Half Bath Renovation: Underlayment

Concrete backer board cut to fit? Check. Thinset mortar mixed and ready to be troweled? Check. Experienced installers? Be gentle, this is our first time.

My partner decided he wanted to get the underlayment on the floor, and I agreed. It was ready to go and wouldn’t take that long to install, so he mixed the thinset and allowed it to rest for about ten minutes before he began troweling it directly onto the decking subfloor. This worried me at first, me not standing over his shoulder questioning every little thing he was doing because I am incredulously annoying like that, but I figured I’ve never done this either, so I should maybe keep my big mouth shut just this once.

After he put the first piece in, I quickly went in to help screw the backer board into the decking to secure it even further. I started with just a couple screws in each back corner then screwed into every board, staggering them into a wave pattern to prevent any cracking that might occur, on the front of it so that when the second piece was installed we’d know where the decking was to screw into.

So I stepped out of the half bath and allowed my partner to trowel on the thinset to the other half so we could install the second piece. We dry fitted this, so it was bound to all fit together perfectly, right? Ha hahahahaha! Surely you jest! It became increasingly obvious that we needed to cut off about a quarter inch from one side in order to make it fit into place, so with thiset already gooping up the backer board, my partner takes it into the garage and proceeds to trim the piece. Of course, now that it is cut and laid to the point where we can’t pry it out without breaking it, where the two pieces meet are overlapped and not moving.

“Rubber mallet!” I proclaim. “I need a rubber mallet!”

“Where is it?” my partner asks as he walked into the garage.

“In the canning pantry, left side, third shelf down next to the toolbox,” I inform him with my eyes closed to imagine its exact location.

Within seconds he returns with the rubber mallet and a few quick whacks is all it takes to set the backer board into place. I begin screwing it into the decking, using the screws from the first piece as a guide to where the subfloor boards are, and work my way around before screwing the center areas. I also put in a few extra screws in the area where the floor was saggy and around the toilet.

When I was done screwing… that sounds dirty… I grabbed the joint tape, stuck it onto the seam, and troweled on some thinset over it. Underlayment laid and screwed and ready for tiling, only one more test before doing so… walk on the sag. So I step onto the area where it sagged and lo and behold it didn’t move. Now my partner tests out the area and it still doesn’t move. Success! Operation Underlayment complete! Now we have to dry fit the tile and mark any pieces that will require cutting. Ugh.