Duroc question?

I am in the process of making a shower/bathtub tile surround. I thought I could install cement board dirrectly on studs. Now I read that drywall goes on the studs then cement board on to drywall then tile. I am having a hard time understanding why this is the proper procedure. Thanks in Advance!

 

skip the dry wall ,but dont forget a moisture barrier behind the CBU.

 

Cement products are not waterproof, only water resistant. Cement wicks moisture very well. A barrier is needed behind the cement board to stop the water from going any further. Even a tiled wall is not waterproof. While the tiles themselves may stop water the grout does not. Look at the problems caused by tiling on drywall. The water gets through, the drywall softens, and the tiles come off. Tarpaper was used for many years as a barrier; there are newer materials that are as good or better. By lapping the membrane under the shower floor behind the membrane on the wall water will be shed to the drain without going undesirable places.