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Installing an Ornamental Tile Floor, continued Drop cloths. Although good builders seldom ignore these basic steps, I quite often find that they've dropped the ball during drywall finishing by failing to protect the floor from paint overspray. This guarantees a tile bond that is only as strong as the paint on the floor. In such cases, I can't proceed until the floor is sanded clean.

Isolation membrane. On this job, I used a slightly different subfloor system. For added insurance against cracking and other potential problems, the owners asked me to install the tile over an isolation or uncoupling membrane called Ditra Mat (Schlüter Systems, 800/472-4588, www.schluter.com). This plastic sheet material has a waffle-like grid with undercut cavities that lock onto the dried cement in dovetail fashion; its other side is covered with a fleece-like material. The 1/8-inch-thick membrane is easy to install and allows most tile to finish flush with abutting 3/4-inch wood flooring.

According to the manufacturer, Ditra can provide a crack-free installation when applied directly over a single layer of 3/4-inch tongue-and-groove plywood decking on joists spaced up to 19.2 inches apart. The American Tile Association approves this approach for ceramic tile, although stone tile will still require a double-layer 1 1/8-inch substrate. (To be conservative, we used a double-layer 1 1/8-inch substrate beneath the Ditra Mat even though we were using ceramic tile, not stone.) Ditra Mat can also be used in place of backerboard under tile countertops (see "Tiling Over a Laminate Countertop," 3/03).

To install the membrane, I used a 3/16x1/4-inch V-notched trowel to spread a high-grade latex-modified thinset (Tec FullFlex, Tec Specialty Products, 800/832-9002, www.tecspecialty.com) mixed rather loosely to aid distribution. I snapped parallel lines at 40-inch intervals — about the width of the mat — to guide spreading the thinset. After laying the membrane, I used a 75-pound linoleum roller to drive out air pockets and voids and embed the fleece side of the mat in the thinset.

Planning the Layout

A distinguishing feature of the second-story installation was the "rug pattern" planned for the central area of the great-room floor. An 18x18-inch rectangular tile border edged by a 6-inch decorative tile band would frame an inside field of the same 18x18-inch tiles laid diagonally. A pair of patio doors in the middle of the room's front wall served as a focal point for the pattern, so I centered the border tile and field pattern on the mullion between the doors. On this job, the baseboard hadn't yet been installed, but the door casings landed directly on the underlayment. I cut the trim to allow the tile to slip underneath, using a tile and a piece of corrugated cardboard to space the blade of my Bosch 1640VS undercut saw off the floor (Figure 2).

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Figure 2.Rather than cutting tile around door casings, the author cuts a clearance gap beneath with an undercut saw guided by a simple tile-and-cardboard gauge.

Getting square. The first and most important step was to check the room for square and parallel. I did this by snapping a centerline down the room's long axis, then snapping a second line at 90 degrees to the first, using my folding layout triangle aSQUARE (C.H. Hanson, 800/827-3398, www.asquaretools.com; $50) to check the angle (Figure 3). If the room is a little out of square or parallel, it's not a big deal. Working the tile pattern from a centerline allows any discrepancies to play out inconspicuously along the walls.

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Figure 3.

After covering the subfloor with a plastic uncoupling membrane, the author squares up the layout for an ornamental rug pattern (top). Full-sized tiles are dry-fitted in both directions from a line centered on the door mullion, followed by a 6-inch decorative tile band (above left). A diagonal half-tile border sets up the diagonal field tiles in the pattern area (above right).

Cutting and dry-fitting. I checked my measurements for the rug layout by dry-fitting single rows of tile in both directions, working out from the centerline, using the proper joint spacing — in this case, 3/16 inch. To give the rug pattern a look of planned symmetry, I wanted to complete the edges of the diagonally laid field tile with full half-tiles. This layout dictated the run of the surrounding decorative band. The interlocking zigzag decorative border tile came ready to install and included special pieces to complete inside and outside corners. Theoretically, the pattern could be laid to complete the full band without having to make any cuts. However, the field tile layout stretched the decorative band layout, resulting in an undersized gap, too small to fit a full decorative tile. The owners were adamant about maintaining uniform grout lines, so I couldn't make up the difference by slightly spreading the overall layout. The irregular shape of the proprietary corner tile also ruled out any clever mitering tricks. Instead, I downsized a couple of tiles adjacent to the corner tile and split the difference between slightly reduced, recut pieces.

Cutting a zigzag line through ceramic tile is a job for a ring saw — mine is made by the Gemini Saw Company (310/891-0288, www.geminisaw.com). The saw is vaguely similar to a band saw, but the ring-shaped, diamond-coated wire blade cuts in all directions and revolves through a water bath (Figure 4).

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Figure 4.By expanding the border dimension of the rug pattern, the author created a space in the decorative tile layout too large for a single tile to fill yet too small for a full tile. Laying the cut-down tile at the border's corner effectively disguises the adjustment. A diamond-coated blade in a ring saw permits fast and accurate irregular cuts.

Scroll cuts are simple to make with a ring saw; any shape or radius can be cut, greatly simplifying custom inlay work.

I cut the field tile with an electric wet saw equipped with a diamond blade (Figure 5). It's possible to trim the tiles with a snap cutter, but when cutting large, 18x18-inch tile, a water saw produces less waste. The machine I use, a Tile Master XL High Production Tile Saw (Felker Tile Saws, 800/938-7925, www.felkertilesaws.com), allows me to cut larger tile on the diagonal in a single pass, rather than having to cut halfway and then flip the tile to complete the cut. When cutting on the diagonal, I draw a guideline on the tile with a permanent marker and a straightedge, from corner to corner. This helps to keep the cuts accurate as I "freehand" them through the saw blade.

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Figure 5.After drawing an accurate diagonal line on the tile, the author makes freehand cuts on a diamond-blade-equipped wet saw.