Q The ceramic tile in my house was installed over a concrete slab. Several areas of 1/8-in. to 1/4-in. wide cracks have developed right through the tiles. They seem to follow cracks in the underlying concrete. Before I replace the tile, is there any prep work I should do to prevent cracks in the new tile?
A Yes, you can protect tile by employing a couple of the tricks professionals use when laying tile over concrete that has cracks. The technique you should use depends on the type of cracks in the concrete: random vs. intentionally placed control (or expansion) joints. Random, crooked cracks develop from ground movement or the curing process and can show up almost anywhere in any slab. Most of these appear within a year or so of the slab’s being poured.
Control joints (photo below) are scored lines put in by the cement finisher to provide a predictable, weakened straight line for the concrete to break along as it cures, shrinks and settles.
If you’re planning to tile over a floor that has random cracks, read on. To prepare a floor that has control joints, see the box below.
How To Prepare A Randomly Cracked Floor For Tiling
Click image to enlarge.
Install an isolation membrane (also called an anti-fracture membrane) that will span random cracks in the floor. It will isolate the tile from the concrete with a cushion of plastic encased fiberglass fibers. The membrane absorbs the stress-not the tile and its thin-set adhesive. First, carefully examine the entire concrete surface to find and mark all the cracks, then follow these steps.
1. Use a 1/8-in. notched trowel to spread a 30-in. wide layer of thin-set (tile adhesive) centered over the crack.
2. Embed a 24-in. wide strip of isolation membrane (available at tile specialty stores) into the wet thin-set. Force the membrane into the thin-set with a wide, flat knife or trowel, squishing out the excess thin-set from the sides. Gently feather and thin the excess from the edge of the membrane out onto the untreated concrete.
3. Let the patch harden overnight, and then you’ll be ready to set the tile. When you apply thin-set over the floor, put a thinner layer over the patch by holding a 1/4-in. trowel at a lower angle while you’re passing over the feathered edge. That way, the thin-set bed will be a consistent height and the tile surface will be flat.
How To Prepare A Floor That Has Control (or Expansion) Joints
To lay tile over a floor that contains control joints, plan carefully when selecting tile sizes and layout patterns. Since control joints are there by design and almost guaranteed to crack or move, you’ll have to tile around them-not over them. Plan grout lines to fall directly over the joints. That means you have to measure distances between joints and select tile sizes so that the edges will nearly fall on joints. You may have to tweak the grout spacing until the tile lines fall perfectly over the joints.
Sometimes you can’t get everything to jibe or something’s running at an angle, and you’ll have no choice but to cut tile to create a grout line directly over a joint. If that’s the only alternative, try to lay out tile so half is on one side, half on the other (instead of a quarter of a tile on one side and three- quarters on the other, for example).
Once you’ve laid out and installed the tile, tuck foam backer rod into the control joints below the tile before grouting. Select rod diameters so that the top of the foam projects a little higher than the bottom of the tile. Fill the crevice with tile caulk that matches the color of your grout, then sponge the surface to shape and clean the joint. The backer rod thins out the center of the caulk to create a thinner bridge of caulk in the center of joints that will flex with slab movement. A thick bead of caulk would pull away from the tile instead of stretching in the center.
You’ll find foam backer rod at home centers, but shop at full-service tile stores for tile caulk and isolation membrane. Be sure to read the directions on the membrane you buy, because installation techniques vary.
