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I'm remodeling the bathroom in my house that was built in the 1930s. I stripped out the old flooring down to the 1x8 wood planks (about 1/4" between each).

I have 1/4" Hardie Board to put down in preparation for tile and I'm a little confused on how I should do this.

Some resources I've found have said to lay down mortar first, then screw the board into the floor. Other resources have said that you can just screw it into the floor in appropriate locations. Others yet have recommended laying down plywood first, then Hardie Board, then tile. I'm really not sure what to do here.

The wood floor outside of the bathroom is only about 3/4" higher than the plank subfloor in the bathroom. I'm concerned about ending up with a bathroom floor that's considerably higher than my wood floors.

Thanks for your help.

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I have done several tile floors by just screwing the hardie board down and then laying thinset and tile right on top. I only just heard recently that some people recommend that you should lay down thinset under your hardie board.

Since you are putting down hardie board on boards with small gaps, I would not recommend trying to put down thinset under it. It will make an enormous mess under the house (or whatever is under your bathroom). It is also more likely to make the floor uneven since it is hard to level thinset that is separated by gaps.

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The installation manual for HardieBacker suggests to embed it in thin-set and it should be installed over plywood.

Thus I would suggest removing the current wood planks flooring, install a 3/4" outdoor grade plywood, thinset and then your HardieBacker (screwed down).

All of this is to prevent as much movement in your substrate and avoid cracks in your tiles and in your grout.

Here is the link to the installation guide: http://www.jameshardie.com/d2w/installation/hardiebacker-us-en.pdf

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I normally would use 1/2" HB for this. However there is a lot that goes into making flooring calculations so you might be OK with 1/4" (but I would go with 1/2").

You will just screw the HB into the planks with backer screws. Try not (don't) screw into the joists below the planks. This should be pretty easy since you have 1/4" view holes in between each. Give the HB about 1/4" on the outer edges between the wall and about 1/8" between each other. You do not need to tape or thinset. The thinset you use for tiles will cover any gaps (and gaps don't need to be filled).

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I suggest you lay down plywood over the planks with subfloor glue and decking screws. After that I would use a thin layer of thinset under the hardiboard (per its installation instructions) and also screw it using the hardi-screws.

Install the backer tape as you tile

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