TPO Retrofit Over a Wet Gravel BUR on a Food-Processing Warehouse

The proposed R-value is short of the requirement. Two inches of polyiso at R-6 per inch is R-12, and 1/2 in. high-density polyiso cover board adds roughly R-2.5, for about R-14.5 total, not R-20. The contractor would need additional insulation, such as another inch of polyiso or a thicker base layer, and should show the calculation by layer. The moisture risk is more serious. A 55% indoor relative humidity in a food plant means the dew point can fall inside the roof assembly in winter. A vapor retarder belongs on the warm side of the insulation, normally at the deck, and must be continuous. Leaving the existing gravel and BUR in place may trap moisture and makes mechanical attachment unreliable; wet insulation near the drains must be removed and the deck examined. The designer should either remove the existing roof to the deck and build a known assembly, or use a verified retrofit assembly with a vapor retarder and a way to handle trapped moisture. Covering wet insulation is not acceptable.

Mechanical attachment must be based on the project's wind-uplift design pressure, not on a generic fastener pattern. The contractor should obtain the membrane manufacturer's current approval for the specific assembly, including the 22-gauge deck, insulation type, cover board, fastener, and stress plate. Fastener pull-out values should be verified by field tests on the actual deck, because 22-gauge steel is thin and pull-out can govern the pattern. The layout must account for perimeter and corner zones, which usually need denser fastening than the field. Fasteners must be long enough to engage the deck properly without bottoming out or stripping, and the cover board must not be so soft that plates pull through. The contractor should also verify that the existing deck is not rusted or deflected and that the gravel is removed where fasteners penetrate. A manufacturer's letter and a marked fastening plan should be submitted before installation, and the inspector should verify fastener density and engagement during the first roof section.

At 30°F to 40°F, 60-mil TPO becomes stiff, difficult to unroll flat, and more prone to wrinkles and seam stress. The manufacturer's adhesive limit of 40°F and rising means bonded seam adhesive should not be used at 30°F, and even at 40°F the crew must follow the manufacturer's temperature and humidity windows. Hot-air welding can often be used in cold weather, but the welder settings must be adjusted for the ambient temperature, with preheat and a test seam made and peel-tested at the start of each shift. Membrane should be stored in a heated trailer or warm area and not folded sharply in the cold. The crew should avoid installing over frost, ice, or wet deck, and should not walk on newly installed membrane before seams are cooled. Schedule welding for the warmest part of the day, use wind breaks, and increase seam inspection. If conditions stay below the manufacturer's limits, the work should be stopped rather than risk seams that fail later.