Because a hearth is in front of a major focal point, you want it to appear symmetrical. If you're working with high-end art tile, you're in luck: Manufacturers or dealers will often create a layout for you and order the proper amount of materials in sizes that require a minimum of cutting. But more often, you'll need to play around a bit with placement and pattern to make sure tiles look even, neat, and aesthetically pleasing.
Before the final design is put in place, however, the structure needs to be sound, because a hearth receives constant abuse. A lot of that starts with the substrate, which should be even and level. "Just like any good construction, if the foundation is true, the rest will follow," says tile contractor Joe Ferrante, who has worked on several This Old House TV projects. If the area has a concrete slab, you may need to smooth it out with a skim coat of thinset after removing the old hearth. Newer houses, on the other hand, may have a plywood subfloor under the old tiles. To create a noncombustible substrate for the tiles, you'll need to put down a piece of cementitious backer board. You'll also have to bring the hearth dimensions up to code: Most localities require hearths to be 16 inches deep and extend 8 inches beyond both sides of the firebox.
Once you have a good base for the tiles, it's important to adhere them to the substrate with thinset, not tile mastic, which can't hold up to the heat of a fireplace. But before you put the tile down, take the time to perfect a dry-laid version of your design. Try different configurations and patterns—a 3-by-6-inch subway tile, for example, lends itself to a bricklike running bond, but simple square tiles might look best in a grid—and make sure everything fits well, with even lines. "There's no such thing as checking too much," says Ferrante. Then, as you set the tile, work slowly and deliberately to stick to your design.
Tiling can seem daunting, but the great thing about a hearth is that it's essentially a flat rectangle. You have room to play around—with different borders, with different patterns. And the best part is that thinset is very forgiving. As long as it's wet, you can always pull up a tile and redo your work, making sure that all the puzzle pieces fit together perfectly.