Corner Uplift on a Mechanically Attached TPO Roof
A 90,000 ft² TPO roof on a 10-story office building 1,200 ft from the Gulf Coast is mechanically attached over 1/2-in. recovery board and 3-in. polyiso on steel deck. Design wind speed is 120 mph. The installer fastened the field membrane at 12 in. on center along seams and 24 in. on center in the field, using 2-3/8-in. barbed fasteners with 2-3/8-in. round plates. At the perimeter, they used the same 12-in. seam spacing and a standard 24-gauge edge metal with 6-in. fasteners at 12 in. on center. In a 95-mph gust during construction, a 20 ft by 30 ft section at the northwest corner peeled back, exposing the insulation. The building official is asking for a wind-uplift analysis and corrective scope.
1. Identify the wind-uplift deficiencies that most likely caused the northwest corner to peel.
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The attachment scheme is essentially uniform, but wind uplift is not uniform. ASCE 7 and the manufacturer's uplift chart divide the roof into field, edge, and corner zones; corner zones experience the highest suction. Using the same 12-in. seam spacing and 24-in. field spacing at the northwest corner leaves the corner under-designed. The 2-3/8-in. round plates may be acceptable in the field, but the corner usually needs closer fastener spacing, larger plates, or both. The 24-gauge edge metal with 6-in. fasteners at 12 in. on center may also be inadequate for a 120-mph design wind without an engineered edge detail and continuous cleat. Peeling at the corner, rather than random field failure, is consistent with a zone-specific attachment deficit.
2. What corrective scope and verification should the consultant require before the roof is accepted?
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Corrective scope should start with removal of the failed section and inspection of the insulation and deck for damage. The designer should produce a wind-uplift calculation for the actual building geometry and exposures, then enhance the corner and perimeter attachment to match the manufacturer's chart. That may mean closer fastener spacing, larger seam plates, additional rows at the corner, and an engineered edge metal with the specified fasteners and cleats. New membrane should be installed with seams oriented to shed water and with seam plates covered by the lap. Verification should include fastener pull tests, seam probe and test cuts, and a manufacturer's inspection letter. The building official should receive the uplift calculation and attachment schedule, not just a promise to add fasteners.