Why is my grout discolored?

Q.   Why is the grout in my shower getting discolored?

A.  Grout gets discolored when mold starts to grow in it.  How does mold get to grow in it? Well it's all to do with water and moisture management.  Sure your shower gets wet all the time, but it also dries out between each use too.  And if you've got discolored grout you've probably tried using bleach, which works to brighten the outside, but it doesn't last long.  The problem is that there is moisture coming from somewhere else and that place is INSIDE your shower wall. The problem comes from a number of possible places, and to understand that we have to understand shower stall construction..

First point of understanding: Stone, porcelain & ceramic tile, grout and cement are all POROUS. That is to say they will all allow moisture to migrate through them over time.

So understanding that water is going to get into your wall changes how you anticipate the structure should perform, and the different methods of building the shower in the first place.


Here are the 4 main methods you'll see in Southern California.

  1. Lath + Plaster+ paper.  The old fashioned way to do it.  Water in water out. No problem.
  2. Cementatious backer board. I.e. Hardiebacker. A major problem because this stuff holds water like a sponge. So much so that a dry board will wick water UP into your wall.
  3. Tile on Gypsum sheet rock.  Nightmare scenario. Nothing is worse. Gypsum is basically a sheet of chalk sandwiched between two sheets of paper.....You'll be able to pull tile off the wall if you have this in your shower.  Expect life forms inside.
  4. Waterproof membranes.  Kerdi, Nobel and Wedi. There may be others.  A plastic waterproof membrane that is applied to a finished wall, to waterproof it. We're all trained in the Schluter Kerdi system, which is bomb proof.

Lath and plaster, is basically the same process as the stucco found on the exterior of a building. Osmosis draws water through the mortar until it hits the vapor barrier formed by the tar paper on the inside. The moisture condenses and drips down the paper and it is eventually caught by the shower pan and routed to the drain and special holes called Weep Holes.  This system is old, and works very well. It not supposed to be water proofing, it’s water management.  The drawback is that the shower stall will be very heavy when complete, and it takes time and skill to build.  The advantage of the system is that the finished wall with be completely flat if it’s done right.

Backer board was invented to speed up the process of building the wall, which it did. The backer board replaces the floated mortar wall and there is still a vapor barrier of tar paper behind the board.   It started to go wrong at the moment the backer board is installed, when the screw that fastens the board to the wall penetrates the vapor barrier to reach the stud…but this isn’t the Achilles heel.  As the water migrates down, it exits into the shower pan like the lath and plaster wall. However, the backerboard composition makes it act like a sponge and it retains a great deal of water. So much so that once the wall has appeared to be dry, the boards still a reservoir of water static in it.  This is the water that allows the mold to grow. 

Drywall in a shower stall? Why? Who? When? What for? Whatever.  We’ve taken showers apart that have had tile applied straight onto the sheetrock. They always fail in the end. Hotmop? Yes. But gypsum holds water really well and turns into cheese if it does get wet..  Just NO !

Waterproof membranes.  The concept here is new. Water proof the wall before the tile is applied. Waterproof the wall to the top of the tiled area and carry that waterproofing across the shower pan into the drain on the floor. The water never gets to go into the wall in the first place.. To learn more read ou rblog about Kerdi shower waterproofing systems from the German company Schluter Systems.

So while we won’t guarantee that your grout will never discolor, the water doesn’t get to hang around to keep the mold alive.