Flood Test Passes but the Curb Wicks Moisture
A contractor completes a tiled shower with a PVC liner, clamping-ring drain, and a 4 in. tall curb built from three stacked 2x4s covered with cement backer board. A 24-hour flood test was performed before tile and passed with no drop in water level. After tile and grout, the homeowner notices efflorescence and a dark stain on the drywall on the outside face of the curb, about 3 in. above the bathroom floor. The stain grows slowly over a month. The liner was not inspected for fastener penetrations on the inside of the curb. The installer says the flood test passed, so the receptor must be watertight.
1. Explain how a receptor can pass a flood test and still leak at the curb, and identify the most likely defect.
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A flood test only proves the pan floor and its seams hold water under a static head. It does not test the curb's vertical faces, the top of the curb, or the transitions where the liner turns up.
The most likely defect is fastener penetration through the liner on the inside face or top of the curb. Common failure modes:
- Backer board screwed or nailed through the liner on the curb's interior and top surfaces.
- Liner not carried over the curb with the required overlap and no dam corner / pre-formed corner at the curb-to-jamb intersection.
- Liner cut rather than folded at the inside corner, or the curb top not protected by a sloped mortar bed and membrane.
- No weep path at the liner-to-drain connection, causing water to stand in the bed and migrate outward through the curb by capillary action.
The slow growth and efflorescence indicate ongoing wicking, not a one-time spill. The flood test is simply the wrong test for this failure location.
2. Describe the diagnostic steps to confirm the source before opening the curb.
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- Isolate the receptor. Plug the drain, fill the pan to just below the curb top, and mark the level. Observe for 24 hours - this re-tests the floor only.
- Test the curb separately. With the pan drained, apply water only to the curb top and inside face (a controlled spray or a dammed trough on the curb top) and watch the exterior stain for change over several hours. A response confirms curb transmission.
- Check the showerhead and valve wall. Run the shower normally with the drain open and observe; a leaking valve body or missing wall membrane above the curb can mimic a curb leak.
- Moisture meter the curb from the outside face at several heights to map the wetted zone.
- Remove a small section of tile and backer on the inside face of the curb at the stain elevation to expose the liner and look for fastener heads, cuts, or missing corner pieces.
Document each step with photos and readings before demolition so the cause is established rather than assumed.
3. Recommend the repair and the preventive detail for the rebuild.
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Repair: remove the tile and backer from the curb's inside face and top, expose the liner, and repair or replace the liner section with a properly lapped, solvent-welded or adhesive-bonded patch per the liner manufacturer. Install pre-formed dam corners at both ends where the liner meets the jamb, and carry the liner up and over the curb with the required height above the drain-side floor. Do not fasten through the liner on the inside or top of the curb; fasten only on the outside face of the curb, above the finished floor line, and cover the fasteners with the membrane.
Preventive details for the rebuild:
- Slope the curb top toward the shower (about 1/8–1/4 in.) and set a solid, sloped mortar bed over the liner on the curb top.
- Use a surface-applied waterproofing over the curb and up the jambs, tied into the pan waterproofing, if the system allows.
- Maintain a continuous weep path at the drain: protect weep holes with gravel or a weep-guard, and use a drain that permits drainage from the mortar bed.
- Keep the liner continuous behind the backer board at the jambs; do not pierce it below the curb height.
Then re-flood-test the pan and perform a separate curb water test before reinstalling tile.