Shower Mold Growth - Identification & Solutions

Black mold growth on the surface of a shower surround

Depending on the type of materials used, mold growth can occur within the stone / tile itself.  The photos below show heavy mold growth in travertine tile, which is very porous.  Unless it is properly sealed, the moisture can penetrate the stone, creating conditions conducive to bathroom mold growth.

Mold growth on caulking

Shower caulking must endure heavy moisture conditions on a daily basis.  It’s not surprising this is one of the most common areas of mold growth within a home.  This growth often occurs within the caulking itself, which is why it is difficult to clean.  Replacing the caulking is usually necessary.   The best long term solution?  Use the highest quality caulking you can find.  The best products are formulated to withstand years of moisture saturation without developing mold growth.

Mold growth on grout

Mold growth can occur within the grout used in the spaces between the tiles.  Like mold growth in the caulking, this is very difficult to clean.  In its natural state, grout is highly porous and will absorb enough moisture to propagate mold growth.  Preventing this growth requires a high quality grout sealant (and periodic maintenance).  In the image below the grout has failed entirely in sections, allowing moisture to penetrate behind the tile.  

Mold damage behind the shower surround

Surface mold growth is annoying, but rarely more than an aesthetic nuisance.  However, if enough moisture has penetrated the wall cavity behind the shower surround, structural damage can occur.  Three causes are possible.

1. Leaky cold & hot water supply lines.
2. Cracked tile & grout.
3. Vapor drive

Failed shower tile

A failure of the shower surround will affect the backer board first.  Often the first indication of the problem is a loose tile.  This occurs when the backer board swells or loses it structural integrity, causing the glue/mastic to fail.

Mold Damage Behind Failed Shower Tile

The moisture doesn’t always penetrate the underlying framing.  In the right hand photo the moisture meter shows elevated water content in the backer board.  In the left hand photo, the underlying stud is dry and structurally sound.

Diagnosing hidden mold and rot behind shower surrounds

Assessing a shower for hidden mold is difficult. Moisture meters, the most important tool in a mold inspectors kit, is of limited value on shower surrounds.  Tile and stone often provide false positives due to the conductivity of the material. Fortunately, well trained mold inspectors have other avenues to identify moisture issues.

• Investigate the crawlspace or room below.  If the leak is significant, often the materials below the shower will show signs of water damage.
• Investigate the adjacent wall.  If the shower shares a common wall with a bedroom or closet, an inspector can use a moisture meter to check for elevated water content.  Additionally, a small hole can be cut in the sheetrock to provide a visual inspection.  A boroscope can then be used to look within the wall cavity.   This type of inspection is much less invasive than removing tile and can often be performed without drywall repair.

Below is a before and after shot of a tile shower.  Note the extensive damage to the joists.  Very little was keeping this shower from falling into the crawlspace below.  The damage was fundamentally due to insufficient waterproofing behind the tile.  The grout and caulking failed (as it eventually does in all cases), allowing water to pass into the backer board.  In a correctly designed shower, this backer board is coated with a thick waterproof coating.  This is not merely a backup in case the grout fails.  It should be thought of as the primary water barrier.  In fact, skilled tile contractors will fill the shower pan with water prior to installing the tile to prove the efficacy of the waterproofing.

Shower during original inspection

Shower with suspected water damage.

Mold & rot visible after removing shower tile and pan.

Clean up and remediation

Unless the problem is very small, mold problems in the shower wall should be addressed by a certified mold professional.  The following are steps an experienced mold contractor will follow:

• Setup containment. (HEPA air scrubber, negative pressure, etc.)
• Remove shower surround.
• Remove tile backer board if present.
• If necessary, remove tub or shower pan.
• Clean underlying framing and flooring.
• If dry rot is present, remove and replace damaged framing.
• Address plumbing problem.

Is shower mold a health problem?

If the mold growth is occurring inside the wall cavity (behind the shower surround), health issues are unlikely.  Because the shower surround acts as an air barrier, the mold spores cannot migrate to the indoor environment.  However, if the shower mold is removed without professional containment in place, the indoor air quality will undoubtedly become compromised.

What type of surrounds are most conducive to shower mold?

Shower surrounds vary wildly in the tendency toward failure.  Unfortunately, the designs and materials often considered most aesthetically desirable are also the most likely to fail.

1. Stone tile. Stone surrounds, while beautiful, carry all the problems associated with grout and caulking and have the additional challenge of mold growth within the stone itself.
2. Porcelain Tile.  Porcelain tile is much less porous and will rarely propagate mold growth itself.  The main challenges are the potential failure of the grout and caulking.
3. 3-piece fiberglass surround.  Failure points are limited to the caulking at the seams.
4. 1 piece fiberglass surround.  Virtually bulletproof, a 1-piece surround provides very few failure points.  Moisture can leak through the door, but this is usually an installation failure.

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