Cold-Morning TPO Seam Failures at a Distribution Center
A 180,000 ft² distribution center in Columbus, Ohio, received a 60-mil TPO mechanically attached roof in late October. The contractor started welding at 7:00 a.m. when ambient temperature was 38°F and the membrane surface measured 34°F; by afternoon the surface reached 60°F. The crew used a hot-air welder set at 1,000°F with a travel speed of 12 ft/min and a silicone roller. During a walkthrough two days later, the owner's rep found open laps along the west elevation and several blisters near the seam edge. A rounded-tip probe can be inserted 3/8 in. at the open laps, but seams in the center of the field pass a probe. The membrane is dry; no rain occurred during installation. The contractor wants to re-weld the failed areas and move on. The roof is under a manufacturer's 20-year NDL warranty.
1. What conditions and installation variables most likely produced the open laps at the west elevation while center-field seams passed? Explain your reasoning.
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The pattern - failed laps at the west elevation but passing seams in the center - points to a combination of low membrane temperature and heat input, not a single bad welder. At 7:00 a.m. the membrane surface was 34°F, only 4°F below ambient, so the surface may have been at or near the dew point and carrying condensation; hot-air welding cannot tolerate moisture at the lap interface. The west edge likely stayed cooler and windier longer, and the crew may have started there first. A 1,000°F set point and 12 ft/min travel may be adequate on warm membrane but marginal on cold membrane, especially if roller pressure was light and the seam was left open too long before welding. Blisters near the seam edge suggest moisture or trapped air. Center-field seams welded later, when the surface warmed, passed the probe because they received more effective heat. The contractor should not assume the probe alone proves weld quality; probing checks only the exposed edge.
2. What should the contractor do before re-welding, and how should the repair be validated?
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Before any re-welding, stop production and establish a test weld on the same membrane and substrate. Record nozzle temperature, air volume, travel speed, roller pressure, and membrane surface temperature; verify the welder is calibrated and that the membrane is dry. Cut out the failed lap rather than cap-welding over an open seam, because an open edge can hide contamination and a cold weld. Re-weld a full-width lap using the manufacturer's cold-weather parameters, typically slower travel and adequate preheat, and use a silicone roller immediately. Probe all seams and take test cuts at the intervals required by the manufacturer or specifier. Repair test cuts with approved patches. Document ambient and surface conditions, welder settings, and test results for the warranty file.
3. What should the consultant recommend regarding the 20-year NDL warranty and the contractor's proposed quick fix?
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Notify the manufacturer before repairs and follow its written repair and inspection requirements; an NDL warranty often requires approved contractors, materials, and inspection records. The consultant should require a seam survey, test cuts, and a repair plan that addresses both the failed west elevation and the seams that passed only a probe. The manufacturer may accept localized repairs if test cuts show proper weld width and peel strength; if not, it may require removal of the affected membrane. The contractor should also explain why cold-weather controls were not followed. Re-welding without test cuts would leave the owner with an undocumented repair and a weak warranty position.