Caulked. Any joint where two planes meet — wall to wall, wall to floor, tile to tub — moves independently and will crack rigid grout every time. Grout belongs between tiles on the same plane, and nowhere else.
The rule, and why it exists
These are called change-of-plane joints. Two surfaces meeting at an angle expand, contract and settle at different rates — because of temperature, because of the building moving, because a tub flexes when it fills with water and a person. Grout is a rigid cement product. Put it in a joint that moves and it cracks. Not sometimes. Always, eventually.
Industry standard has required flexible sealant at change-of-plane joints for decades. It is one of the most reliably ignored rules in residential tiling, which is why cracked corner grout is one of the most common bathroom complaints there is.
Doing it properly
- Rake the old grout or caulk out completely. A grout saw or an oscillating tool with a grout blade. Halfway is worse than not starting.
- Vacuum, then wipe with denatured alcohol. Silicone will not bond to dust or soap film.
- Use a 100% silicone rated for kitchen and bath, or a sanded siliconised product matched to your grout colour. Not standard acrylic latex — it will not last in a wet joint.
- Tape both sides, lay a thin bead, tool it once with a wet finger or a tool, pull the tape immediately while it is wet.
- Leave it 24 hours before the shower gets used. This is the step people skip and it is the one that determines whether the joint lasts.
The tub joint specifically
Fill the tub with water before you caulk it, and leave it full until the caulk has skinned. A full tub sits lower than an empty one, so caulking it empty means the joint gets stretched every time someone takes a bath, and it tears within months. It feels like a fussy detail. It is the difference between a joint that lasts five years and one that lasts one.
When cracked corners mean something worse
If you rake out cracked grout in a corner and find the substrate behind it is dark, soft, or crumbly, stop. Re-caulking over wet substrate seals the water in. That is the point where the wall needs opening up, and it is exactly the situation where a small job found early is very much cheaper than the same job found in two years.
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