Overflow Scupper Sealed During Parapet Repair
A 60 m × 40 m retail store has a mechanically attached TPO roof with a slope of 1/4 in/ft to four primary roof drains and four overflow scuppers. During a 100 mm rain, water ponded 75 mm deep at the northeast corner. A primary drain strainer was clogged with debris, and the overflow scupper at that corner had been sealed with membrane flashing during a recent parapet repair. The penthouse door has a 50 mm curb, and the bottom of the door is 25 mm above the membrane. The property manager asks you to “raise all overflow scuppers 50 mm and add a new drain” so the tenant can reopen. The store opens in 48 hours.
1. What failed, and what immediate actions are appropriate before the store reopens?
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The primary drain was blocked, and the independent secondary drainage path was defeated when the overflow scupper was sealed. The water rose until it reached the penthouse door area. This is a life-safety and property-damage risk, not just a ponding nuisance.
Immediate actions:
- Clear the clogged primary drain strainer and remove debris from the roof.
- Open the sealed overflow scupper immediately; do not wait for a permanent repair. If the scupper cannot be opened safely, provide temporary secondary drainage such as a controlled pump or temporary scupper, and monitor the roof.
- Inspect the roof structure and deck for deflection or distress from the 75 mm ponding, and keep people out of the area if the structure is suspect.
- Verify that the door threshold and curb are not being overtopped; place temporary water barriers if needed.
- Document the water level, clog, sealed scupper, and any interior damage with photographs.
Do not simply add a new drain at the same low point without re-establishing the overflow. The overflow must remain independent of the primary drain.
2. Evaluate the request to raise all overflow scuppers 50 mm. What is the correct relationship between the primary drain, overflow, and door threshold?
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Raising the overflows 50 mm is likely wrong. The overflow must be set above the primary drain so it does not operate during normal rain, but low enough that water cannot rise to the door threshold, curb, or any vulnerable penetration before secondary drainage begins. Here the door bottom is only 25 mm above the membrane, so a 50 mm raise could put the overflow above the door and allow water to enter the penthouse before the overflow functions.
Correct relationship:
- Primary drain: lowest point, with strainer, taking normal flow.
- Overflow: higher than the primary drain but below the lowest vulnerable opening, including the door threshold and any curb.
- Door threshold/curb: must have enough height above the finished roof and overflow level to prevent water entry; verify against the applicable building code and manufacturer details.
The actual fix is to re-establish the overflow at the correct elevation, not to raise it. If the door cannot be protected by elevation, a new curb or a relocated door may be required. Adding a new primary drain does not substitute for a functioning independent overflow.
3. Outline the permanent correction and the verification you would require before sign-off.
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Permanent correction:
- Remove the membrane flashing that sealed the scupper and inspect the opening and wall for water damage.
- Install a proper scupper sleeve with a clamping ring, flange, and screen, flashed into the TPO per the membrane manufacturer's detail.
- Re-establish the overflow at the correct elevation above the primary drain and below the door threshold; provide a visible discharge to a safe location.
- Clear and repair the primary drain; replace the strainer with a dome strainer sized for the roof area.
- Consider adding a secondary overflow drain at the low corner if the existing scupper cannot be corrected, and verify the roof slope and ponding.
- Correct the door threshold condition by raising the curb, adding a step, or relocating the door as required.
Verification:
- Flood-test the primary drain and overflow separately to confirm flow and visible discharge.
- Confirm the overflow operates before water reaches the door threshold.
- Inspect membrane terminations and sealant details; confirm no sealant is used as the primary waterproofing.
- Photograph the final elevations and obtain acceptance from the owner and, where required, a roofing consultant or engineer.