We are in the midst of adding a bathroom to our second floor and we’re to the point that we are drywalling, etc. I would like to put cement board over the plywood subfloor before tiling the floor (we are definitely using the cement board for the shower stall and the mold-resistent drywall for the rest of the room). My Father-in-law thinks the cementboard is just one more step he doesn’t want to do (my husband’s in Iraq, so he’s helping out). He says it’s unnecessary and will make the floor higher. I say another quarter inch or so won’t make a difference and I don’t want to have to take up the floor in a few years when moisture has gotten to the wood (like our downstairs bathroom that is next on the reno list). What do you think? Am I being paranoid, or can we get away with no cement on the floor?

I assume that you are referring to a ceramic tile or something other than vinyl. For a vinyl tile, I would just tile over the subfloor using a good quality epoxy and tile. I always put a layer of 1/2″ cement board over the plywood subfloor prior to installing ceramic or natural stone tile. The thinset will adhere better to the cement board and the layer of cement board will help keep the tiles from cracking.

By adding cement board to your Bathroom floor is an excellent way to make sure no moister penetrates the floor and it is great for setting tiles.You will need to seal the seams in the cement board with tile set cement or anything similar.Your toilet probably will need longer screws and a double wax ring .It definitely is a lot more work but it is worth it.

I think it depends. If you’re putting down ceramic/porcelin tile, it’s a great idea. It will strengthen the surface and be protected for years from day to day foot traffic and moisture that penetrates the grout. If you’re putting down vinyl in a sheet, it’s probably not necessary. You’re right in that a 1/4 in. won’t make much difference. But if you add tile, it may add another 1/4-1/2″ as the tile would have to be adhered with thinset mortar. Sometimes it means you’d have to cut a door at the bottom and add thresholds at the bathroom entrance. Another caveat would be replacing the seal on your toilet as the higher floor would cause the seal to break. All extra work, but well worth it in the end.

There are a couple of alternatives to Cement Board. One is a product called Ditra the other is Unimat. Both require a good solid floor. There cant be any spring in the floor. Either plywood or OSB is ok. This comes in rolls and is about 1/4 inch thick. It’s a lot like putting down the underlayment for engineered flooring. Roll it out flat and then lay the tile with Thinset.