I have been to dozens of sites and there are dozens of recommendations. We are about to lay good ceramic tile through much of the house, including bathrooms, kitchen, hallways and a family room. These areas are all interconnected. We are laying three tiles sizes, 18x18, 12x12, and 18x12. We are laying around 700 sq ft. of tile, with some areas being over 24 ft across. We are not laying any tile on concrete. We are taking up the lino. We have 5/8th inch plywood subfloor. On top of that is 1/4 inch plywood subfloor. We purchased the "top of the line" "One step polymer fortified tile adhesive". (I see no reference to laytex on the bag.) We have purchased everthing from a local tile company and went with their recommendations. My research says we need at least 1 1/4 inch subfloor. When we went to the hardware store to buy more subfloor they said 1/2 inch OSB could be put on top of the existing plywood floor and that would be fine. What's the real story here? What sub-floor should we put down? Do we need a waterproof membrane? If yes, can we use the roll-on? Do we have to use Ditra? Would an option be to just put 1/2 inch concrete board on top of the existing sub-floor?
In the kitchen we are not replacing the cupboards. I do not have the option of going under the cubboards. We will, of course go under the fridge and stove and I would like to go under the built in dishwasher (it is time to replace it anyway.) What do you recommend to deal with the change in height? Thanks to anyone who can help.

This message has been edited. Last edited by: Keelie,

If you try setting tile over what you have, that tile will come up almost as fast as it goes down. First of all, the 1/4" has to come up. 1/4" ply is notorious for delaminating under tile. Pull it and replace it with 1/2". After that, you can either go over it with 1/4" cement board, or Ditra, if you want to try and kep the height down (the Ditra only raises it 1/8"). Whoever told you you could set tile over 1/2" of plywood and nothing else has no idea of what they're talking about.

Thanks Bill. I just clarified by first post. What they said was we could put 1/2 inch OSB on top of the existing subfloor and that would be fine. Other sites have said OSB is not a good sub-floor. Some sites say you should have a water proofing membrane and others say its not needed. A number of sites talk about Ditra but none of them say much about the roll on waterproof membrane. I just want to know what are the options, what works. Thanks again to anyone who can help.

If you're going to install the 1/2" layer anyway, might as well do it right and go with exterior grade plywood. OSB WILL suffice, but the plywood will give you more strength to the floor. As for what you can do from there, yes, you can use one of the roll on waterproofing membranes, such as Custom's Redgard or Laticrete's 9235 over the plywood. I prefer, however, if you're going to use a membrane as a bonding surface, to use a sheet membrane, LIKE the Ditra, or Noble's CIS ( http://www.noblecompany.com ). The other alternative, as I said before, is to use 1/4" cement board, thinsetted and screwed (or nailed) to the subfloor.

Thanks. Your reply was helpful. Just to be on the safe side we are gong to go with the plywood as you suggested. In the bathrooms and around the kitchen area and laundry room we will put down a vapour barrier. We are trying to find the product you suggested but so far none of the local hardware stores carry it. We are checking out some speciality tile stores today. Thanks again.

You should be able to get any one of the four membranes (Redgard, 9235, Ditra, or CIS) from a tile showroom. If you go to Home Depot, they'll have the Redgard AND Ditra. If you go to Lowes, they'll have the 9235. If you go to http://www.noblecompany.com ,you can buy the CIS right off their website.