Caulk or Grout in Shower Corners?

Q: My tile installer recently told me that he wanted to use caulk in the corners of a tiled shower instead of grout. Is that necessary?

A: Tom Meehan, a second-generation tile installer and co-author of Working with Tile who lives and works in Harwich, Mass., responds: The vast majority of showers that I’ve tiled have had no problems when I use grout in the corners. But on that subject, the Tile Council of North America (TCNA) offers the following recommendation: “Technically, anywhere there is a change in substrate or backing surface such as the joint between walks and floor and wall joint, caulk should be used in place of grout since these surfaces move independently of each other.” But the TCNA then goes on to list five reasons that installers use grout instead of caulk. Matching the grout color—both when the caulk is new and as it changes over time—are major reasons. Caulk’s tendency to support mold growth as well as its greater need for maintenance are others.

The key word in this recommendation is “technically.” Clearly, the TCNA recognizes that many tile installers use grout alone to seal shower corners. But the point of allowing surfaces to move independently of each other is an important one. To that end, I am very careful when prepping the backerboard in a shower. I finish all backerboard corners and seams using a special moisture-resistant mesh tape bedded in latex-modified thinset mortar. I never use the standard mesh tape used for regular drywall. Before applying the tape, I put a layer of thinset on both mating surfaces without filling up or “packing” the joint. This joint space allows for slight movement, which helps to keep the grout from cracking, in most cases.

In those extremely rare cases where cracks develop in the grout in a corner, I thoroughly scrape out the old grout and completely clean the joint. Then I run a bead of caulk in the corner. To minimize the chances for mold, I use nothing except 100% silicone caulk. In addition to silicone, the TCNA recommends urethane or multi-polymer caulks. It cautions against acrylic caulk, which it says can break down in horizontal wet applications.

As the TCNA recommendation notes, it can be difficult to match the color of the caulk with the color of the grout used in the rest of the shower. I’ve found that caulk manufacturers always seem to be expanding the color choices for their caulk to meet this challenge. As a word of caution, however, note that some caulks change color in the process of curing, becoming opaque or turning clear. So it’s always a good idea to test a small area before caulking the entire seam. If I can’t find a close color match, I often resort to using clear silicone.