Up and Adam [Ries]

As the title suggests, this post refers to the next step in finishing the bathroom floor. Because it involved laying down material, I really really wanted to title it “Getting Laid,” but my better judgement stepped in and changed it. Oh well, perhaps a future title.

What I laid was cement board. To achieve the 1-1/4″ total subfloor thickness before I can lay the finish tile floor, I put down 1/2″ cement board over the 3/4″ OSB I installed a few weeks ago (read about that huge step forward over here). The Durock Next Gen brand cement board came in 3′ x 5′ sheets, so it took 4 total to cover the entire floor. I purposely arranged them in a way that they would overlap the joints of the OSB underneath, so not to have seams directly on top of one another. First I dry fit each sheet, cutting them to size and then cutting out the hole for the toilet drain and tub drain access.

While some sources say to simply screw the cement board into place, a professional contractor here in town advised me to set them into mortar thinset, just the same as if I were laying the finish tile floor, and then screw down into place. Why? He said imagine the two layers of subfloor as if they were two regular pieces of lumber. If they are screwed together, the two boards are stronger than they were apart. But if they are laminated together, they are even stronger yet. The same for the cement board and OSB underneath: using mortar thinset “laminates” these two layers together, making the subfloor stronger and less likely to move or flex (which is what causes a tile floor to crack or grout to crack and fail).

I used a trowel to spread the mortar about 1/8 – 1/4 inch thick on the floor, in a square just large enough for one sheet at a time – this is so I wasn’t stepping in the wet mortar. Once I felt I had an even bed of mortar, I used the notched trowel to make ridges and then carefully laid each sheet down, making sure it was pushed flat against the outside wall studs.

I placed screws every 8 or so inches apart, on a grid across each cement board, maybe more along the edges of each board. Unfortunately, the bathroom floor will be about 1/2″ taller than the kitchen floor where they meet. Not ideal in my book, but a custom threshold strip stained to match the wood floor in the kitchen should make a nice transition. So now this floor is ready for tile! Onto the walls.