Condo Blues: How to Prep a Wood Bathroom Subfloor for New Tile




Sunday, January 26, 2020

How to Prep a Wood Bathroom Subfloor for New Tile

Just like painting a wall, the success or failure of your floor tiling project depends upon how well you prepare the surface for the tile itself. The more care and consideration you take when installing the tile underlayment, the better.

Flooring underlayment is the layer of material that sits in between the subfloor and the flooring material. As tempting as it may be to skip the underlayment – don’t. The underlayment layer is what keeps your floor tile from shifting and cracking over time.

And trust me, after the killer backache preparing and tiling this floor gave me, it is NOT something I want to completely redo if I don’t have to!

Fortunately, preparing a floor for tile by installing underlayment isn’t difficult to do and once you finish the job you can immediately go into tiling the floor.



how to install waterproof bathroom tile underpayment
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How to Lay Bathroom Tile Underlayment the Easy Way!

First you will need to demo the old flooring and the underlayment. I removed the vanity and toilet and stuffed a rag into the opening of the toilet plumbing to keep sew gas from coming into the the room. I used pry bar and rubber mallet to remove the wood underlayment and vinyl flooring that hadn’t been removed immediately after a bathroom leak as well as the base molding that I never liked anyway.

how to remove old vinyl floor tile
Buh-bye! Smell ya later!


Next, I had to remove any remaining staples that held the wood underlayment to the wood subfloor with a staple remove tool like this one which I highly recommend to save yourself from the heartache of trying to pull flooring staples out of a wood subfloor with pliers. It won’t go well. Trust me on this. (Disclosure: I am including affiliate links in this post for your convenience.)

Before adding to the now clean subfloor, use a level to check if your subfloor is level. If your subfloor is not level, you will need to take the extra step to level the subfloor otherwise your flooring may crack over time. I recommend using a self leveling underlayment like this which does all of the tricky bits of leveling an uneven floor for you.

Once your floor is level and clean (you will be one with your wet dry shop vac during this project. I recommend the Craftsman wall mount shop vacuum because has a decent capacity and yet light enough for a short gal like me to heft up and stairs to empty it and comes with attachments to clean strange areas where remodeling dirt accumulates.) It is time to measure, cut, and dry fit your tile underlayment.

You have two choices for tile underlayment that will withstand bathroom moisture:
  • Cement backer board which is also called cement board (learn more about cement backerboard here) which requires cement backer board screws and can be tricky to cut correctly if you are tiling a small room with a lot of fixtures like a bathroom.

  • Waterproof floor uncoupling mat underlayment which is a lightweight honeycomb polymer mat that uses thin set to attach to the subfloor and can be easily cut with a utility knife or scissors while you are doing your dry fit. Uncoupling mat underlayment is moisture resistant which makes it perfect for bathrooms. This is what I used and it is absolutely worth the extra money and I’m never ever going back to using cement backer board again. There are two major brands: Dural Durabase (learn more here) and Schluter Ditra (learn more about it here.) For this project I am using Schluter Ditra.

Measure, cut, and dry fit the underlayment of your choice in your bathroom. I was able to mark and cut the unpopular mat underlayment for the plumbing and to size while I was dry fitting it on the fly instead of trooping up and down stairs to the garage to cut cement board. No matter what type of underlayment you use, I suggest you wear construction kneepads like these because you will be spending all of the tile floor prep, tiling, and sealing on your knees which is killer – ow. Also if you plan to cut cement backer board, you should were a respirator (it looks like this) to protect your lungs from cement dust.

how to install waterproof bathroom tile underlayment
Dry fit time!

When you are happy with the dry fit it is time to adhere the underlayment to the subfloor using either cement backer board screws or thinset mortar if  you are using mat underlayment. I used this exact premixed thinset mortar because I can use it to adhere the Schluter Ditra to the subfloor as well as my porcelain tile to the mat underlayment.  Also I didn’t want to have to deal with the mixing thinset too thin, adding more powder that makes it too thick, then adding more water that makes it too think dance. Ya feel me?

I used the trowel to spread an even smooth coat of thinset mortar to a section of the subfloor just like icing a cake. Then I used the notched edge of the trowel to comb the thinset to make a series of rows that will allow the thinset mortar to really grip onto the felt side of the underlayment mat and stay stuck to the floor.


how to tile a wall

I don’t’ have an in progress photo of applying and troweling thinset to the floor. This photo from another tiling project shows the combing technique once the thinset is applied to the surface. The only difference here is I’m using the small notched side of the trowel to lay small tiles instead of the large notch side I used to lay the mat underlayment.


Once I was happy with combing a section of thinset I rolled the felt side down of the underlayment mat onto the combed thinset and used a clean trowel and my body weight to really press the mat underlayment into the thinset mortar since I am working in a small room. If you are titling a larger area, I suggest you use a weight floor roller like this one.


I repeated the process to seal the seams in between the pieces of mat underlayment with Kerdi-Band Waterproofing Strip (learn about it here) to keep water from potentially leaking through the seams before, during or after the tile is installed.

At this point the bathroom floor is prepped and ready to immediately tile!

Looking for more helpful tiling project tools and ideas? Check out the following options - and more! - below!

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