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Caulk vs Grout in a Shower: Which Goes Where?
Short answer
Grout goes in the joints between tiles on the same flat surface. Caulk goes wherever two surfaces meet or can move: inside corners, where tile meets the tub or shower pan, and around fixtures. Grout is rigid and cracks at those joints, while caulk flexes.
Why corners get caulk
A shower moves more than it looks like it does. Walls, floor and tub expand and shift at different rates with heat, water and weight. Rigid grout in a corner cracks along that line, usually within a few years. The Tile Council of North America's movement-joint guideline, EJ171, calls for soft joints at changes of plane for this reason.
How to tell which one failed
A long, clean crack running straight up an inside corner usually means grout was used where caulk belonged, or the caulk there has split. A crack that wanders along the small joints between tiles, or grout that's crumbling or missing in spots, is a grout problem. Check whether the material gives under your fingernail. Caulk feels rubbery, and grout feels like hard sand.
Which repair you need
Failed caulk needs to be cut out and replaced, and that's the whole job we do. Failed grout needs grinding out and regrouting by a tile setter, which we don't offer. A small area of cracked grout doesn't always mean a big repair, but widespread loose grout or tiles that move when you press on them can mean the wall behind has been getting wet. Our guide to signs of water damage behind a tub covers what to look for.
Don't paint one over the other
Spreading caulk over cracked grout, or grout over old caulk, might look fine for a month. Neither one bonds well to the other, so the fix peels and traps water. Health Canada's moisture guidance lists damaged tile, grout and caulking around showers and tubs as things to repair or replace. Remove what failed and put the right material back.