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Unread 05-20-2008, 04:31 PM   #1
Join Date: May 2008
Location: Campton, NH
Posts: 2
Backer board use over below grade concrete floor

I came across a posting to this forum that had to do with the use of cement board over a basement, concrete floor. A gentleman indicated that he planned on using DriCore finished-off with carpeting for the greater part of his project. But, for a bar area, he wanted to use cement board to raise the level of the tile so it approximated that of the carpeted area.

Advice given ranged from: 1. essentially live with the height difference using transitioning threshholds, 2. forget the DriCore, or 3. mud up the bar area so as to match the carpeted area. EVERYONE pretty much said NOT to use backer board as a substrate over a concrete floor for this purpose.
(There was mention of Wedi board that apparently could be used except that it was difficult to find.)

My situation and dilemna is similar to this man's. I am converting a walkout basement to living space. I have installed Platon (an Armtec sheet product similar to the PVC part of DriCore) plus 5/8" ply. A difference is that my tiled area is probably considerably larger...200+ sq ft. It would seem a lot to mud ...or, am I too timid?

The answers to that previous poster never said WHY NOT to use backer board. Is it that the manufacturer doesn't want to be held liable should something go wrong. Are the fears founded? Are there valid structural concerns? I discovered one "heretic" (name left out by me) cited as "a tiling contractor in MA who has worked on many This Old House TV projects" (on the TOH website). He advised someone to do just what most posters said NOT to do. He said "Even if the bond between the thinset and basement slab eventually fails, the boards will 'float' and the tiles will stay secure." Is this an issue...that in fact if this were to occur, the tiles would NOT stay stable? I would like to hear your thoughts. Thanks!
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Unread 05-20-2008, 04:40 PM   #2
Veteran DIYer- Schluterville Graduate

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Join Date: Jul 2004
Location: Nashua, NH
Posts: 13,835
Nobody recommends installation of cbu over a slab. It is not possible to ancor it to the slab mechanically, and you could introduce voids that could allow movement. How much height do you need?

Wediboard is not normally installed with fasteners (on a floor, if I remember correctly), so it doesn't really know whether it is a subfloor or a slab.

Schluter has a new version of Ditra that is about twice as thick, putting it around the same as 1/4" cbu, that might be an option. Or, EasyMat can be obtained in various thicknesses, and it would add some insulation to the floor.

The last thing I'd add is cbu. Without fasteners, I think you'd end up with slight differences in the height between sheets, and give yourself fits with then trying to tile, let alone possibly losing the bond.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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Unread 05-21-2008, 07:36 AM   #3
Join Date: May 2008
Location: Campton, NH
Posts: 2
Thanks for the reply JadNashua,

I figure the combined height of the one floor would be:

Platon 1/4" + Ply/OSB 5/8" + low pile carpet 1/4" = 1-1/8"

Tile 3/8" + thinset 1/8" (?) = 1/2"

This would leave about a 5/8" difference in floor level assuming the thinset and tile thickness are typical.

With 1/4" Ditra it would bring it down to about 3/8" which seems acceptable although I would prefer that they were even in the best of all worlds. I like the idea of the Ditra which allows for moisture migration as does the Platon. My only question about it has to do with the way it is taped to the walls with KerdiBrand. Wouldn't this defeat the moisture migration and trap the moisture causing mold and deterioration of the sheetrock paper?

From a quick check of EasyMat it would gain 5mm or about 1/16" less than Ditra.
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Unread 05-21-2008, 07:56 AM   #4
Tile Contractor -- Seattle, WA.
Join Date: May 2007
Location: Seattle
Posts: 1,291
A guy I used to work for used cbu on basement floors. He used mushrooming spikes. It seems like a ridiculous amount of work(drilling with rotohammer) and money-those little spikes are spendy.

Use the thick ditra or double it up, maybe?

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