Walls

• Restrictions apply to cement board wall usage. For example, for strength reasons, manufacturers mandate 1/2-inch (and 5/8-inch) board on walls. The exception to this rule allows 1/4-inch cement board on walls only where there is minimum 5/8-inch exterior-grade plywood sheathing or Oriented Strand Board (OSB) subfloor. Maximum stud spacing on walls is 16 inch off center. Additionally, fastener spacing should not exceed 8 inch off center. Manufacturers require installation of a moisture barrier (4-mil plastic or heavy felt construction paper) under the cement board in wet applications such as a shower surround. Cement board manufacturers also stress that cement board should never be used as a load-bearing member.
  
  

Floors

• Both 1/4- and1/2-inch cement board are appropriate for floors. To install cement board on floors, cement board manufacturers mandate a 5/8-inch plywood subfloor or OSB underlayment. They also require a 3/4-inch subfloor where tile size exceeds 12 inches. Manufacturers also recommend bonding cement board to floors with a 1/4-inch polymer-modified thinset coating. Additionally, experienced tile setters find that 1/2-inch cement board provides a superior sound/noise barrier in upstairs applications.
  
  

Manageability

• One thing that do-it-yourselfers should consider is the difference in manageability between 1/2-inch and 1/4-inch cement board. For example, 1/4-inch board weighs less, is easier to affix to the substrate (with either screws or nails) and is easier to cut. On the other hand, 1/2-inch board can be a real bear to cut and manage at the job site.
  
  

Additional Cement Board Benefits

• Cement board is extremely hardy, adding to the longevity of your shower. Unlike traditional wallboard (such as drywall and green board), cement board is water- and mold-resistant. It doesn't swell in high-moisture areas and is also fire-safe. Cement board also provides a far superior bond to tile than traditional fiberboard.