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I sometimes visit a theatre which is located in a shopping mall some thirty miles from my home. The mall was built over very marshy land I'm told and now there are visible cracks in the marble floor tiles in front of the theatre entrance. What would be the solution to repairing those cracks,what would the cost be roughly, and would the cracks return?

Take2

The situation you describe could be quite serious. Our advice would be to simply remove and replace the tiles. Cost should be around $5.00 per sq. foot, plus the cost of the marble tiles. The problem is that if the building is still moving because of the marshy ground, the cracks will probobly come back. However, it is possible that the cracking is from settlement, and the building may have settled as much as it's ever going to. In this case, the new floor should be good for a long time.

Take2

While we're on the subject of marble floors, about 15 years ago, we used a lot of re-constituted marble tiles for cinema lobbies. These are the 12 x 12 tiles made of marble chips in an epoxy matrix. These tiles proved to be very unsatisfactory. First, they cracked very easily, and up here in Canada where we get lots of snow, the city uses salt on the streets to melt the snow. When people come into the theatre with salt and snow on their shoes, it corrodes the tile so they disintegrate after one winter. Another problem where we used these tiles outdoors in places like Texas with very strong sun, the sun would bleach the colour right out of the tiles. If you are going to use marble, use the real thing, not the reconstituted type.

Another issue to be addressed when placing tile over a cracked concrete substrate is to put an isolation membrane directly over the crack to keep it from telegraphing through the new tile. This is usually a tape like product that prevents the thin set mortar from bonding to it. This works if the substrate is in a stable vertical plane , but has some horizontal movement. In extreme cases of settling ,mud jacking is also used to lift the concrete slab to its original position. Many times it is just easier to remove tile and substrate and start over.