Question

What is the best backing material for shower walls?

Asked by gary
Chesapeake, VA

It must be water and moisture resistant.

Answer

Polly Osborne, FAIA, LEED AP

Answered by Polly Osborne, FAIA, LEED AP

Los Angeles, CA

Polly Osborne Architects

November 3, 2010

Waterproofing showers and tubs correctly is the most important interior moisture-proofing you can do, and perhaps the one most often done wrong. When we remodel a home, this is the area we most often see completely rotted away.

Like with any waterproofing, the goal is to get the water away from the structure, so waterproofing a shower is like the little brother to waterproofing your roof.

The shower floor is done very much like a flat roof, with a hot-mopped layer or a membrane over the shower pan. The walls begin with a membrane to protect the structure and are then overlayed with a:

  • concrete float,
  • concrete board, or
  • waterproof gypsum board.

Waterproof gypsum board, or “green board” is not allowed in some jurisdictions because it has not proven to be reliable. A better solution is a cement board and the best solution is an onsite concrete “float.”

  • When a cement board is used, screwing it to the studs and careful sealing and mudding of all joints is important. 
  • When using a float, the concrete is hand-toweled over the membrane and a wire reinforcing mesh.
  • One of the reasons a float is considered the best solution is any imperfections in the framing can be smoothed out, making the best base for ceramic tile. It is best to have an experienced craftsperson do this.

The thinset and grout used with the ceramic tile applied over these bases can have an embedded waterproofing in it to further protect the walls beneath, whether they are backer (cement) board or float. Obviously this is only a surface waterproofing. With the perforations made by plumbing, you need waterproofing behind as well.


For more information:

Read Rick Goyette's Q&A "Doing a bathroom remodel and need to replace shower pan, tile, etc. What is the greenest choice?"

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