The Tile Council of North America is one of the top resources for tiling recommendations in the country, and perhaps unsurprisingly, it doesn't offer any recommendations for someone who wants to tile over a marble fireplace. This is probably because the desire to cover over the elegance of marble with a permanent layer of tile is a somewhat unusual one.
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The lack of a yea or nay from an authority like the TCNA leaves you pretty much on your own if your plans for a marble fireplace makeover include tiling. Some contractors would refuse to take the job because of the risk of tiles falling because of poor adhesion. However, the TCNA does offer pointers for tiling over vinyl, terrazzo and existing tiles, so you can breathe easier if you apply these to your tile-over-marble job. If you really want to be on the safe side, you can always install a cement backer board before you lay the tiles.
Overcoming Problems With Tile Over a Marble Fireplace
Marble is smooth and nonporous, which are both good qualities for a finished surface but poor ones for a tile substrate. Add a layer of soot to the natural smoothness of marble, and you'll be hard-pressed to find a mastic that will stick permanently. The mastic may adhere while it's wet, but it will have a more difficult time doing so when it solidifies.
The situation calls for a three-part strategy:
- Clean the marble – Wash the marble with a solution of 1 cup trisodium phosphate (TSP) per gallon of water. Wear gloves and a mask because this is a caustic detergent that won't only remove soot, grease and etch the marble, but it will also give you skin burns.
- Grind the marble – Scarify the entire marble surface with a grinder and a cup stone wheel. The goal is to deeply etch the surface and remove all sealers, soot, polish, and other contaminants that can interfere with the tile mortar bond.
- Use the right adhesive – Check the manufacturer's recommendations on a mastic or thinset product before you buy it. If it's recommended for tiling over a marble floor or other smooth surfaces, such as vinyl or terrazzo, it will probably work on your fireplace. Some polymer-modified thinset mortars are suitable for this application.
Playing it Really Safe
If you follow the three-part strategy above, your tiles will probably stay put, but if "probably" isn't good enough, consider installing a cement backer board on top of the marble. This should be secured with mastic as well as with screws driven into predrilled holes in the marble. If you do this, you can install the tiles just as you would on any other vertical surface.
The problem with this approach is that the backer board underlayment adds an extra 1/4 to 1/2 inch to the tile installation. This could bring the tile surface out beyond any molding on the corners or the mantel, and you may have to compensate by changing the molding.
Back to Business as Usual
Once you've addressed the problem of adhesion, the job of tiling over marble to revamp the marble fireplace becomes like any other tiling job. Spread the mastic with a notched trowel, lay the tiles and use spacers to keep the lines between them uniform. Don't forget to remove the spacers before the mastic sets completely or you may find them difficult to extract.
After laying the tiles, wait for about 24 hours before grouting. Apply the grout with a rubber float and use a wet sponge to remove excess from the tiles. After the grout sets, wash the haze off the tiles and seal the grout with an acrylic grout sealer.