The purpose is less adhesion than filling...assuring that it is in good and
continuous contact with the sub-strate.
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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
firstname.lastname@example.org (Amuzed2death) wrote in message
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.
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.
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
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.
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