.


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## MattCoops (Apr 7, 2006)

if its on a basement floor and there is paint or a waterproofing agent dont use thinset underneath backboard, it wont adhere properly

use screws with cement backerboard


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## Mike Finley (Apr 28, 2004)

Isn't trowel size determined by the thickness of the material you are setting and then only because of it coming up between the grout lines?


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## R&D Tile (Apr 5, 2005)

MattCoops said:


> if its on a basement floor and there is paint or a waterproofing agent dont use thinset underneath backboard, it wont adhere properly
> 
> use screws with cement backerboard


Matt, thinset under CBUs is there to fill any voids, not adhere them to the floor, oh and NEVER install a CBU over a concrete subfloor, only over plywood or OSB, wood floors.:no:

Mike, to a point that can be correct, there are other issues that also determine the size of the trowel to be used, but in this case, we're talking about trowel size for installing a sheet of backerboard, not the tile itself.


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## MattCoops (Apr 7, 2006)

*CBU over cement is OK*

The first job I ever did was a basement.
I was a helper with this old yankee, with 28 years in the union
he had me mix thin-set with laytex add-mix and spread over concrete slab, then beat in cement backerboard with a mallet, then apply tile.

We went back to that job a year later to do her kitchen


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## R&D Tile (Apr 5, 2005)

MattCoops said:


> The first job I ever did was a basement.
> I was a helper with this old yankee, with 28 years in the union
> he had me mix thin-set with laytex add-mix and spread over concrete slab, then beat in cement backerboard with a mallet, then apply tile.
> 
> We went back to that job a year later to do her kitchen


Why in the world would you want cementboard over a concrete slab, not only was that an incorrectly done installation, it also voids the manufacturers warrantee, unless he wanted more height, and in that case there are other products to do that correctly.:no:


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## MattCoops (Apr 7, 2006)

you can't tile a concrete floor if it has a coating of paint or waterproofing agent :no: 

Sanding would be a big mess, that's for sure, and even then you can't be sure the tiles will hold. 

Even if the bond between the thinset and the basement slab eventually fails, the boards will "float" in place and the tiles will stay secure

you dont want tile floating

Now,
If it were just a concrete slab, clean of paint and all, you could use RedGuard (2 coats) then thin-set and apply tile


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## Rich Turley (Apr 9, 2005)

MattCoops said:


> you can't tile a concrete floor if it has a coating of paint or waterproofing agent :no:
> 
> Sanding would be a big mess, that's for sure, and even then you can't be sure the tiles will hold.
> 
> ...



Floating tile????

A little experiment:
Tile some CBU
Lay it on the floor letting it "float"
Walk on it
Watch it fall apart over time

And why use RedGuard (a waterproofing membrane) for a non wet floor?

Go to www.tileusa.com and check out the link to publications. You can learn the industry standards and ANSI standards. 

Can you break the rules and the installation not fail? Like sticking CBU to the cement? Sure, it is called getting lucky. 

To offer installations that will hold up for 20 years consistently you can't rely on luck.

Rich


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## Mike Finley (Apr 28, 2004)

CBU, Hardi, DensShield all have to be mechanically fastened.



> I was a helper with this old yankee, with 28 years in the union


That's where the problem was, you were a helper and didn't know any better, he was 28 years in the union and has spent the last 20 years making sure he doesn't learn anything new. Matt - don't follow in his footsteps.


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## R&D Tile (Apr 5, 2005)

MattCoops said:


> you can't tile a concrete floor if it has a coating of paint or waterproofing agent :no:
> 
> Sanding would be a big mess, that's for sure, and even then you can't be sure the tiles will hold.
> 
> ...




Your not serious, are you? 

If you shotblast or scarify the paint off and the slab absorbs water, you will have no problems installing tile or a membrane first over it, or just install a mud job over the slab, tar paper, lath and 1 1/4" minimum of mud, this will float as one, DO NOT ever do it with a CBU, please.:no:

And as far is Redgard is concerned, yes, it's also used as a crack isolation membrane, as well as a waterproofer, but I would always rather have a sheet membrane installed for cracks, as opposed to a liquid applied one.


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## MattCoops (Apr 7, 2006)

The reason why you dont install tile right over concrete in basement is that moisture rises up through the ground. Hence the reason for RedGuard as a waterproof agent. Down south there is a lot of muddy earth and a lot of water in basements. That's why most houses are built on slabs with no basement. RedGuard blocks out water from penetrating through concrete. Of course there are other waterproofing methods. That is just method I prefer and find to be easiest and cheapest.

TCA states that all cement backerboard be installed over thin-set and then screwed.

If your using DensShield, even the instructions on sticker say to apply a sheet over thin-set and then screw.


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## R&D Tile (Apr 5, 2005)

Matt, I understand how CBUs are installed over floors, we're talking about slabs here, YES?, NO CBUs ever over a slab.:thumbdown 


As far as Redgard is concerned, your better off with a sheet membrane on a floor, also if there is a moisture problem from Hydostatic pressure, that should be addressed first, Redgard will fail in this case, the floor shouldn't be covered until the problem is solved, better off not using a membrane, as long as the floor is dry when installing tile with thinset, moisture isn't an issue after it cures.


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## dayspring (Mar 4, 2006)

Have you ever seen CBU fail if it didn't have thinset under it? Screwed to wood floor. I saw a tile man do this once about a year ago.


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## MattCoops (Apr 7, 2006)

i would not lay CBU on slab. I would lay it in basement. But i would lay redguard over concrete regardless what sort of tile to be laid. There is nothing wrong with your way. I just prefer RedGuard.

And no, CBU wont fail if you dont lay thin-set underneath. It is just the factory recommendation. I don't think I even screw it to factory recommendations.


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## R&D Tile (Apr 5, 2005)

MattCoops said:


> i would not lay CBU on slab. I would lay it in basement. But i would lay redguard over concrete regardless what sort of tile to be laid. There is nothing wrong with your way. I just prefer RedGuard.
> 
> And no, CBU wont fail if you dont lay thin-set underneath. It is just the factory recommendation. I don't think I even screw it to factory recommendations.



Boy, this saga never ends, all the basements I've been in have concrete floors, SAME as a slab.:wallbash: 

And YES, if you don't install thinset under backerboards, your floor has a very good chance of a failure, it's not a recommendation by the manufacturers, it's needed and they tell you so, you also void the warrantee if not used, and the fact that you don't even screw it per specs is foolish and those that listen to this should not follow in your footsteps.:wallbash:


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## MattCoops (Apr 7, 2006)

the reason why I lay CBU in basements is due to water.
I feel more comfortable with tile installed over CBU in basement than right on slab. Basement floors eventually will be susseptable to moisture. Laying an underlayment over the basement concrete is a good idea (whether it be CBU, Ditra, DensShield, paper). It also keeps floor covering warmer by insulating concrete.
Plus, when concrete dries and sets, over time, it contracts and cracks. This will have an adverse affect on tile. All the more reason to lay it on top of CBU and RedGuard.


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## R&D Tile (Apr 5, 2005)

You know what, do it your way, some people just don't want to listen and learn, I GIVE UP, just hope no one follows that advice.:no:


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