It all depends on the deflection and subfloor thickness.

Ceramic requires L/360, but marble requires TWICE THAT, L/720.

While ceramic tile can be installed directly over a wood subfloor system, the sheet wood needs to be AT LEAST 1.5 inches thick and they MUST use a high grade modified mortar.

Very few wood subfloor systems ill have this thickness unless specifically consructed for this direct tile installation prerequisite.

In no case should a natural stone or marble product be installed directly on a wood substrate ... ever.

If it lasts the legally required guarantee period, it wont last much longer, and expect lots of cracked grout joints.

Personally, we never install over wood ... ever ... not without a cementious backer unit like durock or wonderboard, or a membrane like schluter ditra.

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There are two ways to do any job. The right way and the wrong way. Do it right everytime.

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http://flooringworld.org/

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my company never goes directly over wood with tile or stone installations. Rooms subject to moisture are best protected by using 1/4" wedi board. it is 100% waterproof, mold resistant, crack suppressing membrane and much easier to install than CBU. It only weighs 8 lbs. per sheet and cuts with a utility knife.

I would get written guarantee from your tile guy because I do not believe that the bond between the thinset or mastic will hold. Contact info@gstallc.com for information on wedi systems products if you decide to have him do the job right.