Grout is a mortar. Sand, cement, pigment, sometimes a polymer, and once it has cured it does one thing very well: it fills a gap and stays precisely where it was put. It has no give at all. Put it in a joint that moves and it cracks — every time, in every house, and the only variable is how many months it takes.
So the crack in your shower corner is not a bad batch of grout, and it is not the color you picked. It is grout sitting in a joint that was never a grout joint. It got there because running a float straight through a corner is faster than masking the corner, letting the field cure, and coming back another day with a sealant gun. One trip instead of two. You pay for the saved trip later.
What grout is actually for
Grout fills the joints between tiles that share a plane. It supports the tile edges, keeps the layout reading as a grid, and keeps dirt off the substrate. That is the whole job.
What it is not matters more. Grout is not an adhesive — the thinset under the tile does the holding, and a tile that sounds hollow is a thinset problem no grout will rescue. It is not waterproofing; a cement joint passes water and vapor steadily, which is why what sits behind the tile is what keeps your framing dry. And it is not a sealant. It has no elasticity in tension. Stretch it and it breaks.
Sanded, unsanded, epoxy
Sanded grout carries graded aggregate and goes in wider joints, where the sand resists the shrinkage that comes with a lot of cement paste. Unsanded goes in narrow ones, and it will crack in a wide joint for the same reason. The crossover width sits on the manufacturer's data sheet — a spec, not a preference. Sanded grout will also scratch a polished marble or a soft glaze if it is dragged across the face carelessly.
Epoxy grout is a different material: resin and hardener instead of cement. It absorbs almost nothing, holds its color and never needs sealing, and it costs more in labor because the working window is short.
What it does not do is solve the corner. Epoxy is still rigid. In a joint that moves it does not stretch, it relocates the failure — instead of a hairline down the grout, the tile edge chips, or the whole strip releases from one face and stands proud. Harder is not softer. If someone offers epoxy as the answer to a cracked corner, that is a tell.
Every change of plane is a movement joint
The corner is not special because it is a corner. It is special because it is where two things that move differently meet.
- Wood framing takes on moisture in a humid summer and gives it up through the heating season. Studs and joists shrink and swell across the grain, the same mechanism that moves your cabinet doors and hardwood. Tile does not move with it.
- A tiled wall gets hot water on one face and a cold cavity behind it. Tile, thinset, backer board and framing each expand at their own rate.
- A floor that flexes underfoot carries the tile with it, and the plane change at the wall is where that flex concentrates. That is why a stiff, flat subfloor matters more to a tile job than the tile does.
- A tub or a shower base is a separate object bolted to the structure. It deflects when someone stands in it and settles when it fills with water. The wall above it does neither.
The tile industry's own installation handbook has long carried a movement-joint detail: a flexible joint at every change of plane, at every perimeter, and wherever tile runs into something that restrains it. Not a premium detail. The baseline one that gets skipped.
Where the sealant belongs
In a normal bathroom, the soft joints are:
- Every inside vertical corner in the shower and the tub surround.
- The wall-to-floor joint and the wall-to-pan joint, which take more water than anything else in the room.
- Tile to tub, tile to shower base, and every plane change on a curb — its top meets the inside face and the outside face at more corners again.
- The perimeter of a niche where the box meets the field wall, and the joint where the sloped shelf meets the back.
- Tile to any other material: drywall above a surround, a window jamb or stool, a vanity top meeting the wall, a threshold meeting wood flooring.
Shower doors have their own rule and it gets reversed constantly. Seal the outside of the jambs and the outside edge of the bottom track. Leave the inside open. Water gets under a sweep whatever anyone tells you, and the inside of that track is how it drains back to the shower. Seal both sides and you have built a trough that never dries.
Where it does not belong
Caulk in a field joint is not a repair. If a joint between tiles on the same plane has cracked, something under the tile moved — deflection, a hollow set, a substrate that was never flat. A bead smeared over it hides the only evidence you had.
Caulk over old grout is not a repair either. Silicone bonds to tile edges, not to a crumbling ribbon that is itself letting go. The corner gets raked out with a carbide rake or an oscillating tool and a grout blade, then cleaned.
A tub gets caulked full, not empty. An acrylic or steel tub drops under a load of water. Seal the joint with the tub dry and the first bath opens the bead along its top edge. Fill it, seal it, let it skin over, then drain it — that is the sequence, and it is one of the reasons a tub-to-shower conversion changes the joint detail as well as the layout.
Silicone will not stick to a damp face, to soap film, to a mildewed surface, or to old silicone. Any of those and the new bead peels off in one rubbery strip inside a season, which is usually what people mean when they say caulk never lasts in their bathroom.
The toilet base is the honest "it depends". Sealing it to the floor keeps mop water and worse from getting underneath; leaving a gap at the back means a failed wax ring announces itself on the floor instead of quietly rotting the subfloor. Some jurisdictions want the fixture sealed. That is a question for the inspector on your job, not for a blog post.
Choosing what goes in the joint
Pure silicone stays flexible for years, bonds well to glazed tile and glass, and takes constant wetting. It cannot be painted, and it has to come off completely before a new bead goes on.
Siliconized acrylic — the tubes with silicone in the name and latex in the formula — tools easily and cleans up with water, which is why it gets used. It also shrinks as it cures, chalks, and gives up early in a wet corner. It belongs on painted trim, not in a shower.
Color-matched sealant, sold by the grout manufacturers to match their own colors, is usually the right specification here — not because it performs differently, but because the corner then reads as a grout line. A soft joint that looks like a grout joint is one the next person is less likely to fill with grout.
Mildewcide in the tube is standard and it is not magic. The black that returns after cleaning is usually surface growth feeding on soap residue and moisture that never leaves the room — a ventilation problem, not a sealant one, and the fan matters more than most people expect.
How a corner gets built properly
The sequence is unglamorous and it is most of the answer. Set the tile. Let it cure. Grout the field and keep the grout out of every change of plane. Let the grout cure. Rake and clean the soft joints down to bare tile edges. Then seal them.
Two details separate a bead that lasts from one that does not. The joint needs dimension: tiles butted hard into a corner leave nowhere for a bead to live, so what you get is a smear that peels at its feathered edge. Holding the corner joint to the width of the field joints is a tile-setting decision made when the first course goes up, not something fixed at the end. And the bead should bond to the faces either side of the joint, not to the back of it — a deep joint gets a bond breaker so the bead can stretch. Stuck to all three faces, it tears instead.
Then it gets tooled, pressed into the joint rather than heaped over it. A proud bead collects soap and lifts at the edge.
Reading your own cracked corner
What the crack looks like tells you which conversation you are in.
- A fine, even hairline down an inside vertical corner: grout in a movement joint. Cosmetically minor, and water has been getting past it since it opened.
- The wall-to-floor joint powdering out: same cause, worse location, because that joint takes the most water in the room.
- A crack running across several tiles in a straight line through the field: structural. The floor or the substrate is moving, and the tile is reporting it.
- Tiles that sound hollow when tapped near a cracked joint: thinset coverage. Those tiles are moving on their own.
- Caulk lifting in one continuous ribbon: it went over grout, over damp, or it was the wrong product.
What a repair is, and when it stops being one
Removing the old material mechanically, cleaning the residue, drying the joint and running a new silicone bead is a real fix for the first two cases above. It is also maintenance — sealant in a wet corner is a wear item, not a permanent finish. Expect to redo it.
It stops being a repair the moment the joint is a symptom. Press a probe into the corner. If the board behind it is soft, if the tile below sounds hollow, if there is a damp line at the bottom plate or a stain on the ceiling below, a new bead is cosmetics over a failure. We would rather say that standing in the room than after the tile is off. Sometimes the honest answer is that the shower comes out. Sometimes it is a tube of silicone.
That gap is worth understanding before anyone quotes you. Independent research on this market — Angi's Indianapolis cost work and published Greater Indianapolis contractor cost guides — puts a typical bathroom remodel around $10,000, with most between roughly $5,700 and $14,700. A cracked corner is not that. It becomes that when it has been left long enough to reach the framing.
The short version
Grout goes between tiles on the same plane. Flexible sealant goes everywhere two planes meet, everywhere tile meets a different material, and everywhere a tub or a base can move independently of the wall. Getting that boundary right costs an extra trip to the job and nothing in material. Getting it wrong stays invisible for a season, and then it is a corner you look at every morning.
If a corner in your house has already opened, the useful next step is having somebody read it rather than reseal it. The sequence we work to on a bathroom remodel puts waterproofing and joint sealing in their own stage, before any finish material arrives — the joints are decided long before the tile is. If you want us to look at yours, ask for an estimate and we will walk it with you.