Grout Stays Wet or Dark After Showering: Causes and How to Diagnose

Cement grout normally darkens when wet and returns to its dry color within about 2 to 12 hours, depending on ventilation. Grout that is still dark the next morning, or stays dark for days, is being fed water from below: a saturated mortar bed, clogged weep holes, a failed shower pan or membrane, or a plumbing leak. A timed dry-out test tells you which.

Normal darkening versus a problem

Cement grout is porous. Water fills the capillary pores, the surface reflects less light, and the joint looks one to three shades darker. That is ordinary behavior, explained in grout porosity and absorption, and it is why grout does not waterproof tile. Water that soaks into the grout during a shower should evaporate back out of the face once the room dries.

How long that takes depends on the room. In a bathroom with a working exhaust fan and an open door, a sealed cement grout joint usually looks dry in 1 to 4 hours. In a closed room with no fan and steamy air, 6 to 12 hours is common. Floors take longer than walls because water collects and sits on them. Epoxy and most urethane grouts barely absorb water, so they should not darken much at all. If epoxy grout looks wet for long periods, look at the tile edges and substrate, not the grout.

Causes, ranked by likelihood

1. Unsealed or worn grout plus poor ventilation

The most common and least serious cause. Grout that has lost its sealer absorbs more water and holds it longer, and a room without a fan simply cannot dry it. Signs: the darkening is even across the whole shower, it fades by the next day, and water beads poorly on the grout. A water drop test confirms it: if a drop darkens the joint within a minute, the sealer is gone.

2. Saturated mortar bed with clogged weep holes

Traditional shower floors have a liner under a sloped mortar bed. Water that passes through the grout drains along the liner to small weep holes in the drain body. When thinset, debris or mortar block those holes, water fills the mortar bed and the floor grout stays dark for days. This condition is very common in showers built over a traditional liner. Signs: floor grout darkest near the drain and fading outward, a sour or musty smell, efflorescence at the joints, and dark grout that never fully dries even during a vacation. Wall grout looks normal.

3. Water trapped under the tile from poor coverage or a blocked path

Hollow spots under tile, from poor thinset coverage, collect water. On surface-bonded membrane systems, water should not get below the membrane, but it can sit in voids between the membrane and the tile. Signs: isolated dark patches that match the shape of a tile or a group of tiles, a hollow sound when you tap with a coin or the handle of a screwdriver, and tiles that flex or click.

4. Failed pan, liner or membrane

A torn liner, a liner that was nailed through low on the curb, a membrane seam that opened, or a curb or niche with no waterproofing lets water escape the system. Signs: dark grout at the curb, at the bottom row of the walls or around a niche, stains or soft drywall outside the shower, swelling baseboards, and stains on the ceiling below. Dark grout on the outside face of a curb is a classic sign of curb waterproofing failure.

5. Plumbing leak

A leaking valve body, shower arm connection, supply line or drain connection supplies water whether the shower is used or not, or only when it is running. Signs: a dark area centered on the valve or shower head rather than on the floor, dark grout that gets worse after the shower runs even when no spray hits that area, a water meter that moves when every fixture is off (supply side leak), or a damp area that appears only after the drain is used (drain side leak).

The timed dry-out test

This is the single most useful test a homeowner can do. It separates surface absorption from a supply of water below the tile.

1. Take a normal shower. Use the shower as you usually do, then squeegee walls and floor.
2. Ventilate. Run the exhaust fan and leave the door open. A small fan pointed into the shower helps.
3. Photograph at fixed times. Take photos from the same spot at 1, 4, 12, 24, 48 and 72 hours with the same lighting. Put a strip of painter's tape in the frame as a reference.
4. Do not use the shower or any nearby fixture. Keep the shower out of service for the full test, and avoid flushing a toilet or running a sink that shares the same wall if you can.
5. Compare the pattern. Note which areas dried first, which dried last, and which never dried.

Reading the results: grout that dries evenly within 24 hours is a sealing and ventilation issue. Floor grout that takes 3 or more days to dry, or never fully dries, points to a saturated mortar bed. Dark areas that stay put while the shower is out of service and are near supply plumbing suggest a pressurized supply leak, because that water arrives with or without use. An area that dries and then darkens again right after a shower, while the rest of the shower stays dry, points to water escaping at that spot.

Using a moisture meter

A pinless (capacitance) moisture meter reads a relative value about 3/4 inch to 1 inch deep without damaging tile or grout. It does not give a true percentage on tile, so do not trust the number alone. Compare readings instead.

• Get a baseline. Read a dry area of the same tile and substrate outside the shower, such as the floor tile in the bathroom or a wall section well away from the spray.
• Map a grid. Read every 6 to 12 inches across the floor and the lower 2 feet of walls, plus around the valve, niche and curb. Mark the high readings with tape.
• Repeat over the dry-out. Read the same spots at 24, 48 and 72 hours. Readings that drop toward baseline mean surface absorption. Readings that stay high mean water held below.
• Check outside the shower. Read the drywall beside the curb, the baseboard and, if there is one, the ceiling below. Elevated readings outside the shower mean water has escaped the waterproofing.

Matching the dark pattern to the cause

Simple leak checks before calling a pro

Water meter test. Turn off every fixture in the house and note the meter reading or watch the leak indicator (a small dial or triangle) for 15 minutes. Movement means a supply leak somewhere. It does not prove the leak is in the shower, but combined with a dark patch at the valve it is strong evidence.

Drain flood test. Plug the drain, or fill around it with a towel dam, and let 1 to 2 inches of water sit on the floor for a few hours while you watch the ceiling below or the area outside the curb. This tests the pan and liner, not the grout, and it can push water into a damaged structure, so stop at the first sign of a leak. A plumber or shower contractor can run a controlled version with a test plug.

Fix summary by cause

• Worn sealer and poor ventilation: clean, let the grout dry fully (2-3 days with the fan running), then reseal. Add or fix the exhaust fan. See sealing shower grout.
• Clogged weep holes: sometimes cleared through the drain body by a plumber, but in many showers the holes are not reachable without removing tile around the drain. When the mortar bed has been saturated for a long time, the bed itself is usually contaminated and the floor gets rebuilt.
• Voids under a few tiles: remove and reset the affected tiles with full coverage, then grout. If the voids are widespread, the installation is the problem, not the grout.
• Failed pan, liner, curb or membrane: tear-out to the waterproofing layer and rebuild. Regrouting, sealing or caulking will not stop water that is getting past the waterproofing. This is the situation described in when regrouting won't work.
• Plumbing leak: fix the plumbing first, through an access panel if possible. Let the wall dry and recheck with the meter before deciding whether the tile needs to come off.

When it is time for tear-out

Surface fixes are worth trying only when the evidence points to absorption. Plan on removing tile when any of these are true: meter readings stay elevated for 72 hours or more with the shower out of service; dark grout is accompanied by loose or hollow tiles over a large area; there is staining, swelling or soft material outside the shower; the shower smells musty even after cleaning (see smelly grout); or the floor grout keeps producing white deposits, which is covered in efflorescence on grout. Delaying tear-out rarely saves money: water held against framing and subfloor spreads damage beyond the shower.

Prevention

• Build showers with a waterproofing layer that meets ANSI A118.10 (or a correctly installed traditional liner) and flood test it for 24 hours before tile goes on.
• Protect the weep holes on traditional drains with pea gravel or a weep hole protector so thinset and mortar cannot block them.
• Get full thinset coverage under shower tile. Industry practice calls for at least 95 percent coverage in wet areas.
• Slope the shower floor and the sill of any niche toward the drain (1/4 inch per foot is the standard minimum for floors).
• Run the exhaust fan during and for 20 to 30 minutes after showers, and squeegee walls.
• Choose low-absorption grout in wet areas; epoxy grout and A118.7 grouts hold less water. More on choices in shower floor grout.