Low-Slope Re-Cover With Ponding and Hidden Moisture
A 3,200-square-foot low-slope roof on an occupied 1986 office building has a ballasted EPDM membrane over 1.5-inch gypsum cover board, fiberglass insulation, and a 22-gauge metal deck. After heavy rain, water stands about 1.5 inches at the northeast drain and extends 20 feet along the parapet; the membrane is wrinkled, and two seams near the drain are open. The owner wants to save money by installing 0.060-inch mechanically attached TPO directly over the existing roof, adding R-25 insulation, and leaving the existing drains. The building is three miles from the coast and has no record drawings. The owner needs the building watertight before tenant move-in in two weeks, and a moisture survey is not in the budget.
1. What investigations should the contractor require before recommending a recover over the existing assembly, and how should the results change the recommendation?
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A recover is a judgment about the existing assembly, not just a new membrane. The contractor should require at minimum: a visual and tactile survey of the membrane, seams, flashings, drains, and parapet; core cuts at representative locations and at the ponding area; a moisture scan such as infrared or nuclear if core cuts suggest widespread wetness; and an evaluation of the metal deck and insulation attachment. The absence of record drawings raises the risk that the deck gauge, insulation type, or fastening pattern is not what it appears to be.
If the insulation is wet over more than a limited area, or if the deck is corroded or deflection is noticeable, a recover is not appropriate. A common decision rule is that if roughly 25% or more of the insulation is wet, remove the roof to the deck and replace the wet materials; verify that threshold with the membrane manufacturer and a designer. If the assembly is dry and the deck can carry the added weight and uplift loads, a recover may be viable. The contractor should put the investigation scope and the recover limitations in writing so the owner understands that hidden moisture can change the price and schedule.
2. How should the contractor address the ponding and drain details if the owner insists on covering the existing roof?
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Ponding of 1.5 inches that remains long after rain is a slope and drainage defect, not a membrane defect. Covering it with new TPO will not correct it and may add load and hide deterioration. The contractor should first verify the drain is open and the scupper is not blocked; then survey elevations to find the low spots. The repair should create positive slope to the drains, typically with tapered insulation or a sump, and should not insulate over the drain clamping ring. Add a secondary drain or scupper if the roof area and code require it, and use crickets or saddles along the parapet where water stands.
The drain detail should include a proper TPO drain insert or clamping ring compatible with the existing drain body, a sealant bead, and a reinforced membrane flange. The contractor should not simply run membrane over the existing drain opening. If the deck cannot be tapered economically, the contractor should recommend a structural or drainage retrofit rather than a cosmetic overlay. Document the ponding before and after and explain that a 15-year warranty may exclude ponding-related damage if the slope is not corrected.
3. What should the contractor consider for wind uplift, attachment, and energy code when specifying mechanically attached TPO in this location?
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The contractor should not select fastener spacing from habit. Mechanically attached TPO in a coastal North Carolina location must be designed for the applicable wind speed, exposure, building height, and roof zone using ASCE 7 or the code-prescribed method, then installed per the membrane manufacturer's approved detail. That means verifying deck pull-out capacity, using the correct fastener and stress plate, and increasing density at corners, edges, and perimeter zones. A ballasted assembly should not be substituted in a high-wind area.
The contractor should also verify the current energy code requirement rather than assume R-25 is sufficient; commercial roof insulation requirements may be higher and may require continuous insulation or a specific thermal barrier. If the existing assembly is a recover, the new insulation may need a cover board or slip sheet per manufacturer requirements. Finally, confirm that the TPO and attachment method have the required code approvals and that the warranty covers the actual assembly, including the recover and the fastener pattern.