Trust, But Verify

Waterproofing looks good? Great. But the only way to know it works is to test it—before tile goes on.

Once tile is installed, you can’t fix waterproofing without tearing everything out.

When Flood Testing Is Required

TCNA Requirements

Mortar bed shower pans:
Flood testing is required (TCNA Handbook method B421)

Bonded waterproofing membranes over solid substrate:
Flood testing is recommended but not always required

Local Code Variations

Some New Jersey municipalities require flood testing for all shower installations. Always verify with your local building inspector.

How to Flood Test (TCNA Method B421)

Materials Needed

  • Drain plug or test balloon
  • Hose or buckets
  • Marker or tape
  • Camera (for documentation)
  • 24-48 hours of patience

Step-by-Step

  1. Plug the drain
    Use a test balloon or watertight plug

  2. Fill the shower pan
    Fill to the height of the curb or drain threshold (typically 2-4 inches)

  3. Mark the water level
    Use tape or marker on the shower wall

  4. Photograph everything
    • Water level mark
    • Overall pan
    • Drain area
    • Curb/threshold

  5. Wait 24-48 hours
    Do not disturb. No one uses the bathroom during this time.

  6. Check water level
    • No drop = PASS ✅
    • Any drop = FAIL ❌
  7. Document results
    Photograph final water level

Acceptable Results

PASS: Water level remains constant (within 1/8”)

Minor evaporation is normal in dry climates, but level should not drop significantly.

FAIL: Water level drops more than 1/8”

This indicates a leak. Do not proceed with tile installation.

What to Do If It Fails

  1. Drain the pan completely
  2. Allow to dry (24-48 hours)
  3. Locate the leak:
    • Inspect drain connection
    • Check corners and seams
    • Look for punctures or gaps in membrane
  4. Repair per manufacturer instructions
  5. Re-test

Do not skip re-testing. A “patch job” without verification is guesswork.

Bonded Membrane Systems (Kerdi, RedGard, etc.)

Flood testing bonded membranes over solid substrate (cement board, foam board) is less common because:

  • Membrane is bonded (no water path between layers)
  • Drain integration is mechanical (clamping ring)
  • System is designed to be waterproof when installed correctly

However: Testing is still recommended, especially:

  • First time using a system
  • Complex drain/niche details
  • Homeowner requests verification
  • Local code requires it

Visual Inspection (If Not Flood Testing)

At minimum, visually inspect before tiling:

✅ All corners reinforced
✅ Drain properly integrated
✅ No gaps or voids in membrane
✅ Seams overlapped per manufacturer specs
✅ Penetrations sealed
✅ Curb fully wrapped

Photograph everything. This is your insurance policy.

Documentation

What to Record

  • Date of test
  • Products used (membrane, drain, etc.)
  • Initial water level (photo + measurement)
  • Final water level (photo + measurement)
  • Duration of test (24 or 48 hours)
  • Pass/fail result

Why This Matters

  • Warranty claims: Manufacturers may require proof of testing
  • Insurance claims: Documentation of proper installation
  • Resale value: Shows quality workmanship
  • Peace of mind: You know it works

Common Mistakes

❌ Testing too short (< 24 hours doesn’t prove anything)
❌ Not marking water level (can’t tell if it dropped)
❌ Proceeding after failed test (hoping it “seals itself”)
❌ No documentation (no proof test was performed)
❌ Testing after tile installation (too late)

Final Verification Checklist

Before tiling begins:

✅ Flood test performed (or visual inspection documented)
✅ Test results photographed
✅ All corners, seams, drains verified
✅ No gaps or voids in waterproofing
✅ Slope verified (1/4” per foot minimum)
✅ Substrate flatness checked

If all boxes are checked → proceed with confidence.