12x12 onyx on top of existing tile floor

[captfishhead]

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I am about to install 12x12 caramel colored onyx tile on top of an existing tile floor. The home was built in the early 80s and the existing tile floor is laid on a layer if gypcrete and is very stable. my question is what is the best process for doing this job? Should I use a bonding agent on top of the floor before I begin or will thinset adhere to the existing tiles. Should I use epoxy on the floor? If I grind the surface of the existing tiles will a thinset bond be good enough? The grout joints will be 16th of an inch is that a problem?I have set thousands of feet of floor tiles in the past but this onyx seems fragile , glassy smooth, soft and extremely expensive. I only want to do this job once so I have to get it right the first time.Any and all advice will be appreciated

[mctile]

Well, first of all, you HAVE to make sure that your joist system will support your onyx. Use the deflecto above to see if you floor has the proper deflection rating. You are right, these tiles are extremely fragile. You may get consiterable chipping on your cuts so no exposed cuts. If you have to expose a cut, cut it about 1/8" big and use polishing pads (dry marble pads will work fine) to bring it down to the desired size. Next, make sure you burn the backs of the tiles. This will do two things. It will get you a superior bond as well as make the tiles look uniform due to the transparent nature of onyx. And finally, 100% coverage is very important with onyx. If you have weak coverage, you will have broken tiles, period.

[captfishhead]

Thanks for the timley reply. So you think that its ok to just scuff up the old tile floor and use thinset to to set the new onyx? Also the idea of polishing the exposed cut edges is a good idea as much of the cuts will be coverd by shoe moulding. Your comment about burning the backs is not clear. Do you mean to coat the backs with mud before I lay them?Thanks for the response.

[Mike2]

Your comment about burning the backs is not clear. Do you mean to coat the backs with mud before I lay them?

You've got it, Capt. Skim-coat a thin layer of mortar onto the backs using the flat side of the trowel.

Have you checked out your joist structure using our Deflecto tool? I'd highly advise you to do that since onyx requires a much stronger floor than ceramic or porcelain tile.

Gather up your joist information and enter that into the Deflecto tool found on the blue bar above. Then come back here and tell us what results you get.

[captfishhead]

Well, the deflecto chart does not include deflection for tgi members but if I use the calculator for 2x10 fir then the rate of deflection is 857 which would be more than sufficent for natural stone. Thanks

[Mike2]

You're right, Capt., Deflecto only works with sawn lumber. You can't even use it to approximate what you might have with any given "I" joist size and span.

Deflection data for your TJI joists must come from the manufacturer. We do have links to some "I" joist span data tables in the Liberry. You might be able to find yours there. http://johnbridge.com/vbulletin/showthread.php?t=9355

[captfishhead]

I looked at the links and have determined that the structure is sound. I will take plenty of precautions when installing this stuff because it costs about 32 bucks a piece. I do not know alot about onyx so my main concern is adhesion and durability. Is there a better thinset to use over another?Oh one other thing the tiles have a small bevel on the edges, can they be installed butted to each other? Thanks for the advice ,captfishhead

[Mike2]

Knowing how soft and easily scratched onyx is, how do you think it's going to work out on your floor, long term?

[MICK1]

Also you may want to use white thinset, the caramel onyx or Amber Onyx it is an expensive Onyx, if you are polishing the sides you want to make sure that you are using pleanty of water, you do not want those edges to get hot or they will crumble.

Mick

[captfishhead]

Thats a good question. My clients know that the material is soft but like the idea of the floor having that "used" look after few years. I worry about the floor having a "used" look after a couple of weeks. The distributer says that the material has been compressed for hardness so he says. Who Really knows?They like the material. What do you think?

[MICK1]

are those tile coated with resin in the back?
are you layng only on floor or on walls also?

Mick

[Davestone]

Compressed for hardness? Is it cultured onyx maybe?That might change things some.Don't know how one might compress a real onyx,maybe have elephants sit on it.

[Mike2]

...or maybe they get Edna in the back room to sit onit.

[captfishhead]

Yes mick some of the tiles have a mesh on the back but only the ones that are likely to have natural fisures that may not be sound. And no, no tiles on the walls

[captfishhead]

Yeah the compression thing sounded like alot of hooey to me also. But when I look at the stuff there is some filler on the surface that may have been put into some sort of vacume chamber to get it to suck into the surface. I don't know I will do the best with it and see if it holds up and if not its just a weeks work and about six thousand bucks worth of materials.Thanks for the help. I will let everyone know what happens.