Rebuilding a Shower Pan With a Clamping Drain
A remodeler is rebuilding a 4 ft x 6 ft walk-in shower on a wood-framed floor. The existing clamping drain is reused. The old pan had no preslope; the liner was nailed to the curb, and greenboard extends to the floor. The owner selected 12 x 24 porcelain for the walls and 2 x 2 mosaic for the floor. The remodeler plans to install 1/2 in. cement backer board over the greenboard with screws 12 in. on center, place a new PVC liner flat on the subfloor, and tile the curb by bonding tile directly to the liner with modified thin-set. The proposed curb is 2 in. high. The shower is on a second floor above a finished ceiling.
1. Identify the defects and code or TCNA violations in the proposed shower assembly. Explain why each matters.
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The pan has no preslope, so water that passes through the grout and mortar bed will sit on the flat liner rather than drain. The liner is nailed to the curb, creating penetrations below the required height and a leak path. Greenboard is not an acceptable substrate in a shower wet area, and installing cement backer board over greenboard creates a double layer that does not solve the moisture problem. The plan to set a new liner flat on the subfloor repeats the preslope omission. Bonding curb tile directly to the liner with thin-set is wrong: the liner is not a stable bonding substrate, fasteners or thin-set can puncture it, and the curb needs a mortared or structural core. The 2 in. curb should be checked against the local code and the drain height; it may be too low. There is no flood test, and the finished ceiling below makes a leak especially costly.
2. Describe the correct sequence for rebuilding the pan, curb, and walls.
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Remove the old shower to framing. Inspect and block the stud bays. Install a preslope of mortar at 1/4 in. per foot from the drain to the perimeter, with the drain clamping ring set to the finished elevation. Install the PVC liner over the preslope, run it up the walls at least 6 in. above the curb, and do not fasten it below 2 in. above the curb top. Fold the corners rather than cutting them. Install a mortared curb over the liner, with the liner protected and no penetrations through the top. Flood-test the pan for at least 24 hours. Then install cement backer board on the walls with a gap at the pan, embedding it in thin-set and fastening it without penetrating the liner below the curb. Tape and waterproof the walls per the system. Set the mosaic floor to the drain with weep holes protected, then tile the walls and curb. Leave flexible sealant at changes of plane.
3. What drain and flood-test details must be verified before the tile is installed?
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Before tile, confirm the drain clamping ring is set so the finished tile is flush or slightly above it and the weep holes are open. Protect the weep holes with pea gravel or a drain protector so mortar does not block them. Verify the preslope and final mortar bed both slope continuously at about 1/4 in. per foot, with no birdbaths. The liner must be continuous, lapped over the curb, and free of penetrations below the curb. Flood-test by plugging the drain and filling the pan with water to just below the curb top; mark the level and check after 24 hours for loss. Look below the floor if accessible. Do not cover the liner until the test passes. Confirm the curb height above the drain and the local code requirement before setting tile.