A reader wants to know how to repair cracks between a shower pan and the wall tiles. (Reader photo)

Q: Our bathroom remodel was finished last spring. Within a few months, cracks developed between the shower pan and the wall tiles. The separation is at the drain end of the shower pan only (so far). The contractor is willing to fix it. What is a permanent solution?

A: Building materials expand and contract as temperatures fluctuate, and different materials do this at different rates. So showers — and many other building components — need what some builders call “movement joints” and what others refer to as “expansion joints.”

In showers, the movement joints are all the corners, vertically and horizontally. Grout, which is a cement-based material, is great for filling the spaces between tiles. But it isn’t flexible enough to stay intact where a shower pan or tub meets surrounding walls. Nor is grout the right material for filling the corner joints on the walls. For these spaces, the right material is caulk.

Caulk comes in a bewildering array of types. Acrylic caulk is the easiest to apply neatly, and it’s paintable, which makes it the best for sealing most joints before you paint. But it shrinks as it cures, and it’s likely to crack in a shower, where moisture and temperature fluctuate dramatically. Caulk that’s 100 percent silicone doesn’t shrink and stays flexible, but it is so sticky that it’s tough to install tidily. Siliconized acrylic caulk is somewhere in between: easier to apply and more flexible than pure acrylic, but still not as flexible as pure silicone.

Besides choosing caulk that performs well over many years, you also want a product that blends in with the rest of your shower. The pictures you sent show relatively wide joints, with sanded, tinted grout. You will want sanded, tinted caulk, such as Color Sil Silicone Sealant, made by Color Rite in Yukon, Okla. (405-354-3644; colorriteinc.com ). January Colbert, a tech spokeswoman for Color Rite, recommends this product over the tinted, sanded siliconized acrylic caulks that her company also makes because Color Sil is 100 percent silicone. “It’s liquid rubber,” she said. “So you get a whole lot more movement over it.”

There is no way to successfully caulk the cracks in the existing grout. “Never, ever do that,” Colbert said. Instead, you or the installer will need to remove all of the grout from the joints you need to caulk. Pros usually use a grout-removal attachment on an oscillating tool. (Milwaukee’s ­
18-volt lithium-ion cordless model is $119 at Home Depot.) Be careful to avoid scratching the shower pan; you might want to cover the pan edge with masking tape first.

When the grout is gone, clean all the dust and other debris and let the surfaces dry thoroughly. Then you can smooth painter’s tape along both sides of the joints before you add the caulk. Or you can skip the tape and just spread the caulk and go over it in a single pass to smooth it. For novices, taping is extra security against creating a mess. But silicone caulk skins over quickly — in about five minutes, for Color Sil — so unless you remove the tape as soon as you finish each seam, the caulk will wind up with slightly raised edges, which easily break off or collect moisture, leading to mildew.

Whether you opt for tape or skip it, Colbert strongly recommends using two auxiliary products that her company sells: pretooling mist for silicone caulk and the Perfect Bead tool. Silicone caulk won’t stick to whatever you mist, so if you spread a bead of caulk and then immediately mist it and surfaces on either side, you can smooth the joint flat without having any excess stick to the tile or the shower pan, Colbert said. The bead tool presses caulk into the joint evenly and allows you to create a flat joint even with the face of the tile. If you were to use a damp finger instead, you’d probably create an inward-curving surface that collects water, and you’d push the caulk toward the back rather than against the side surfaces where you want it to stick, Colbert said

To avoid getting the mist on joints you haven’t caulked yet, she recommends starting the caulking along the bottom edge and continuing that bead up a few inches in each corner. Don’t attempt to caulk all seams in one pass. Do each joint separately, but one right after the other.

Caulk sticks best when it connects two surfaces, not three. You want it to bridge edges of the tile and the shower pan but not connect to the cement board or other surfaces behind the tile. If joints are wider than about one-fourth to three-eighths of an inch, stuff the seams first with caulk backer rod, a foam-type material that comes in rope form. Leave the front one-fourth inch empty so you can fill that with caulk.

Color Rite sells the mist, bead tool and 10.3-ounce tubes of Color Sil through dealers nationwide and its website. (The order form is under “Contact.”) The best deal online is to get a tube of caulk, the tool and the mist as a package, for $33.03. One tube is enough for about 11 to 100 feet, depending on the joint width.

To clean fresh smears of silicone caulk, use mineral spirits or lacquer thinner. But test a tiny area of the shower pan to make sure the solvent doesn’t damage it. Or, better yet, ask the installer to call the shower-pan manufacturer and ask whether the solvent will damage it.