Recover Over Wet Modified Bitumen
A 22,000 ft² single-story warehouse has an existing two-ply modified bitumen roof over 1½ in. polyisocyanurate and a steel deck. The membrane is 19 years old, has scattered alligatoring, and holds ponding water ⅜–½ in. deep in two bays after rain. The owner accepts a low bid to install 60-mil mechanically attached TPO directly over the existing membrane, with no tear-off and no recovery board. Six weeks after completion, the crew reports two 8-ft blisters along a seam and a 14-in. seam peel near a curb. An infrared moisture survey shows wet areas under the new membrane at four locations, all corresponding to old ponding zones. Core cuts reveal damp insulation and a strong asphalt odor. The building is in a mixed climate with summer highs near 95°F and winter lows near 10°F. The TPO manufacturer's warranty requires a moisture survey and limits the assembly to one recover over a sound substrate.
1. Was the recover assembly as bid acceptable under typical code and manufacturer requirements? Explain the specific failures.
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No. The bid skipped the two conditions that matter most on a recover:
- Substrate soundness and moisture. Reroofing provisions such as IBC 1511 allow a recover only when the existing roof is sound and suitable. Infrared and core cuts show wet insulation at four locations. Wet insulation cannot be left in place; it must be removed and replaced whether or not the membrane is covered.
- Manufacturer conditions. The warranty expressly requires a moisture survey and one recover over a sound substrate. Neither condition was met. Coverage may be voidable even if the TPO itself was installed correctly.
- Drainage. Ponding ⅜–½ in. deep after rain indicates the roof does not drain properly. Positive slope and a 48-hour ponding limit are typical code expectations; covering ponding areas traps water and adds load.
- Compatibility. TPO over an asphalt membrane generally requires a separation layer or recovery board to prevent asphalt oils from migrating into the TPO and to protect the new membrane from irregular substrate. The bid omitted both.
The blisters and seam peel are symptoms of trapped moisture and restraint, not random workmanship defects.
2. Explain the mechanism for the blisters and seam peel and outline a verification and testing program.
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Mechanism. Moisture trapped between the old membrane and the new TPO is heated by solar gain. Water vapor pressure rises under the TPO, which is vapor-retarding. The pressure concentrates at seams, laps, and fastener plates, lifting the membrane into blisters. The seam peel near the curb is a stress concentration: the seam is at an edge where movement and ponding pressure are highest, and the weld may also have been contaminated or under-welded.
Verification program:
- Perform a grid moisture survey using infrared plus capacitance or nuclear meters; mark boundaries of wet areas.
- Take core cuts at each suspect zone and at a control location. Measure insulation moisture and photograph the deck.
- Probe all seams within 10 ft of blisters and the peeled seam using a rounded probe; any probe opening is a failed seam.
- Cut a 12-in. square around one blister to inspect the back of the TPO and the old membrane for moisture droplets and asphalt bleed.
- Record ambient temperature during the survey; infrared readings taken under direct sun can mislead.
The output should be a scaled moisture map, not a single reading, because repair quantities depend on the wet-area boundaries.
3. Recommend a remediation plan and the sequencing.
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Sequence the work to avoid re-covering wet material:
- Stop and notify. Notify the manufacturer and owner in writing; preserve the moisture survey and core photos.
- Remove affected TPO and wet insulation. Cut back to sound, dry substrate with a minimum 2-ft margin beyond the moisture map. Inspect the steel deck for corrosion; replace deteriorated deck.
- Correct drainage. Install tapered insulation or crickets to the drains so no ponding remains. Verify drain strainers and scuppers.
- Rebuild the assembly. New insulation, a recovery board or slip sheet compatible with the existing membrane, then new TPO. Mechanically attach per the manufacturer's pull-out values for the deck.
- Seams and details. Weld seams with calibrated equipment, probe all seams, and reinforce curb and penetration details.
- Verify. Water-test repaired areas, repeat infrared after a sunny day, and document seam probe results. Do not accept the area until the moisture map is clear.
A spot repair of only the blisters is not adequate because the wet zones extend beyond the visible symptoms.