Flooring Tile - Under Layment for Stone Tile

I am putting stone tile in my front hall. The sub-floor is 1 1/4" plywood. My joists are 2x10 and 16" apart on center. They run about 8' from the foundation to a steel I-Beam, so I think I'm OK on sag.

I need an under layment to put on top of the plywood. I want to add as little height as possible. I could use a thin backerboard. The Tile Shop recommended two other options.

First is a wire mesh that is secured to the plywood and then go over that with a thin coat of thin set and then the tile.

Second, is a product called Uni-Mat. You use an adhesive to stick it to the plywood, then you put thin set and tile over it.

Has anyone ever used this product called Uni-Mat? Would you recommend it?

I would appreciate any direction concerning these three options: 3/8" backerboard, wire mesh, and Uni-Mat.

Which one is best, pros and cons of each.

Any information is greatly aprreciated.

Tripper

The wire mesh option is not an option. It is not an approved installation method and has been known to fail in a short period of time.

Ive not used the Unimat product, but understand that it is an isolation membrane like Schluter's Ditra. I use ditra on occassion - seems more and more these days, and its a great product. I would recommend it over the unimat, only because I use Ditra and know it works great. The Unimat may be just as good, but I dont have any first hand experience with it. The advantage of these membranes, is that they are thinner, lighter and easier to work with. They do a great job of isolating seasonal subfloor movement from tile. They are more difficult to put down a chalk line grid to follow for setting the tile, however it can be done. Only disadvantage I know of is they are not cheap. Ditra, and Unimat are designed in such a way that the thinset locks into dovetails in the top of the mat and bonds to the tile. The thiset doesnt actually bond to the membrane, it locks into the dovetails. Sounds kinda funny but it works real good.

Cement backer board is also a good choice. It is not as easy to work with as the membranes, as its a lot heavier and harder to cut. It also will increase the height of the finished floor as compared to a membrane. It is a good surface for the tile to bond to, and will isolate the subfloor movement somewhat, but not as good as the membranes.

More and more pros that I know are using the membranes these days. It has obvious benefits and does just as good, and maybe even better a job than the cement board does. Some old timers like myself still like the cement board because we have used it for a long time and we know it works well.

Years ago the wire mesh method was widely used and worked well. But, it is no longer an approved method. I asked a tile rep once why that was and he told me that wood, because most of it anymore is grown on farms and is force grown, doesn't have the density it once had and the mesh method will no longer hold up under the greater expansion and contraction factors. It made sense so I quit using the mesh method and went to concrete board. I know from installing carpet that the less dense thing is correct. When putting tack strip in a really old hose, the nails take more effort to sink and they hold very well. But, in a new or recently built one, the nails go in almost like they're sinking into cardboard and I have to add reinforcing nails or the stretcher pulls the strip right off the floor. I use concrete board exclusively. I knew pretty much nothing about the membranes before getting involved with this site and suppose I should try one, but, as Henry Blake said on Mash, "better the devil you know" and I'm familiar with concrete board so I stay there. It comes in different thicknesses and you should use the 1/4 inch version on the floor.

Johnny and Smoke,

Thank you so much for your quick and informative responses! It is greatly appreciated.

I think I'm going to go with the membrane....just to try it. The dovetail thing makes sense to me.

Trip

PS- Gotta love Henry Blake....

Tripper

Thats a good decision.