Marble floor nightmare

[pageintl]

We moved into our new house in New England last year and are facing our 3rd (yes, 3rd) marble floor installation in our 300sq ft foyer. It's Thassos White (no veins); first floor was ripped up because the floor "moved" after it was laid, by the time it was ripped up you could trip over the lippage(not certain how the floor was installed). The next installer nailed down the durock, used thinset, and unsanded white grout but this floor is cracked clear accross the floor in a mirror image of the plywood subfloor, has lippage though not as bad but it still "moved", and the grout lines are too thick (1/8"). The GC is going to reinforce the floor from the basement, and use Schulter instead of Durock for the next installation; here's my Questions For The Experts:
1)Is this marble appropiate for this size foyer? Are we crazy to lay it again?
2)Can the cracking be prevented with the reinforcements?
3)The grout is impossible to keep clean. Is there a white grout that would? And wouldn't 1/16" lines be better?
4) Should the marble be sealed?
5)It seems obvious that the subfloor is not perfectly flat; what needs to happen to install this marble so it stays flat? It seemed flat at the last installation, but in a metter of months, it wasn't..
6)The installer did not "dry-lay"the floor out or use grout spacers, he just eye-balled it. Is this normal?

Sorry this is so long; I appreciate any insight.
Tom

[Scooter]

Sounds like the floor moved on you. You need what we call an isolation membrane, something to isolate the plywood floor from the tile. The plywood expands and contracts at a different rate than the durock and than the tile itself. The issue is not the installation per se, but separating the substrate (plywood) from the setting bed (durock or mud). Schluter is a fine company with great products that perform this exact function. "Ditra" is one.

I would have your installer speak with one of the many fine Schluter Reps out there. Their technical service people are, in my judgment, some of the best, most helpfull, and responsive in the business.

If you are concerned, I am sure that the Schluter guys would be glad to talk with you, and recommend some products to discuss with your installer.

I did not address the flatness of the floor, because you did not mention how "off" it was. I'd like to take an 8 foot level to it, and see how far it is off.

[Bud Cline]

You should quiz your builder on the amount of "load" your floor is rated for as it has to do with "deflection"..

Natural stone (which marble IS) requires twice the strength of that required for a ceramic tile installation.

Sounds to me like even the reinforcement would be an exercise in futility unless your builder really REALLY understnads the importance of the structures strength when it comes to movement (deflection).

Floor areas to which ceramic tile is to be applied are to have a deflection not greater than 1/360 of the span. Marble is twice that ie: 1/720 of the span.

This deflection is measured under a concentrated load of 300 pounds. Tell you builder to take a look at ASTM C627.

Your going to have to get back to basics if the floor is going to last. Believe it or not your builder may not even understand this.

[John Bridge]

Welcome aboard.

The marble is Greek, I think, and many white Greek "marbles" are mostly crystal, which fractures easily.

Marble in general is about the softest stone known to people like us, and as Bud said, it takes a lot of underpinning to make a sucessful installation.

What we would need to know:

What size are the joists or trusses that support the subfloor?

How far apart are they?

What does the subfloor consist of, i.e., plywood (how thick, how many layers?).

Was the concrete backer board bedded in thin set mortar and then fastened?

What is the overall span of the joists or trusses from support to support?

Are the joists or trussed "bridged" at mid-span or anywhere in the span?

If we know all of the above, we will be able to take a GUESS at the amount of deflection you are likely to accrue.

If it's over 1/720th of the span, we'll recommend going to an alternate flooring.

Please give us a short name to call you by. It can be an alias.