Ideal for the application of thinset mortar, installed backer board forms a stable cement surface.
Although cement backer board manufacturers recommend fastening cement board directly to wall framing, they do not explicitly warn against installing backer over drywall. While properly prepared drywall can serve as a substrate for cement board, it presents both waterproofing and tiling challenges. If you learn about standard cement board installation procedures and the reasons that manufacturers recommend direct fastening to studs, you can decide whether to leave your drywall in place or tear it out before tiling.
Drywall Substrate Challenges
Drywall's thickness presents a challenge during many cement board projects. Added to the thickness of cement board, mortar, waterproofing materials and tiles, drywall's thickness often pushes the wall's finished surface beyond the desired dimension. Drywall typically ranges from 1/4 to 3/4 inch thick. Because tiled walls sometimes must abut counters, tubs or shower stalls at specific positions, drywall's thickness is unsuitable for many wall tiling projects. Additionally, while cement board resists moisture, underlying drywall might become damaged from moisture or contribute to wood rot or structural damage.
New Construction vs. Remodel
Working on new construction, builders have the opportunity to easily install cement backer board without a drywall underlay. During a remodel, builders might face an old drywall installation. If your project presents you with a choice between installing cement board over drywall or removing drywall to begin clean, consider the finished thickness of the tiled wall compared to the thickness of adjacent wall surfaces. If drywall pushes your tiled wall's finished surface beyond the level of adjacent surfaces, consider both the aesthetic and practical impact of the transition before installing the cement board. Whereas adjacent, flush surfaces create a smooth, nearly seamless transition, uneven surfaces create a bold visual change. Practical considerations include deciding what type of transition or edge tile will span the gap and seal, waterproof or cover the gap.
Wall Framing
Cement board manufacturers advise builders to install cement board only over structurally sound framing systems. Acceptable framing systems include standard wood and metal stud walls. Standard wood studs, called "2 by 4" studs, measure roughly 1 1/2 inches by 3 1/2 inches. Manufacturers recommend a minimum of 20 gauge studs for metal stud wall installation; a metal stud's gauge refers to the thickness of the metal stud's flanges and web. Additionally, the required stud spacing is 16 inches on center. Many builders fasten a waterproof membrane or vapor retardant covering to the studs before applying cement board.
Cutting and Fastening Cement Backer Board
It is important to know that cutting cement board with a power saw releases potentially harmful silica dust. Cement board manufacturers recommend dividing cement board using a utility knife and the score and snap method. If you decide to cut cement board with power tools, manufacturers provide product-specific guidelines regarding proper blade selection and safety procedures. In all cases, cutting cement board requires skin protection, approved respiratory protection and safety goggles.
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