Bathroom mold: grout, caulk, hidden rot, and what it costs

Why bathrooms grow mold

A bathroom gets wet on purpose several times a day, and a lot of bathrooms have no way to dry out between uses.

Steam is the main source. A hot shower fills the room with water vapor, and the vapor condenses on the coldest surfaces: the exterior wall, the window, the ceiling above the shower, and the back of the toilet tank. With no fan, or a fan nobody runs, that water sits on the paint and the grout until the next shower adds more. A fan that vents into the attic moves the problem upstairs.

Grout and caulk are the second source. Both wear out. Grout cracks and absorbs water. Caulk at the tub-to-tile joint pulls away, and every shower sends water behind the surround. The drywall or greenboard behind the tile gets wet from the back, then the framing, and the growth happens where nobody sees it until a tile comes loose.

Plumbing is the third. A toilet wax ring that fails leaks a little with every flush into the subfloor. A supply line under the vanity drips into the cabinet floor. A shower valve leaks inside the wall. A tub drain leaks into the ceiling below.

Small bathrooms with a window in the shower, an exterior wall, and no heat source make all of it worse.

Signs you can check in the shower

Start with the caulk line where the tub meets the tile, and the grout in the lower corners of the shower.

• Dark spotting on caulk that does not come off with cleaning. Growth under the caulk cannot be cleaned from the surface.
• Grout that stays dark after it dries, or crumbles when you scrape it.
• Tiles that move when you press them, or a tile wall that feels soft or hollow behind the lower rows.
• Bubbling or peeling paint on the ceiling above the shower and on the exterior wall.
• A musty smell inside the vanity cabinet, with a stained or swollen cabinet floor.
• A soft floor around the toilet, or a toilet that rocks.
• Stains on the ceiling of the room below the bathroom.

Press the tile wall at the bottom of the surround with your palm. Solid tile on sound backing has no give. A wall that flexes or feels spongy has wet material behind it.

What happens if it waits

Surface mold on grout is a cleaning problem. Mold behind the surround is a construction problem.

Water that gets behind the tile wets the backer board, then the studs, then the subfloor at the base of the tub. Drywall and greenboard fall apart when they stay wet. Wood framing rots. A tub surround that has been wet from behind for a long time usually comes out along with the wall behind it, and the rebuild is a tile job on top of the remediation.

A failed wax ring rots the subfloor around the toilet flange, and the repair means pulling the toilet, cutting the floor, and sometimes sistering a joist.

Bathroom air is house air, and the smell and the spores leave through the door every time it opens. Mold can irritate people who are sensitive to it.

Buyers’ inspectors press on tile walls, check caulk lines, rock the toilet, and look at the ceiling below. Loose tile and a soft wall are findings that lead to a bathroom remodel in the negotiation.

What bathroom remediation involves

Two very different jobs share this page.

The cleaning job covers tile, grout, caulk, painted walls, and the ceiling. The crew removes the old caulk, scrubs the grout, cleans the hard surfaces, applies an antimicrobial, and recaulks. Porous grout that cannot be cleaned gets scraped and regrouted. This is a surface scope, and HomeWyse prices cleaning-only work with favorable conditions at $2.30 to $2.80 per square foot.

The demolition job starts when there is growth behind the surround or under the floor. The crew sets containment at the bathroom door, runs negative air, and removes the surround, the tile, the backer board or drywall, and any wet insulation. The framing gets cleaned, treated, and dried. Subfloor that has rotted gets cut out. A moisture meter confirms the framing is dry before anything closes up.

The leak gets fixed in the same visit, or by a plumber as a separate line. Then a tile contractor rebuilds the surround. The rebuild sits outside the remediation band.

A bath fan that vents outside is the usual moisture fix. Installing one, or rerouting one that ends in the attic, is an electrician or HVAC line.

What moves the bathroom price

The band shown above, $500 to $1,500, reconciles This Old House at $500 to $1,000 with Fixr at $500 to $1,500. It matches the small-area band for a visible, contained, accessible job, which fits most bathrooms.

The main variable is whether anything comes out. Cleaning tile and grout in one shower sits at the bottom. Removing a tub surround and the wall behind it reaches the top, and the tile rebuild sits on top of that.

Square footage is the second variable, at $10 to $25 per square foot of affected surface. A bathroom is small, and the area is usually under the 50 square feet that This Old House prices at $500 to $1,250.

Hidden extent is the third. Growth that runs from the shower into the wall shared with a bedroom or a closet moves the job to the behind walls band at $1,000 to $12,000.

Subfloor damage adds carpentry. A failed wax ring that rotted the floor around the toilet is a floor repair before it is a mold job.

Small areas under the EPA threshold of about 10 square feet are ones most homeowners can clean themselves. A few spots on the caulk line are in that category. A whole wall of soft tile is a professional job. Read the EPA 10 square feet rule for how that line works.

Get a planning range from the estimate tool, and see the bathroom mold removal cost page for the full breakdown. Planning ranges come from national data. A remediation company confirms scope and price on site.

Keeping the bathroom dry

Run the fan during every shower and leave it running afterward until the mirror clears. A timer switch does this without anyone remembering.

Vent the fan outside through the roof or a wall, and check the attic side once.

Squeegee the shower walls after use and open the door or curtain so the enclosure dries.

Recaulk the tub joint when it pulls away or stays dark. Reseal the grout when water stops beading on it.

Fix drips as soon as you notice them. A rocking toilet needs a new wax ring before the floor softens.

Keep the bathroom door open when the room is empty, and keep the house humidity in the EPA range, below 60 percent and ideally between 30 and 50 percent. The full list is in preventing mold from coming back.