Flooring Tile - backerboard question

Hi all,

This weekend we totally gutted our kitchen (no turning back now). All that's left in the room are the walls and the wooden subfloor. Putting in the ceramic tile floor is our first project. I know that we need to install cement backerboards. This is what I've found out so far, and I hope that you can confirm or issue better advice. Also if you have advice to make the installation easier- by all means, please give it!

buy 1/4" cement backerboard
make cuts with a scoring knife
attach to subfloor with thinset mortar
attach again with screws or nails

Questions that I have are:
- Is it wise to use mortar on top of subfloor, or do we have to put down an underlayment first? If underlayment is advised, what kind and what's the thinnest we can go?
- What's the purpose of both mortaring and screwing it down? Why isn't one or the other enough?
- About how long do you think it'll take to do a 10x13' room?

Thanks!
-Stephanie

Whether or not you can put mortar directly on the subfloor depends on what the subfloor is made out of ("wooden" is not sufficiently precise).

The mortar does not adhere the cement board to the subfloor. Only the screws/nails do that. That's why you use so many of them. The purpose of the mortar is to provide a firm base and eliminate air gaps. The cement board and the subfloor will move independently of each other (slightly) with various temperatures and humidities (mostly humidities).

Thanks so much for taking time to write back, I appreciate the advice.

This is for clarification:
wooden subfloor: The planks are ~5" wide and I think 1" high. It's really difficult to tell height because although the planks seem to have a 1/4" space between them, you can't look down into the basement. There's more wood. Maybe back in 1948 (when the house was built) the wooden planks had tongues on them?? The joists underneath are ~16" on center and 2" thick- all in really good shape.

Regarding the mortar:
Please forgive my lack of full comprehension... mortar will adhere cementboard to tile, but not adhere cementboard to wood? I understand now about why the mortar is necessary. However, we'd like to make it relatively easy for the next poor soul if they want to entirely replace the floor. The old linoleum had a waxy paper under the underlayment. Is it possible to put down something similar so that the mortar fulfills its function with the cement board but doesn't become embedded in the subfloor grooves?

Thanks!
-Stephanie

P.S. We will not be the poor souls who next gut the kitchen!!

Hi
Just to clarify...you shouldn't put the cement board over the 5 inch wide planking alone. The cement board doesn;t have any structural strength, so you should add extra plywood first and then the cement board. Screw down the planking as best you can to stop movement...into the floor joists. THe next layer of plywood can then be screw down too. Then when you add the layer of 1/4 cement board(along with the mortar under it) you should try to avoid the joists for this layer and only screw it down to the plywood underneath. This helps isolate the cement board layer from the stresses from the rest of the floor(as much as possible) I know it's a lot more cost and subfloor thickness, but it;s a step that is needed to avoid cracking in the future.

oh yeah, the extra layer of plywood should be at least 3/8 or 1/2 ply.