Once you've determined the floor is strong enough and smooth enough for tile, it's time to think through the layout.

There are three basic considerations:

1. Avoid ending up with narrow tile slivers along one or more walls—with one exception: You are sure that one edge of the floor will always be hidden, say, by furniture. Then you may choose to install slivers there in order to have wide pieces along the opposite wall.

2. If you can do so without ending up with slivers, center the tiles so that the cut tiles on opposite edges of the room are the same size. This is most important if the room is small.

3. If a room is out of square, adjust the layout to hide this imperfection as much as possible. Sometimes you can decide which edge of the floor will have tiles that slowly increase in width along its length; if so, choose the edge along the least visible wall. Make the tiles along this edge as wide as possible, since narrower tiles will make the imperfection more obvious.

Checking if a Room Is Square
Lay sheets of plywood (which have perfectly square corners) on the floor with the edges 1 inch away from both walls. Measure to see if either wall goes out of square along its length. Also check for waves in the walls.

Alternatively, use the "3-4-5" method. Mark one wall precisely 3 feet from a corner and the other wall 4 feet from the corner. If the distance between these two marks is exactly 5 feet, then the corner is square. If the room is large enough, use multiples of 3, 4, and 5, such as 6, 8, and 10 (as shown above) or 9, 12, and 15.

Laying Tiles in a Dry Run
It's easy to make a mistake when figuring a layout mathematically. Take no chances: Lay complete rows of full-sized tiles, with spacers, on the floor in at least two directions (as shown at left). This will tell you the exact size of the cut tiles along the edges. Chances are, you'll make an adjustment or two once you see the layout. Use this dry run to mark the floor for working (layout) lines.

Drawing Out-of-Square and Complicated Areas
If a room has more than four corners or is more than 1 inch out of square, make a drawing to help you visualize the job. Lay 10 tiles in a dry run with plastic spacers, measure the distance the tiles cover, and add the thickness of one grout line. Divide by 10 and you've got an accurate per-tile length.

Make a drawing on graph paper. Decide on a scale that is easy to use—say, one tile (including the adjoining grout line) per graph square, or one tile per four squares. Be sure to draw the room accurately; measure so you can show any out-of-square walls precisely. Make several photocopies of this room drawing so you can experiment with different tile layouts.

If a layout is complicated—particularly if you are tiling adjacent rooms—you may need to make compromises. After you've made one drawing, you may decide to move all the tiles over an inch or two to avoid or hide slivers or to minimize the visual impact of an out-of-square wall.

More about tile flooring preparation and installation:

• Preparing a Floor for Tiling
• Cutting Floor Tiles
• Installing Cement Backerboard
• Installing Tile Flooring Membrane
• Laying a Ceramic Tile Floor
• Laying a Mosaic Tile Floor
• Laying a Stone Tile Floor
• Tiling: Applying Grout
• Tiling: Applying Mortar
• Tiling: Finishing Grout
• Tiling: Setting Tiles