Grout that stays dark and damp-looking long after everyone has showered is one of the most common questions we get on Cape Cod. It is also one of the most useful warning signs a bathroom can give you, because it is telling you water is sitting somewhere it probably should not.
There are two likely causes, and they lead to very different repairs: the room is not drying out, or water is coming up from behind the tile. Here is how to tell them apart at home before you call anyone.
How grout is supposed to behave
Cement grout is porous. It absorbs water during a shower and releases it as the air in the room dries and the components underneath direct water to the drain. In a bathroom with decent airflow, grout should look uniformly dry within 12-16h. On a humid August day on the Cape it might take longer, but it should still dry.
If grout is still dark the next morning — or is dark in specific patches while the rest of the shower is dry — something is feeding it.
Cause 1: Ventilation (the room can't dry)
This is the cheaper problem. Signs that point to ventilation:
- The whole shower stays damp evenly, not in one patch.
- Mirrors and windows fog heavily and stay fogged.
- You see mildew on the ceiling, the top of the walls, or the window trim — places water never splashes.
- The exhaust fan is small and noisy.
- The fan runs for the shower and gets switched off right after.
- The bathroom has no fan, or the fan vents into the attic instead of outside.
The fix is airflow: a properly sized fan (for most bathrooms, 50–80 CFM minimum, more for large rooms or long duct runs), a timer switch so it runs 20–30 minutes after the shower, and the both shower door and bathroom door cracked open. Many Cape Cod bathrooms were built with undersized fans or none at all, and the newer, tighter houses trap moisture even more.
Cause 2: Water trapped under the tile
This is the one that gets expensive over time. Signs that point to water behind or under the tile:
- Grout stays dark in one area — usually the shower floor, the bottom course of wall tile, a corner, or around the drain — while the rest dries normally.
- The dark patch is still there after you've run the fan for a full day, or after a few days of nobody using that shower.
- Grout in the wet patch is soft, crumbling, or has been regrouted before and failed again.
- Tiles in that zone sound hollow when tapped.
- You get efflorescence — a white, chalky crust on the grout lines. That is minerals being carried to the surface by water moving through the assembly, which means water is sitting behind the tile.
- There is a musty smell, or a soft spot in the drywall or baseboard just outside the shower.
When the shower floor is the wet zone, the usual culprit is an improperly installed waterproofing system, a liner that was not tied into the drain correctly, or a pre-slope that was never built so water sits in the mortar bed instead of draining. On walls, it is usually a failed membrane behind the tile, a missing membrane entirely, or a leak at a niche, bench, or plumbing penetration.
A simple test you can do this weekend
- Clean the grout so you have a clear starting point.
- Stop using that shower for 48 hours. Run the exhaust fan for an hour each morning and evening and leave the door open.
- Look at the grout at the end of the 48 hours.
If everything is uniformly dry, your problem is ventilation and daily drying habits. Fix the fan.
If a specific patch is still dark after two days of no use and active drying, water is coming from behind the tile. That water is not going to leave on its own, and the mold growing on it is not going to either.
What each fix actually involves
Ventilation: a new fan and timer switch, sometimes a new duct run to the exterior. Typically a few hundred dollars to about a thousand with an electrician, depending on the house.
**failed pan/membrane or shower leak (Replacement of the base of the shower and 12” up the walls; Rebuild system properly where its the most critical; this process could take $5-8K + materials, depending on shower size, location and other variables.mOur custom shower rebuilds use a double-layer waterproofing system specifically so this doesn't happen again.
We offer leak diagnostic, which involves a series of tests, including moisture meter and thermal imaging inspections, before we quote anything, so you are not paying for a rebuild when a repair would do — and not paying for a repair that will fail in six months.
Don't do this
- Don't seal, paint, or regrout over a patch that is still wet. You are trapping water.
- Don't put a bead of silicone over a cracked grout line on a shower floor and call it fixed.
- Don't ignore efflorescence. It is the clearest sign there is that water is saturating the system and moving through the tile assembly.
If you've done the 48-hour test and the patch is still dark, request an inspection or call (774) 368-9592. We serve all of Cape Cod, Martha's Vineyard, Nantucket, and Plymouth County.
Not sure if your shower is leaking?
Our multi-factor leak test finds the answer in one visit without removing a tile: interview, photographed inspection, knock test, 22-point moisture map, thermal imaging and a bucket stress test, with a written diagnosis the same day. $299 per bathroom, credited toward the repair. Serving Cape Cod, the Islands and Plymouth County.