Why waterproof?

    This is all that was left of the subfloor under a traditional shower pan, which leaked for many years.

 
 

      Story of two drains.  The drain on the right is the traditional 2-part drain. Most water does down the top grate, but secondary water always filters through the concrete shower pad. This is suppose to be collected and directed by the membrane back to the lower drain but usually pretty much just sits there.  Any small hole will leak.
     On the left is the Schluter drain.  The Kerdi fabric you see (left) is a waterproof membrane which covers the floor and all shower walls.  All water is directed down the grate, nothing to filter through the concrete.

 

Part of the Schluter system is to waterproof the shower walls. Commonly used cement tile backer board will not crumble when wet. But unfortunately, it will wick water very well when given the opportunity. The orange material is the water proofing, Schluter Kerdi,  (also rated for steam rooms).