Ponding Between Drains on a New Low-Slope TPO Roof
A 60,000 ft² TPO roof over a new steel deck was designed with a 1/4-in.-per-foot slope to four roof drains. The roof has two layers of 3-in. polyiso and a 60-mil TPO membrane, mechanically attached. After a 2-in. rain, water stands 1-1/2 in. deep in a 40 ft by 60 ft area between two drains for more than 72 hours. The structural engineer's shop drawings show a 1/2-in. deflection at midspan under dead load; the insulation board has visible gaps and a few crushed edges near the drains. Seams in the ponded area show slight bridging. The owner is concerned about warranty and wants a tapered insulation scheme added on top.
1. What are the likely causes of the ponding, and why is the drain location not the whole explanation?
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Ponding in the middle of a bay, not at the drains, points to structural deflection and insulation fit rather than a single plugged drain. A 1/4-in.-per-foot design slope is the common minimum, but it is a design slope, not a guarantee of as-built slope. The structural engineer's 1/2-in. midspan deflection under dead load can flatten the roof and create a low point between drains. Gaps in the two-layer polyiso and crushed board edges near drains create local depressions and discontinuities that collect water. The 72-hour standing water exceeds NRCA's usual 48-hour ponding criterion. The seams show bridging because the membrane is spanning a depression or because the insulation has settled. The owner's proposed tapered insulation may help, but only after the deflection and drain layout are evaluated.
2. Evaluate the proposed tapered insulation remedy and outline a proper corrective approach.
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Do not approve a tapered insulation scheme as a cosmetic fix. Adding taper and fill insulation adds dead load, which can increase deflection and make the low point deeper. The designer should first obtain a structural review of the as-built framing and the 1/2-in. deflection, then evaluate drain locations and sizes and the as-built slope. The remedy may combine structural reinforcement or supplemental framing, tapered insulation designed to the actual deflected shape, and improved drain sumps. The existing wet or crushed insulation should be removed and replaced, not buried. If the ponding is shallow and drains within 48 hours after correction, repairs to the membrane may be limited; if not, the area should be re-roofed.
3. How does the ponding affect roof system performance and warranty position?
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Standing water increases load on the membrane and seams, accelerates dirt and algae accumulation, and can extract plasticizers or degrade the membrane over time. In freezing climates, freeze-thaw cycles at the seam edge can open laps. The trapped water also raises the risk of insulation moisture gain and reduces thermal performance, which may affect the warranty. The owner should document the ponding with photographs and measurements, notify the manufacturer, and require the designer to confirm that the corrected roof meets the code and the warranty. A waiver or drain maintenance plan is not a substitute for correcting a structural low point; if the deflection is the cause, maintenance cannot fix it.