Grout goes in the regular joints between tiles on a flat surface. Caulk, a flexible sealant, goes where the surface changes direction or meets something else: inside corners, the joint where wall tile meets the tub or floor, the room perimeter, and movement joints. Grout is rigid, so in those spots it cracks as the surfaces move. Custom Building Products’ Polyblend Sanded Grout data sheet says the grout is not for use in movement joints or changes of plane, says to use an appropriate caulk or sealant there, and notes that movement joints are required at perimeters and other changes of plane in all installations, citing TCNA detail EJ171.
Where each belongs
| Location | Use |
|---|---|
| Joints between tiles in a flat field | Grout matched to the tile, joint width and exposure |
| Inside corners in a shower or backsplash | Flexible sealant |
| Tub-to-tile, countertop-to-backsplash, wall-to-floor | Flexible sealant |
| Perimeter and designed movement joints | Flexible sealant, with backer rod in deep joints |
| Grout that keeps cracking in the same line | Find the movement first; it may need to be a movement joint |
Which sealant for movement joints
Custom’s movement-joint paper says ANSI recommends silicone, polysulfide or polyurethane sealant meeting ASTM C920 for tile movement joints. It adds that siliconized acrylics don’t have the flexibility to move with the assembly in commercial applications. Sealants sold in grout-matched colors let the joint blend in.
For a proper movement joint, the paper says to:
- keep the joint free of grout while you grout the rest of the tile;
- once the grout has cured (generally 48 hours), clean out any debris;
- press in closed-cell backer rod so the sealant depth is about half the joint width, for example filling a 1/2-inch joint to within 1/4 inch of the tile surface;
- fill with the sealant.
For more on sizing deep joints, see backer rod and sealant sizing .
Neither one waterproofs a shower
In a wet area, the waterproof membrane or approved backer behind the tile keeps water out of the wall. Grout and caulk are finishes. Loose tile, a soft wall behind the tile, recurring mold or persistent dampness all call for investigation, not just a fresh bead.
Repair tips
Remove failed material without scratching the tile or cutting into the waterproofing. Clean and dry the joint as the product requires, then follow its joint-width, temperature, tooling and cure instructions. When regrouting, match the grout type and color and don’t add extra water. If staining is the only problem, cleaning may be enough: see bathroom grout mold: clean, regrout, or recaulk .