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.

6 Answers 6


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.


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


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


The thinset always go under Hardie or any rockboard. Use a 1/4 trowel and spread a consistant thin layer. This is designed to fill in any low spots and helps create a solid layer when bonded to the subfloor.(wood or not). You have to apply the Hardie screws in the tight pattern as required. This will spread the thinset evenly underneath. You must screw down each Hardie sheet right away before the wood substrate draws out the moisture and stiffens the thinset. This is correct method before tile to prevent grout pops. You can listen to plenty of experts that have done it differently but thinset is the professional way. Not the easy way or the fast way. If it's worth doing, its worth doing right.


I hate thinset under hardi backer board. A friend of mine who is a contractor said don't bother with the thinset but after reading a waranty warning I decided to go ahead and granted I'm a neophyte but I started with a level floor and wound up with not level hardibacker board because I'm new at thinset and the layer of thinset wasn't level. Now I want to take the hardi board off, scrape off the thinshet which doesn't really stick to wood (according to friend) and start over with hardi directly on the osb. I understand that hardi is trying to avoid complaints from customers who have laid the cement board on uneven floors but I say if your floor is level and even don't bother with the thinset.

  • Whether you hate it is not super relevant. What's particularly relevant are manufacturer specifications and recommendations.
    – TylerH
    Commented Jun 22, 2021 at 19:23

Floors are usually uneven upstairs. If you do not put thin set under the backer board, your tiles will start to break loose within the year.

  • "Floors are usually uneven upstairs" [citation needed]. While this may be true for some houses, there's no guarantee it's true for all. Even though this house is nearly 100 years old, it could still be structurally quite sound, or have recently had repairs to get the floor quite flat & smooth.
    – FreeMan
    Commented Feb 28 at 13:48

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