The water resistant barrier is the reason why your shower pan will succeed at retaining moisture within the shower area floor or it could be the reason why your shower pan fails and allows moisture to reach below to the subfloor. It is the most important factor in a built up shower pan and you should have a good understanding of the many options you have to choose from and how they each can support your waterproofing strategy. Furthermore, the water resistant barrier is not optional, it is a mandatory layer needed to waterproof your built up shower pan, choosing to exclude it could lead to severe moisture damage to the subfloor and surrounding area. Finally, the water resistant barrier should not be duplicated in the built up shower pan, this is important to understand, since doing so would not allow moisture to properly evaporate and could increase the likelihood of mold growth.
A liquid membrane is an applied and cured water resistant barrier that has the advantage of being both effective at resisting moisture and easy to apply over your concrete surface. The liquid membrane is typically applied using a paint roller or trowel and per each manufacture specifications, only requires a minimal thickness to be considered effective. The curing process is easy to understand, most manufacturers will provide their product in a pre-applied color with a post-cured color change. The liquid membrane bonds to your surface but also allows for a strong bonding of a tile adhesive. As an added benefit, the liquid membrane also supports crack prevention, in the event that structural movements are present, it will maintain a sturdy and continuous layer.
A sheet membrane is a sheet-applied bonded waterproofing membrane of varying thicknesses that is typically made of a polyethylene core with fiber or fleece laminated on both sides. The sheet membrane is waterproof but not water resistant until all seams and joints have been sealed, this is why it is typically used along with a manufacturer specific mesh band or fiber mesh tape and a liquid membrane that can be applied over the entire sheet installation. A sheet membrane protects from water vapor penetration and also crack isolation to deter future tile protrusions.
The pan liner is an economical, yet effective, water resistant barrier. Pan liners are typically made of either a CPE ( chlorinated polyethylene ) or PVC ( polyvinyl chloride ) material that has the flexibility to be applied over your concrete surface, up the side walls of your shower, and also over the shower curb. Unlike a liquid membrane application, which requires a precise finished thickness, or a combination liquid and sheet membrane installation, which requires precise joints and seems to be waterproofed, the pan liner is a single piece that in installed in your shower floor.
A hot mop water resistant barrier consists of three layers of #15 felt paper with hot tar before, between, and after each layer. The corners are further reinforced with fiberglass that is applied into the asphalt. The hot mop will typically be installed 10-12″ inches up the side walls of the shower and over the shower curb. After installation, the entire shower area is flooded to further set the felt paper and hot tar and test the integrity of the hot mop.