TPO Recover Over Wet BUR
A 38,000 sf warehouse has a 20-year-old gravel-surfaced built-up roof over 2 in. fiberboard insulation and a steel deck. The roof is nearly flat, with four 4 in. interior drains in the middle of each bay and no secondary drainage. After heavy rain, ponding persists 48 hours in two bays; interior columns show rust at deck flutes; tenants report drips at a skylight curb. The owner wants a TPO recover to save money. Drawings show 1/8 in./ft slope to drains. The contractor proposes removing gravel, installing 1/2 in. cover board and 60-mil mechanically attached TPO, reusing existing drains.
1. Should the contractor proceed with a TPO recover as proposed? What investigation is required first?
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- A recover over a 20-year-old gravel BUR is risky, and the ponding plus rust at the deck suggest wet insulation or deck distress. A recover would trap moisture and hide corrosion.
- Require core cuts and moisture surveys. If insulation is wet, remove it in the affected areas; a recover is not appropriate over wet substrate.
- Inspect the steel deck for section loss at the rusted flutes and verify structural capacity. Repair or replace deteriorated deck before any new membrane.
- The skylight leak must be traced; a new membrane will not fix a failed curb or flashing detail.
2. How should drainage be redesigned, and can the existing drains be reused?
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- Four drains for 38,000 sf means about 9,500 sf per drain. The 48-hour ponding indicates inadequate slope, deflection, or drain capacity, not just a slow strainer.
- Verify structural deflection and drain blockage. Provide positive slope, typically with tapered insulation and crickets, and add secondary drainage such as overflow scuppers or a second drain set at a higher elevation.
- Existing 4 in. drains may be reusable only if they can be retrofitted with TPO clamping rings, strainers, and proper seals. Do not simply lay membrane over the old drain.
- Size drainage using the local rainfall intensity and code requirements; do not rely on the old drain count as proof of adequacy.
3. What attachment and assembly issues must be resolved for the mechanically attached TPO?
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- Mechanically attached TPO over steel deck requires verified fastener pullout, deck gauge, and fastener spacing for the wind uplift pressures. The cover board must be attached with a compatible schedule.
- The existing fiberboard insulation is not a suitable substrate if wet or deteriorated. Replace it or specify new insulation and cover board.
- Confirm membrane thickness, seam method, and manufacturer approvals for the assembly. Use hot-air welding and quality control on seams.
- Coordinate penetration flashings, drain retrofits, and edge metal with the new assembly; do not reuse details that were designed for gravel-surfaced BUR.