Underlayment

[woodturner]

BUT, when laying down something like ceramic or stone tiles you are faced with the problem of the subfloor flexing or moving under both the weight of the furniture, possible movement as the seasons change, and of course traffic. The tile itself will not flex and therefore if the subfloor is not absolutely stable, you will get fracturing at both the grouted seams and even cracking of the tile itself.

Not a tile expert, but I read the John Bridge forum ;-)

Most people on that board recommend Ditra or thicker plywood - the key is to limit deflection or flex, as you noted. They have a calculator on that site that will help determine if the floor is stiff enough for tile and can support the weight of the tile.

[chopnhack]

Why not just use transition molding between rooms?

[Helicon1]

I would go with BOTH glue and screws. If you do it, I say do it right and make sure it's solid. I added 1/4" ply to an old 3/4" plank subfloor, and tiled over that. The subfloor obviously wasn't solid enough, and the grout lines cracked and a tile split. Do both and don't worry about it not being stable enough.

Thanks that was what I was going to do. Glue and screws. It is going under hardwood though and not tile, so that is a different story altogether. I had the tile installer use ditra which gives the uncoupling to prevent cracking under the tile.

Why not just use transition molding between rooms?

I could but we don't like them. Replacing all the finish flooring in the house, so why not have an entirely level surface.

As it turns out the tile is even higher than I even expected. So now I can safely go with 3/8 ply glued and screwed down under the hardwood.

My worries were with the 1/4 ply that the screws would pull through if countersunk too deeply and make noise, and alternately if using staples I was worried those would squeak over time as they would be going into 3/4 OSB subfloor. Not as much of a concern now with 3/8.

And I'm not worried about glue since adding the 3/8 everywhere in the house makes the subfloor an even 1-1/8" on all surfaces. If somebody wants to replace down the road, they don't need to go any further than the 3/8 ply.

[gsmittle]

Why not just use transition molding between rooms?

I can't speak for anyone else, but to me transition molding looks tacky.

My floor is finished, no squeaks and only a few quiet creaks where the subfloor creaked already. Someday I'll get around to fixing that, but right now it doesn't bother me.

g.