Almost every tiled shower on Cape Cod will show a few dark spots on the grout at some point. Humid summers, a bathroom with a weak fan, and a shower that gets used three times a day — it adds up. Most of the time it is surface mildew and a good cleaning takes care of it.
But after 1,000+ projects and a lot of repair calls, we can tell you this: the mold you can see is not the problem. The problem is the water you can't see. Here is how to tell the difference, and when it is time to stop scrubbing and call someone who can look underneath.
Surface mildew vs. a moisture problem
Surface mildew grows on the face of grout and caulk because the surface stays damp between showers. It is a cleaning and ventilation issue. You'll notice:
- Spots sit on top of the grout or caulk and wipe off with a bathroom cleaner or diluted bleach.
- The grout returns to its normal color once dry.
- It shows up mostly along the bottom row, inside corners, and the caulk joint at the tub or shower pan.
- After cleaning and a few days of running the fan properly, it stays gone for weeks or months.
A moisture problem under the tile is different. Water has gotten past the tile and grout into the setting bed, the backer board, or the framing. Mold grows there, in the dark, and what you see on the surface is only the leading edge. The signs:
- Grout or caulk darkens again within days of cleaning, in the same spots.
- Grout stays dark or damp-looking long after the shower dries. (If this is your main symptom, read our post on why grout won't dry.)
- Tiles sound hollow when you tap them with a knuckle, or feel slightly loose underfoot.
- Grout is cracking, crumbling, or separating from the tile edge — especially at the shower floor and the first row of wall tile.
- The caulk joint at the tub or pan has pulled away, or there is a soft spot in the drywall just outside the shower.
- A musty smell in the bathroom that doesn't go away with cleaning.
- Discoloration on the ceiling below (if the bathroom is on an upper floor) or on the wall behind the shower.
All these signs could lead to the same problem, which generally cannot be treated by just cleaning the surface.
Why the spots keep coming back
Grout is cement. It is porous. It was never meant to be the waterproofing layer — the waterproofing is supposed to be behind the tile. In a correctly built shower, water that gets through the grout hits a membrane and drains back out. In a shower with no membrane, a damaged membrane, or a membrane that was never tied into the drain correctly, that water soaks into whatever is behind the tile and stays there.
That is why bleach does not solve it. You are killing mold on the surface while the colony behind the tile keeps feeding on a wet substrate. A lot of the calls we get on Cape Cod are from homeowners who have been cleaning the same corner for two years.
What a specialist checks that you can't
When we inspect a shower for a suspected leak or moisture problem, we are not guessing from the surface:
- Moisture meter readings on the tile, the grout lines, and the drywall around the shower, compared against a dry reference area.
- Thermal imaging to find cold, wet zones behind the wall that aren't visible.
- Tap test across the whole floor and the first few courses of wall tile to map hollow (debonded) areas.
- Drain and curb inspection — the two places most showers fail.
- Flood test when needed, to confirm whether the pan actually holds water.
From that we can tell you honestly whether you have a regrout-and-recaulk job, a localized repair, or a shower that needs to come out. Those are very different conversations, and we would rather give you the straight answer than sell you the biggest one.
Keeping your family safe in the meantime
Mold behind a wall is not an emergency by the hour, but it is not something to live with for a year either. Damp indoor spaces are linked to respiratory symptoms, asthma flare-ups, and allergic reactions, and the risk is higher for children, older adults, and anyone with a weakened immune system. While you sort it out:
- Run the exhaust fan during every shower and for 20–30 minutes after. If the fan doesn't hold a square of toilet paper against the grille, it is too weak for the room.
- Squeegee or towel the walls and floor after showering. Thirty seconds that removes most of the standing water.
- Leave the shower door or curtain open to let the enclosure dry.
- Don't paint over, caulk over, or tile over a stain to hide it. You are sealing moisture in.
- If anyone in the house is having unexplained respiratory or allergy symptoms that improve when they are away from home, mention it to your doctor and get the shower looked at sooner rather than later.
When to call
Call a tile and shower specialist if any of these are true: grout darkens again within a week of cleaning; tiles are hollow, loose, or cracking; caulk has separated at the pan or tub; there is a persistent musty smell; or you see staining on the other side of the wall or on the ceiling below.
Batista Tile does moisture testing and shower leak detection across Cape Cod, the Islands, and Plymouth County. If it is just surface mildew, we'll tell you — and tell you how to keep it from coming back. If it's more than that, you'll get a clear written scope so you know exactly what you're dealing with.
Request a shower inspection or call (774) 368-9592.
Talk to a Cape Cod Tile Specialist
Builders, general contractors, and homeowners across Cape Cod, the Islands, and Plymouth County trust Batista Tile for custom tile, showers, repairs, and shower glass.