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1. 
   WoodRivWW | Mar 08, 2008 12:49am | #1

   You can use thinset right over the brick and attach the tiles to the thinset. I did this on my fireplace and it worked great! I've attached an article I found below:

   Q: We’d like to tile over the brick around our fireplace. Can I put cement backerboard on the wall and cover that with tile? 

—Roger Brumm, Sterling,VA.

   A: Joe Ferrante replies: Sure you could, but it’s a lot easier to trowel thinset over the brick and then tile right over that. Here’s how to do it. 

Cover the hearth with a drop cloth and remove the mantel and anything else that’s attached to the brick. Go over the entire area with a wire brush to dislodge any loose mortar, then scrub off the soot with a rag soaked in white vinegar. Let the bricks dry for a day or two, then mix up a batch of white, latex-modified thinset cement to the consistency of mayonnaise. Spread the thinset over the brick with a flat, straight—not notched—trowel and force it into all the grout joints. If the joints are still visible after the first coat cures, skim on a second coat to fill in any depressions. If you do it right, you’ll end up with a surface that’s as flat as backerboard but without any seams. The next day, lay down a new bed of latex-modified thinset combed out with a notched trowel, and set your tiles into that. Don’t use mastic; it can’t take the heat. 

Here’s a trick I use when tiling around fireplaces. The row of tiles directly above the firebox is visually the most important, so it should be absolutely straight. Nail a straight 1x board horizontally across the fireplace, in line with the top of the firebox, and use its top edge as a shelf as you set the tiles. The board also prevents the tiles from slipping before the adhesive cures. Work up from the board first, then, when the thinset hardens, take it off and work down the sides of the fireplace.