The tile substrate is going to be the most important component of your installed shower walls. Choosing the right tile substrate that fits into your waterproofing strategy will be a key factor in ensuring your shower walls provide a stable surface for your tile installation, while also doing it’s part for moisture management.
Cement Backer Board
A cement backer board, sometimes referred to as a CBU or cementious backer unit, is a thin layer of concrete with fiberglass mesh on both sides. The cement backer board intended use is to function as a tile substrate, since it’s not a product of organic material, it does not provide an environment for mold growth. It typically can be scored with a utility knife in order to cut it to size during installation, but a hand saw will provide a much better finished cut through the cement base.
Fiberglass Mat Backer Board
A fiberglass mat backer board is made of a gypsum core with fiberglass mat sheets on either side. The fiberglass mat backer board has the benefit of having a moisture barrier built-in, therefore a vapor barrier is not a requirement and should not be installed to deter from creating a “moisture sandwich”. It also has the advantage of being a lighter tile substrate, while a key disadvantage is it’s susceptibility to soften upon coming in contact with moisture for a long period of time.
Green or Purple Board
A green or purple board is a specially treated gypsum based product that typically is not installed as a tile substrate within the shower area. There are arguments that green or purple board have been effective tile substrates in the past, and combined with more modern liquid and sheet membrane moisture resistant products, they can still be an effective component of your shower wall. The main drawback will be it’s gypsum core, that is if it exposed to moisture for a prolonged period of time, it will begin to soften, putting the integrity of your tile installation at risk.
Seams, Joints, and Fasteners
Your tile substrate will need to be reinforced at each seam and joint. The objective of your installed shower walls are not only to provide a rigid surface, but also build into them flexibility during the normal stresses of your homes movement. Seams and joints should first be filled with a 100% silicone caulk or a polyurethane sealer to prevent moisture from entering behind the substrate. You should also fill the fasteners with the silicone caulk or polyurethane sealer to deter moisture penetration. In between and over each seam and joint, a combination of thin-set mortar and alkaline resistant mesh tape should be applied to create as much of a single wall piece as possible. As soon as this is completed, you can apply a second application of thin-set to the mesh tape to reinforce the seam and joint barrier. Where a fastener exists, mud pack the thin-set to provide full coverage.
Changes of Plane
Where a change of plane is occurring, such as a shower wall to the shower pan or shower curb, you can apply a cut to size alkaline resistant mesh tape that is folded to the contours of the change of plane. You can secure the mesh tape using the thin-set mortar, ensuring that it sits flush with each surface and sits perfectly in each angle.
A true art form, the mortar backed, or floated cement shower wall is an installation method that is not a standard execution as it once was decades ago. You would start off with a 15-lb felt paper installed onto the wood studs, then followed with a wire mesh that would installed over the felt paper. The wire mesh has cupped parts which serve to catch the cement mixture that is floated over it. Two layers are applied, a scratch coat, then followed by a brown coat, each following wood leveling guides that are temporarily placed into the wall in order to ensure they are plum and square. If done properly, the end result is a near perfect platform for your tile installation.