Grout is the material that fills the joints between tiles. It locks the tiles together, stops movement and chipping at the edges, keeps debris and water out of the gaps, and holds the whole installation as one surface rather than a field of loose pieces.
It is not adhesive - that is the mortar or thinset underneath, which bonds the tile to the wall or floor. Grout goes in afterwards, between the tiles, once the adhesive has cured.
Most grout is cement-based, which means it is porous, and that one property explains nearly everything people find annoying about it.
Key Takeaways
- Grout fills the joints between tiles; mortar or thinset underneath bonds the tile down. Different materials, different jobs.
- Most grout is cement-based and porous, so it absorbs liquids and stains. That is why it discolors.
- Epoxy grout is non-porous, waterproof and stain-resistant as installed, and never needs sealing.
- Grout is not waterproof in the ordinary cement form. It resists water; it does not block it.
- Grout is hard and matte. Caulk is flexible with a soft sheen. Confusing the two is the most common and most frustrating mistake.
- Sanded grout for wide joints, unsanded for narrow ones - roughly a 1/8 inch dividing line.
- Cement grout lasts 7-15 years; epoxy lasts decades.
What grout is made of
Ordinary grout is a mixture of cement, water and fine aggregate, sometimes with polymers added to improve flexibility and stain resistance, and pigment for color. It goes on as a paste, is worked into the joints, and cures hard.
Because cement cures as a porous solid, the finished joint is full of microscopic voids. Liquids, oils and contaminants soak into those voids - which is why grout absorbs, stains, discolors and eventually degrades, while the glazed tile either side of it shrugs everything off.
Epoxy grout is a different chemistry entirely: resin and hardener, with no cement. It cures to a non-porous solid, so liquids, oils and chemicals cannot penetrate the joint. It is inherently waterproof and stain-resistant and needs no sealing at all. It costs more and is harder to install, which is why it is less common in homes.
The four types you will meet
| Type | Joint width | Where it is used | Notes |
|---|---|---|---|
| Unsanded (unmodified) | Narrow, under about 1/8 inch | Wall tile, polished stone, glass | Smoother; no aggregate to scratch delicate tile faces |
| Sanded | Wider, about 1/8 inch and above | Floors, most wall tile with wider joints | Sand gives it strength and stops it shrinking and cracking in a wide joint |
| Epoxy | Any | Showers, kitchens, commercial, anywhere stain resistance matters | Non-porous, waterproof, no sealing, highest cost |
| Single-component / pre-mixed | Usually narrow to medium | DIY projects | Convenient, performance varies by product |
Why the sand matters: a wide joint filled with unsanded grout tends to shrink and crack as it cures, because there is nothing bulking it out. A narrow joint filled with sanded grout can be hard to pack properly, and the aggregate can scratch polished marble or glass tile. Joint width is the deciding factor, not preference.
Is grout waterproof?
No - not ordinary cement-based grout. This is worth being clear about, because a lot of people assume a tiled shower is watertight because it is tiled.
Cement grout resists water and slows it down. It does not block it. Water passes slowly through a cement joint over time, which is why a tiled shower relies on a waterproof membrane behind the tile to do the actual waterproofing, and why failing grout in a shower is a real problem rather than a cosmetic one.
Epoxy grout is waterproof. So is a properly sealed cement joint, to a useful degree - but the sealer wears and needs renewing. That is the entire reason sealing exists, and the intervals are in how to seal grout.
If grout is crumbling, cracked or missing, water is reaching what is behind the tile. That is a repair job before it becomes a water damage job.
Grout vs caulk - the distinction that saves years of scrubbing
This is the most useful thing on this page.
Grout is rigid. It fills the joints between tiles on the same plane - across a wall, across a floor.
Caulk (silicone or similar sealant) is flexible. It fills the joints where two planes meet or move - the corner where wall meets wall, where wall meets floor, around a bath or shower tray, around a sink. These joints need to flex as the building and the fittings move slightly, and rigid grout in them would crack.
How to tell them apart:
| Grout | Caulk | |
|---|---|---|
| Feel | Hard, rigid, slightly gritty | Slightly soft and rubbery; gives under a fingernail |
| Finish | Matte | Soft sheen, often slightly glossy |
| Where | Between tiles on one plane | Internal corners, edges, around fixtures |
| When it goes black | Can often be cleaned | Cannot be cleaned - mold grows into the silicone |
Why it matters: the dark line in the corner of a shower, where the wall meets the floor or the bath, is almost always moldy silicone, not dirty grout. Mold grows into the body of the silicone, so no cleaner reaches it. People scrub that line for years, try bleach, try steam, and nothing works - because the only fix is to cut the old caulk out and replace it, which is a straightforward job.
If you have one stubborn black line that defeats everything while the rest of your grout cleans up fine, check whether it is caulk.
Why grout discolors
Four causes, and they need different responses.
Absorbed soil. The default, and a consequence of porosity. Cleanable, and preventable by sealing.
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Get My Instant QuoteMold and mildew. Mostly in showers, which combine constant water, soap residue and poor ventilation. Mold needs humidity above roughly 60% to establish, so ventilation does more than cleaning - an extractor fan run for 20 to 30 minutes after a shower, and a quick squeegee of the walls.
Efflorescence. White, crusty mineral deposits brought to the surface by moisture moving through the installation. Cleaning removes the deposit; only addressing the moisture stops it returning.
Permanent staining. Years of absorbed soil in an unsealed joint. At this point cleaning has a ceiling, and the options are color-sealing - which recolors the joint and hides what cleaning cannot remove - or regrouting.
The full cleaning method is in how to clean tile grout.
Looking after it
- Seal cement-based grout, and test it with the water-drip test: if water beads, the sealer is working; if it soaks in, reseal.
- Ventilate bathrooms. The highest-return habit for shower grout by a wide margin.
- Squeegee shower walls after use. Thirty seconds.
- Use pH-neutral cleaners. Avoid chlorine bleach on colored grout, which lightens the pigment permanently and patchily, and avoid acids entirely if your tile is natural stone, because they etch the stone itself.
- Do not mix cleaning products, especially anything containing bleach with anything containing acid or ammonia.
- Wipe spills on floor grout rather than letting them soak in.
Frequently Asked Questions
What is grout?
Grout is the material that fills the joints between tiles, locking them together, preventing movement and edge chipping, and keeping water and debris out of the gaps. Most grout is cement-based, which makes it porous and prone to staining.
What is grout used for?
It fills and seals the joints between tiles so the installation behaves as a single surface, stops the tile edges from chipping against each other, and keeps dirt and water out of the gaps.
Is grout the same as mortar?
No. Mortar or thinset is the adhesive underneath that bonds the tile to the wall or floor. Grout goes in afterwards, between the tiles.
Is grout waterproof?
Ordinary cement-based grout is not - it resists water but does not block it, which is why tiled showers rely on a waterproof membrane behind the tile. Epoxy grout is waterproof, and sealed cement grout is water-resistant until the sealer wears.
What is the difference between grout and caulk?
Grout is rigid and fills joints between tiles on the same plane. Caulk is flexible and fills joints where two planes meet or move - corners, edges, around baths and sinks. A black line in a shower corner is usually moldy caulk, which cannot be cleaned and must be replaced.
What is the difference between sanded and unsanded grout?
Sanded grout contains aggregate and is used in joints around 1/8 inch and wider, where it resists shrinking and cracking. Unsanded grout is smoother and used in narrower joints and on delicate tile such as polished stone and glass, which sand could scratch.
Does grout need sealing?
Cement-based grout does, because it is porous. Epoxy grout does not. Reseal intervals run from 9 to 12 months on shower floors to 2 to 3 years on bathroom floors and kitchen backsplashes.
How long does grout last?
Cement grout typically lasts 7 to 15 years; epoxy grout lasts decades. Both last longer with ventilation and, for cement, sealing.
When grout needs more than cleaning
Grout has a ceiling. Years of absorbed soil in an unsealed cement joint will not come all the way back with a brush, and professional cleaning is the step that tells you whether you are at that ceiling or just short of it - high-pressure cleaning with simultaneous recovery lifts soil out of the joint and removes it rather than redistributing it in mop water.
See tile and grout cleaning, or stone if your tile is marble, travertine, limestone or slate.
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