Marble Threshold Height Above Tiles

[Newmantjn]

Trying to figure out what to do with my yet to be purchased marble threshold.

What is the minimum/maximum and optimal height that the top of the threshold should be above the tile?

How much height should I allow for the thinset etc.?

Option #1 = Ditra with tile: 1/4" nominal tile thickness + 1/8" Ditra + thinset under ditra + thinset over ditra.
Option #2 = Hardibacker with tile : 1/4" nominal tile thickness + 1/4" Hardibacker + thinset under Hardibacker + Thinset over Hardibacker.

I have the ability to adjust the subfloor a bit under the threshold. I currently have 1/2" plywood (original construction + 3/4" plywood, recently added by me. Next layer is either Ditra or hardibacker,

 

[Reach4]

What is the minimum/maximum and optimal height that the top of the threshold should be above the tile?

While a Neato or Roomba robot vacuum can typically cross a 3/4 inch tall threshold, I would not feel comfortable with such a high threshold. It could be a tripping hazard IMO. A 3/16 or quarter inch seems about right to me to be enough to keep spilled water from passing. I am not a pro.

 

[Sponsor]

 

[Jadnashua]

The marble threshold at my mother's house into the bathroom is in the order of at least 1/2" on the tiled side, and a slight bit taller on the wood flooring in the hall. It's been about 5-years since installation, and hasn't presented itself as a major problem. Visually, it does stick out, so at least when there's enough light, you don't have an issue. I've walked into there a number of times at night when visiting without turning on the lights and have never tripped. If you can manipulate the stackup to make it smaller, it's still probably a good idea. The threshold that was installed at my mother's has a 45-degree angle cut on both sides that's probably close to 3/8-1/2" across, so it's not a straight step up.

 

[Newmantjn]

Thanks. I would to keep it 1/4" or smaller. Is it OK to mount the threshold directly to the the plywood underlayment? The rest of the bathroom would get either ditra or hardibacker.

 

[Jadnashua]

Probably not a problem. Typically, natural stone requires two layers of plywood, and if you tile directly to it, you need a premium modified thinset. Industry standards call for the structure to be twice as stiff as for ceramic tile. Given the thickness of the typical threshold, you could take a chance and ignore those guidelines...sometimes, you get lucky. Being natural stone, it will have some weakness in it in the veins, etc.

Take extra care to ensure you get 100% coverage underneath. It typically takes some effort on a larger 'tile' to smush the trowel ridges and spread the thinset to cover the entire surface.