One-piece shower units are plastic-based showers that have a base which is similar to any other plastic shower pan. There are specific installation instructions that vary from shower type to shower type, depending on the manufacturer, but the general rule is that a mortar needs to be used as a base to support and lock the pan in place. While manufacturer recommendations trump all else, there are some basic rules to keep in mind.
Nature of Plastic
Plastic shower pans have one thing in common, regardless if they are stand-alone pans or pans that are part of an overall one-piece shower unit. All plastic shower pans are flexible, and are thus susceptible to movement. If excessive movement is allowed, cracking will occur. The plastic base for these types of showers needs to be installed in a solid bed that doesn’t allow for movement so that the pan doesn’t have the opportunity to crack out.
Not only are mortar beds the preferred method for installing acrylic tubs, they are also the main recommended method for installing plastic shower pans and one-piece shower units that have walls which are attached to the pan. Thinset tile mortar is the best kind, and it provides a stable, flat base that also adheres to the bottom of the pan, creating a cementitious mortar base that will last for years to come and keep the pan from moving and cracking.
Basics of Foam
Foam is a flexible material that is created through numerous gas bubbles that are trapped inside of a gel film that dries over time after it has been applied in a liquid state. Because it is a flexible material, it is not solid, and thus does not provide the basic support necessary for a shower pan. Should you attempt to use foam as a base for a shower pan, you will have excessive movement that will cause the pan of the shower unit to crack out due to the weight of the human body pressing down at different levels.
Other Foam Issues
Above and beyond the simple fact that foam doesn’t provide a stable base is the fact that foam continually expands over a period of days and even weeks in extremely humid conditions. This means that the foam will be continually pushing up against the base of the shower pan, causing it to “bubble” in certain areas. This leads to warping, which leads to cracking, and can also cause the overall shower unit to shift to one side or the other if too much foam was sprayed in the first place.
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