Hardi board

[houseslave]

Is it necessary to add thinset to the back of hardi board before tile installation. It seems excessive considering that the board gets screwed in every eight inches?

[dadiOH]

The purpose is less adhesion than filling...assuring that it is in good and continuous contact with the sub-strate.

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[Amuzed2death]

First, when you say "hardi board" are you referring to cement board? One brand name would be "Durock" I think?

Keep in mind, I am NOT an expert by any means and I'm just thinking out loud. I'm guessing, the reason you would put thinset behind the board is to fill in any gaps or voids that may be present when installing. In other words, if you are laying this stuff on the floor, as a base for floor tile, the thin set would fill in any gaps between the subfloor and the hardi board. That way, the hardi board couldn't "flex" at all. And you wouldn't want any "flex" if you are putting tile down, as it would be more prone to crack your newly tile floor...or tiles could "pop" out, due to the constant flex when walking on it.

Taking the time to put in a tile floor, only to have tile pop off or cracks form, could be frustrating at best. I would say it's your call, but if you are confident the subfloor is sound and won't leave any voids between it and the hardi board, then it might not be necessary. But again, is that a chance you want to take before putting all that work in?

Hope that gives you something to think about. Like I said, I'm just thinking with you, out loud. I'm not completely sure if that's the right answer.

[Drummer of The Vibe]

All the varieties are known generically as CBB or Cement Backer Board and you're probably talking about the Hardibacker brand. Three are instructional pamphlets available right where you buy it. Whatever the specific brand, you must embed the board into thinset applied with a notched trowel. This keeps the board from spanning any gaps. If you don't do this, you might as well skip the backerboard altogether.

Bobby Bobby

[RB]

Yes. You want to increase the thickness of the floor to increase it's stiffness. By using thinset between the plywood substrate and the cement board (I prefer Durock over Hardiboard) you add 1/2" to the thickness of the floor. Since the stiffness of the floor increases as the thickness to the 3rd power this is non-trivial.

I just finished placing Durock on top of plywood over about 1000 ft2 of plywood in preparation for tiling. I used a latex fortified mortar and the floor is noticeably stiffer now than it was previously. Just screwing the Durock down would not have done the same thing. It's a lot of work but it is worth it.

RB

houseslave wrote:

[houseslave]

Did you use a 1/4" notched trowel? Did you apply thinset to the sub floor and then back of the cement board?

Thanks

[RB]

Mix the mortar so that it will just hold the ridges formed when it is combed out with a 1/4" notched trowel. Thicker mortar will make it more difficult to apply and level. Apply to the subfloor only, first with the un-notched edge and then comb it out.

I like to slightly moisten the subfloor so that the plywood doesn't suck the water out of the mortar.

I prefer to use Durock (dual helix) screws (not drywall screws) but found them hard to find. Home Depot was the only place that stocked them here. My lumber yard had screws shipped in from their USG distributor but they were generic and didn't work as well.

Mortar and screw down one, or at most, two sheets at a time.

I also bought a Makita 6916 battery powered impact driver and it made the job go much quicker. Getting the screws flush without an impact driver is a challenge.

After the Durock is in place tape the joints using mortar and Durock fiberglass tape.

Two cautions. Screw heads need to be flush, if they are proud of the surface you'll have difficulty laying tile flat. Same caution with respect to taped joints, take the time to make sure the joints are not proud of the surface.

Once you have a flat surface laying the tile is a breeze. Before I start laying tile I go over the entire surface with a rubbing block just to catch any high spots. If it's not flat the job is hell.

RB

houseslave wrote: