Water Resistance

• One reason cement board is a popular material is its resistance to water. Even in the presence of a leak it is common for the materials around the cement board to become damaged but for the cement board itself to remain strong. There are even additional coatings you can use to make the board more watertight. However, cement board is not fully waterproof, only water resistant, so in extreme conditions some problems can develop.
  
  

Grout

• Grout can start to rot, crack and eventually crumble away in the presence of consistent moisture problems, especially moisture that works its way underneath the grout or past the grout sealant. If you are dealing with damaged grout, you may need to clear out all surrounding grout and tile to expose the cement board beneath. Carefully clean away the grout to inspect the damage. It may be the crumbling grout that is damaged and not the cement board itself.
  
  

Mold

• If moisture has been present behind the tile for an extended period of time, mold may have started to grow. Mold is notorious for growing into any porous material, and it may have gained a foothold in the cement board. If this is the case, you will need to remove that section of the cement board entirely. The mold cannot easily be cleaned away when it has dug into a surface, and it is safer to replace the board.
  
  

Cracks

• If your cement board is used in outdoor conditions, it may be affected by heat and cold. The changing of the seasons can cause moisture to freeze and melt, and over time the cement board exposed to these conditions can start to crack. Although you may be able to patch these cracks, it may be more effective simply to replace the board altogether.