Adhered TPO with Cold Water-Based Adhesive
A 45,000 ft² TPO roof over a concrete deck is fully adhered with a water-based bonding adhesive. Work began October 15 in Minneapolis. The adhesive was delivered the previous week and stored overnight in an unheated trailer; at 7:00 a.m. the adhesive temperature was 38°F and ambient was 41°F, rising to 58°F by noon. The crew applied adhesive with a 1/16-in. notched squeegee and waited 10 minutes before rolling the membrane. By November 10, the owner reports ripples and hollow spots across the south half; a 4-ft straightedge shows 1/2-in. ridges. The contractor says the adhesive was still tacky and blames the concrete deck.
1. Diagnose the likely cause of the ripples and hollow spots.
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The adhesive was outside its application range and probably never reached a properly coalesced film. Water-based TPO adhesives generally require substrate and adhesive temperatures above about 40°F and rising, and the adhesive should be conditioned, not stored at 38°F overnight. At 41°F ambient the concrete deck may have been colder, and a 10-minute open time may have been too short for the water to flash off, especially in morning humidity. Rolling the membrane over adhesive that is still milky or tacky can trap water, producing ripples and hollow spots as the film dries or freezes. The 1/16-in. notched squeegee may also be wrong for the adhesive's specified coverage. The contractor's claim that the deck is at fault is possible but not supported without moisture testing.
2. What testing and repair procedure should be followed?
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Before repair, test the concrete deck for moisture vapor emission and test the membrane for adhesion in several locations. Cut test patches to see whether the adhesive has formed a continuous film or is still wet or powdery. Remove all unbonded membrane and scrape or grind the adhesive residue; do not re-adhere over a failed film. Reinstall with adhesive conditioned to the manufacturer's temperature range, using the specified notch and open time, and verify bond with pull tests. If the unbonded area is limited and the membrane is not stretched, localized replacement is acceptable. If ripples are widespread or the membrane has taken a permanent set, replace the affected field. The owner should receive a written repair procedure from the manufacturer.
3. What cold-weather controls should have been in place, and what records matter for the warranty?
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Prevention is a written cold-weather plan: store adhesive in a heated trailer, measure adhesive and substrate temperatures, use a thermometer rather than air temperature, stop work when temperatures fall below the manufacturer's limit, and require a test patch before production. The crew should also record open time and weather. The designer should specify a compatible adhesive for the season or require a solvent-based or low-temperature adhesive if approved. The warranty file should include the test patch results. Without those records, the contractor will have difficulty proving the failure was caused by the deck rather than application.