Hi, I am new to this board and came across as I was doing research on a recent problem. We are DIYers remodeling an exsiting bathroom. We were relying on advice from an individual and have just found out that we laid the cement board incorrectly. We laid it down without any thinset. WE put the project on hold and are doing research on every step before we proceed. Will this be a big problem in the future? Any advice with be great!

Yes this would be a problem later.

Cement board is required to be set in a layer of thinset and fastened as per the manufacturers specifications.

Pull up the board and use a 1/4" x 1/4" x 1/4" square notched trowel to apply the type of thinset that is recommended by the cement board company along with the appropriate fastners.

Thanks for your reply. We did use the correct fastners, backerboard screws what kind of problems would have occured if we would have continued and laid down the tile? Just wondering.....

If the area under the CBU is not filled there could be movement between the CBU and subfloor due to any voids or imperfections on the subfloor surface.

If you walk on the CBU and hear a crunching sound this is the first indication of movement.

Look at the manufactures name on the CBU, then go to their website. All the installation specs are listed there.

One of this forum's moderators (Bri) sent me a short video close-up of him stepping onto the cement board. It clearly showed the cement board moving and becoming loose from the screws.
He was able to get a side view which showed the subfloor and cement board with obvious voids between the two materials. If I can load it onto this site it would be a very helpful tool in understanding why mortar must be used.
I offer the same advice as Randall .... remove the screws and apply mortar.

Thanks for all the advice, I will remove it this weekend. I have one more question, since our house is older, our subfloor is made up of wood planks that are about 8 inches wide and there is about 1/2 gap between each plank. Isn't the thinset going to fall through to the crawlspace? Silly question, but I want to make sure we do this right. Thanks again

Hi 402,

Glad you mentioned that little fact too. You MUST install a layer of exterior grade plywood over the planks and then the backerboard. I would recommend 1/2", I think?

First, re-nail or screw the old boards into the joists and replace any damaged or split/cupped pieces. Install the plywood with face grain running across the joists gapping at least 1/8" between sheets and 1/4" around the perimeter. Offset the sheets approx. half their length. Fasten with 1 1/4 screws or 'underlayment' nails, every 5-6" in the field and a bit closer on the edges.

Now you're ready to install the cement board. What brand is it anyway?

Before you start tho...can you tell us the size, spacing and the span of the joists? You need to measure this from underneath, and see if you can read the species and the grade of the joists too.

Jaz