Lots of people want to put natural stone tiles onto heated floors of wooden subfloors. In pretty much all circumstances you can use travertine tiles, marble tiles & limestone tiles on timber floors as long as the floor itself is rigid and has been overlaid with a minimum of 9mm ply. If ply has been laid straight onto timber joists then 18mm marine ply is recommended as a minimum with "noggins" accross where each sheet of ply meets. Even if there is no "bounce" in your floor - this is where if you jump up and down on it, the floor moves downwards and springs back upwards, you can still get issues where cracks form in horizontal lines. This is usually to do with the fact that the wooden subfloor expands and contracts at different rates to the floor finish, your natural stone tiles. Even if you use the right flexible floor adhesive (rapid set adhesive has more flexibility than slow) cracks can still form due to the expansion and contraction of the subfloor. It's more common when underfloor heating is used without insulation boards (this is where the heating element is placed on top of a thermal board to stop the heat escaping through the subfloor).
The way to help avoid this is to use a "de-coupling" membrane also known as "crack matting" or by a brand name like "Ditra Matting" with Ditra being the market leader. This is laid underneath the floor adhesive and effectively separates the subfloor from the finished floor ie. marble tiles, limestone tiles, travertine tiles or even granite tiles. It allows the timber floor to be expand and contract at its own rate without affecting the natural stone tiles above. It also provides an amount of rigidity for floors with bounce in. At the end of the day there's no guarantee that any tile wont crack if your floor has bounce in it but crack matting is probably the best way of helping prevent it. There are many different types with Ditra leading the way, but probably the most expensive. Many architects specify this as a matter of course now if natural stone tiles are being used as a floor finish in new properties.
It is also being recommended for use in newly screeded floors with underfloor heating as this can cause issues as well if the floor hasn't been properly dried out and brought up to temperature before tiles are fixed. Basically what can happen if the floor isn't properly dried out is that it can shrink when heated up and if there are tiles on top, they wont shrink without cracking, even if a good flexible adhesive is used, so the advise here is make sure your floor is properly dried out and heated up before you tile as the investment you make in your limestone, travertine, marble or granite tiles could end up in bits! Need more info, speak to me, Will, in the office as I was installing marble tiles, travertine tiles and limestone tiles for many years and have pretty much seen it all!