Have you ever had to have a leaking shower pan for a tiled shower stall replaced, only to have a recurrence a few years later? If so, it is likely that it was not properly built and/or repaired. In most installations, the shower stall is constructed with an underlying one-piece flexible membrane of PVC that is attached to the wall studs before the backer board and wall tile is installed. No nails or screws should penetrate the membrane below the level of the curb of the shower stall. The only opening in the membrane below the curb must be the hole for the shower drain to connect to the house plumbing. The shower drain is designed to allow water on top of the membrane to flow into the drain via weep holes for that purpose.
In some parts of the country, the practice of the installer is to place the flexible membrane directly down on the flat (i.e., not sloped) subfloor; then a sloped mortar bed is applied for the tile placement. This method is a direct contradiction of the code, which requires the liner to be placed on a bed (subbase) sloped toward the drain. The PVC liner thus should be sandwiched between the sloped subbase and a sloped layer of mortar above the liner, as shown in the accompanying schematic by Fine Homebuilding. If the flexible liner is incorrectly placed on the flat subfloor, water that penetrates the overlying grout joints and mortar bed (and it will, sealed or not!) will not flow properly to the drain once it reaches the flexible liner, keeping the liner chronically wet and deteriorating the grout.
The International Residential Code states as follows in Section P2709 for shower receptors: Lining materials shall be pitched one-fourth unit vertical in 12 units horizontal (2 percent slope) to weep holes in the subdrain by means of a smooth, solidly formed subbase… The International Plumbing Code has similar language.
There is actually a second method aside from the traditional method described; that is, installation of a thin-bed shower pan that will comply with code, sometimes used for handicap-accessible shower stalls.
In our investigations of water damage claims at Warren, we have observed such improperly constructed shower stalls on a number of occasions. If we can be of assistance to you in determining the cause of water damage or evaluation of other construction defects, our team of engineers and construction specialists will provide the expertise and knowledge required to support our conclusions and to determine the scope of damages.
Roger Davis, a senior consulting engineer at The Warren Group, is a licensed professional engineer in South Carolina, North Carolina, Alabama, Georgia and Mississippi. He holds a Bachelor of Science in Mechanical Engineering and a Master of Business Administration from the University of South Carolina. Roger is a certified fire and explosion investigator and certified vehicle fire investigator. He is an accomplished gas and diesel engine mechanic and has more than 30 years of experience with hydraulic plumbing and piping issues. His expertise also includes property damage and personal injury investigations involving municipal utilities. Roger has investigated claims and injuries ranging from pressure piping system failures and material and personnel handling equipment to large engine failures and fires involving machinery, generators and vehicles.