Installing an Ornamental Tile Floor, continued

Installing the Tile

Before spreading thinset for the tile, I snapped control lines on the mat to ensure that the tile courses would remain straight. Thinset should be spread in one direction to eliminate voids and heavy deposits and allow the tile to lie flat. I try not to mix more thinset than I'll use in one hour and spread no more than about 20 square feet at a time to ensure that it stays fresh and tacky (Figure 6).

Figure 6.Combing the thinset adhesive in one direction with a notched trowel distributes the adhesive evenly and allows the tile to lie flat. Limiting thinset application to no more than 20 square feet at a time prevents the cement from drying prematurely.

Checking the bond. To ensure straight grout lines, I always keep the edge of the tile right on the edge of the chalk line. I stand back and eyeball the floor from a distance from time to time to look for tiles that I may have pushed out of line while cleaning up the thinset squeeze-out. As I install each tile, I press it into the thinset and give it a good 1/2-inch slide to ensure complete contact with the cement.

It's a good idea to lift a set tile once in a while to make sure you're getting a complete bond (Figure 7). I do this by sliding a trowel under the tile and lifting it slowly to break the suction. This requires care when the tile is laid over Ditra Mat, because the suction can pull the mat free from its fabric backing.

Figure 7.To prevent cracking, at least 95% of each tile should be contacting the thinset. The only way to check this is to pry up an occasional tile and look — but be careful not to crack the tile or dislodge the isolation membrane in the process.

The tile should have at least 95% thinset coverage. Voids in the bond, or thinset missing under one of the corners, may cause the tile to crack. Replacing cracked tile is a labor-intensive task that can be avoided if you take the proper precautions. The larger the tile, the more important it is to have perfect thinset coverage.

Expansion joints. An uninterrupted field of floor tile should include an expansion joint at least every 20 to 24 linear feet. On this job, the rug-pattern border offered the best opportunity, because it cut all the way across the room, while the field tile ran diagonally on either side of the border. I installed a prefabricated expansion joint, also made by Schlüter (Figure 8). This plastic strip is made to mimic a grout joint and comes in a variety of colors. Its attached webbing extends 1 inch under the tile in either direction. The expansion joint effectively divides the tile field into separate areas, reducing cumulative stress.

Figure 8.Large, uninterrupted expanses of tile should be divided into smaller sections with expansion joints to avoid stress cracks. The author uses a proprietary plastic strip that's available in a variety of colors for a good match with the grout (left). Another strip, placed in front of the metal door sill, protects the adjoining grout and tile from damage caused by thermal expansion (right).

To play it safe, I also installed expansion strips against the aluminum sills of the patio doors. I've often found cracked grout at these junctions, caused by metal expansion and contraction.

I maintained a 1/4-inch gap between the tile and walls around the perimeter of the room. It's important to clear this gap of any debris during grouting. The remaining gap is covered by baseboard.

Grouting and Finishing

Because this job involved such a large area, we began grouting one section before we'd finished laying the tile on the other side of the room. We completed the rug area first, then continued laying the diagonal field on either side. To ensure that the pattern would be consistent on both sides of the rug border, we pulled a chalk line across the rug diagonals (Figure 9).

Figure 9.A line snapped across the points of the completed rug pattern permits accurate layout of the field areas to either side (left). Full half-diagonals initiate the general area installation (right).

My helper, Chuck Young, grouted the floor with Mapei's Ker 200 series polymer-modified gray grout (Mapei, 800/426-3734, www.mapei.com). The key to efficiently grouting a large area is to make the joints as regular and even as possible and to clean the floor well as you go. Because grout that sets up on the surface of the tile becomes difficult to clean off, it's also important to avoid spreading grout over too wide an area — no more than you can clean off within an hour (Figure 10).

Figure 10.To prevent dried grout from sticking to the tile, spread only as much grout as can easily be cleaned within an hour (above). Clean water and regular rinsing with a sponge make for efficient cleanup. As the grout haze dries on the tile surface, the author and crew polish it away with soft rags and paper towels (right).

Before grouting, I give some thought to the temperature of the room. Ideally, it should be a comfortable 65°F to 70°F. In a room with a lot of windows, direct, baking sunlight can cause the grout to dry too quickly. To avoid that, I'll wait until the sun moves off that section of the floor.

By the time Chuck finished spreading the grout, the first section was ready for a wash with clean water and a sponge, regularly rinsed and squeezed out. Using too much water can weaken the cement-based grout. As we completed each area, Chuck polished the floor with a clean paper towel or a rag to take off the dried grout haze. We took care of the small amount of cleaning that remained with a mildly acidic tile cleaner.

The following week, we returned to seal the grout and tile. We coated the entire surface with MiraSeal 511 Impregnator (Miracle Sealants, 800/350-1901, www.miraclesealants.com) using an ordinary pump-up spray applicator, then thoroughly ragged the floor to remove the excess. Proper ventilation is important when using this or any sealer; off-gassing from the uncured product should be considered a health hazard.