As part of your waterproofing strategy, when building a shower pan, it is important to keep moisture inside the pan and out of the surrounding bath area, as well as from penetrating into the interior base shower walls and floor. This is accomplished by adhering to a standard bottom up method of building each layer of the shower pan as a protection from moisture and a cause for it to exit through the drainage system.
Before a pan can be built for your shower, the subfloor must be in a condition to support the several hundred pounds of additional weight. For raised foundation and second floor bathrooms, if you have not already inspected the running boards and joist beams, this would be a good time to determine if supporting wood needs to be replaced. For slab foundations, ensure that the concrete is in good condition to support the construction. You have several options to finish your subfloor, installing over the running boards or concrete slab, a cut to size 3/4 or 1/2 inch plywood or Cement Backer Board or excluding this layer altogether.
A pre-slope is the start of your moisture management, it will establish the negative slope that your shower pan will follow to encourage water to lead down and into the shower drain. The pre-slope is most commonly constructed of a cement dry pack mortar, the correct consistency will allow itself to be balled up in your hand. It is poured onto the subfloor and using a trowel, the dry pack is smoothed downward using a standard measurement of 1/4 inch negative slope for each foot from the shower drain. An alternative option would be a pre-manufactured pre-slope product that takes the difficulty out of preparing a cement mixture and accurately establishing a correct negative slope to the drain.
Perhaps the key component of a built up shower pan is the water resistant barrier, sitting above the negatively pitched pre-slope, which will ensure that moisture does not reach below your shower floor and runs downward. The water resistant barrier comes in many forms, a physical membrane, a liquid membrane, and a combination of both, all of which have their own degree of complexity during installation. The important factor is understanding how your choice of a water resistant barrier works with your overall waterproofing strategy.
While there are liquid membrane water barriers that can be applied to a pre-slope, the common construction technique of a built up shower pan is to apply a second concrete mortar mix above the water resistant barrier that follows the same negative pre-slope to the shower drain. The purpose for this top layer is both as a protector of the water resistant barrier and also as a surface that is ready for the installation of the shower floor tile.
The tile adhesive is not considered to be a waterproofing product, but in the event that it does meet with moisture, it will be one more layer that will help to dissipate in it’s downward motion. The adhesive you choose to install your tile to your shower pan is important, not only for adding another barrier of protection from water absorption, but equally, if not more important, to ensure the tile stays adhered, especially when moisture does become present. While you can use a mastic adhesive, a thin-set mixture mixed on site is widely understood as the standard adhesive for tile.
Your choice of tile that will be installed on the shower floor is the first step in preventing moisture from meeting with each layer of your built up shower pan. All tile is permeable, with resistance levels that seem minuscule at first, over the course of normal usage, enough moisture can be absorbed through it, which could lead to a problem in a shower pan that does not drain properly. Furthermore, the installation requirements for each type of tile are important to understand, given that the spacing will be where vulnerabilities begin.
In between each installed tile piece is going to be a standard width that will allow a mortar grout to be applied and help in resisting water from entering behind it. You have four to choose from: unsanded, fine sanded, quarry type, and epoxy. Each is used for specific tile installations, but all serve to make sure that moisture does not reach behind the tile and into the your walls. While the grout is not water resistant, and is porous, it is still a very important aspect of your shower floor tile installation since it serves to fill in the vulnerable open crevices.
Your method for building a shower pan will rely on many factors, the more important being: state and local building codes, availability of expertise and materials, atmospheric temperature and moisture levels. While the method you choose may or may not be better or worst than any other method, exclusion of specific layers may increase the risk of a shower pan failure, promote the growth of mold, or both. A built up shower pan that does not fail will be due to a method that utilizes each component to strengthen the next.
Mistakes happen but you don’t want them to occur with your built up shower pan. Putting together a good plan to build a shower pan and adhering to standard construction methods will decrease the likelihood of mistakes. It is important to research and seek out the mistakes others have made where a shower pan failure occurred and you will be able to learn from.