Q. I have successfully installed ceramic tile in different homes I have owned, but I have always worked with tiles that are 12 inches square or smaller. In the first level of my new home I want to install 900 square feet of 18- or even 20-inch square tiles on the concrete slab. Everyone tells me this is a task for professionals. Is it that hard to work with these giant floor tiles? What can I do to prevent cracks in my tiles? - S.H., Colorado Springs, Colo.

A. The best analogy I can offer to compare 8- or 12-inch ceramic tiles to the jumbo 18- or 20-inch ones you want to install is this: It's like the difference between running a race in athletic shoes and running one in snow shoes. Wearing snow shoes, you may well finish the race, but it will take much longer and be much more difficult.

One of the greatest challenges you'll confront installing ceramic tile has to do with a characteristic all ceramic and natural stone flooring materials share: These products show great strength when you squeeze them, but they are quite weak when you bend or stretch them. Put another way, if the base underneath your tile is flat and solid, then you can place heavy objects (such as a pool table) on the floor with confidence that the tile will not crack. But if that same tile has a hollow space between it and the subfloor, or if the subfloor flexes, the tile can easily bend and crack.

The susceptibility to cracking grows exponentially as the size of the tile gets bigger. Small ceramic tiles that are 1 inch square can be very forgiving, but large 18- or 20-inch tiles demand perfect installation conditions if you want your new floor to be crack-free for any duration.

Perhaps the most important step is to ensure that the surface on which you install the tile is planar. Some tile installation guides refer to this as the flatness of the floor. The floor doesn't so much need to be level as it needs to be perfectly smooth. Small humps or depressions in the floor can create sinister voids beneath tiles as you lay them. It is imperative you take the extra effort to eliminate these uneven spots by filling in all depressions with a floor-leveling compound.

The best tool to use to find and eliminate high spots and depressions in floors is a precision aluminum or magnesium straight edge. If you don't have one, you can rent one. When placed on a concrete or wood floor, this tool immediately exposes any imperfections in the floor. If you use it with a flashlight shone parallel to the floor, you will quickly discover even the slightest depressions.

Once you have filled in all the low spots with a cement-based floor leveling compound and the floor is smooth as a silk scarf, it is time to install the floor tile. It is extremely important to use the correct notched trowel to spread the cement-based thinset adhesive. I suggest you use a 1/2-inch V-notched trowel. This trowel applies a generous amount of thinset for these large tiles.

The trick is to also skim the bottom of each tile with thinset before you lay it on the floor. Use the flat side of the trowel to apply the thinset to the tiles and then to scrape off the excess. This extra step fills in any hollow areas under the tile with thinset. It also assures you that each tile has nearly 100 percent contact with the floor beneath it.

Another surprise in store for you is the difficulty of cutting large tiles. Many standard wet diamond saws may allow you to cut the tiles, but you may not be able to use the saw's sliding table aspect to help you maintain a straight cut. All cuts may have to be free hand. If you do not have access to a wet diamond saw, you may have to use a traditional glass cutter, unless you can find a large tile-cutting tool made for the jumbo tile. The store where you buy the tile may rent one to you.

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