I tore out a Granite tile floor where the thinset stuck to neither the tile or the hardiebacker. The substrate is good but it appears that the tiles were not back buttered and or the Hardiebacker drew moisture from the grout. The only good thing about it was the fact that it was easy to pull up and clean the grout from the tiles.
I am resetting the tiles and I am going to use Ditra under the tile this time. I understand you use a modified thinset underneath the Ditra and an unmodified above.
My question is whether I should use a concrete adhesive over the Hardiebacker (applied like paint) as a bonding agent before I put down the Ditra?
Thanks in advance,
Jim
Replies
I guess my question would be why leave the backer board down? That like putting on a belt and suspenders except in this case it will make your floor that much higher than the ajoining floor. Ditra is meant to take the place of backer board (and be superior to same.)
Your situation is a perfect poster child for the need to scratch coat all surfaces when installing tile to provide proper adhesion.
I've never seen scratch coated hardi fail to bond with unmodified thinset, let alone modified.
As for pulling up the hardi, I'd leave it in place to help the solid feel of the floor. Every time a tile installer convinces the architect to not use hardibacker the floor has a more hollow sound and feel that to me feels cheap under foot. Membranes are easier for the tile setter, and are great for the long-term life of the tile, but it doesn't add anything to the feel of the floor and that's an important aspect for a high quality tile job.
For anyone not knowing what a scratch coat is, it's using a small amount of thinset that's forced onto the surface of the tile and needs to have a scratchy feel and sound when done right (this sound comes from the sand in the mix being wedged between the tile and trowel). Simply back buttering without a scratch coat is asking for trouble since it doesn't add to the bonding. Same goes for hardi or other backer board - it has to be scratch coated before the main body of the thinset is troweled out.
I'm a little curous as to why you'd want to "prevent proper adhesion".
Darn auto-correct on my phone did it! lol
I should proof read things better :)
scratch coat vs burning in
My version of a "scratch coat" is a thin (1/4" to 1/2" thick) cementitious coating that is placed, allowed to set up, then "scratched" with something like a scrap piece of metal lathe to create some tooth or texture in the layer, then allowed to cure. The tooth allows the finish coat, which is usually applied a day or so later, to adhere to the rough sctratch coat.
I think what you are referring to a scratch coat is what we call "burning in".
When tiling, I'll spread a thin veneer of thinset on the cement board with the flat edge of the trowel and work it into the pores of the cement board, then scrape most of it clean. It's what we call "burning in". It gets a 100% coverage of thinset on the board. Then the thinset gets respread and eventually combed out with the notched edge.
Same with the back of a tile, like a waffle tile, or large format tile, or stone tile. "Burn the thinset in" to the back of the tile. If you want an actual thickness ont he back of the tile, then that's back buttering. If I want thinset on the back of any tile, I'll burn it in, then back butter.
Might be tomato-potato semantics.