QUESTIONWe have an established, very loyal client who wants us to polish their marble foyer, but the whole thing sounds hollow. The homeowner told us that the installers used 4 blobs of mud to attach each tile. It also appears to have been (badly) crystallized at some point. It looks like it would need honing down to maybe a 100. so polishing pads don't seem to be an option. What do you think? Should we go for it, or tell them it all needs replacing? We don't want to further damage their tile with all that moisture.
ANSWERANSWER - First of all a hollow sounding marble floor is not considered a defect in itself. It could be a symptom of a defect. The only way to determine that is to see what is causing the hollow sound. It might not have anything to do with the marble tile assembly.
Another consideration is whether there is any resultant damages caused by the condition that causes the hollow sound. There are cases where a bonded mortar bed is not bonded to its substrate that causes a hollow sound, but the tile assembly is structurally sound.
Spot bonding marble slab or marble tiles is not an approved method of installation. The excess voids diminish potential bond strength, it leaves voids that could collect moisture in some situations that could cause moisture issues, and the stone is unsupported at the voids.
If there are no resultant damages from the spot bonding, then I wouldn't automatically condemn the floor. If the marble is 2 cm (3/4") thick or thicker it might perform ok in a residential application in a spot bonded method. On the other hand it could be more susceptible to live load damage if it is subjected to heavy loads or impacts. Also when the floor is restored and subjected to a lot of water, the water could migrate through the grout joints into those voids under the floor and cause efflorescence staining or even spalling in a worst case scenario.
If the marble floor has no resultant damages from the spot bonded hollow sounding floor, you could restore it. If you do restore it, I would have the client write a disclaimer that you won't be responsible for any resultant damages as a result of the stone being spot bonded.