Concrete roof decks on buildings constructed during the mid-20th century consist primarily of cast-in-place concrete decks without stay-in-place metal forms or precast concrete elements covered with site-placed topping slabs. Both types of construction allow excess concrete moisture to eventually evaporate from the underside of roof decks.
Before the proliferation of modern roof systems, a typical roof system installed onto concrete decks included a continuous layer of bituminous adhesive that bonded the insulation to the deck. This water-resistant adhesive, applied at about 30 pounds per 100 square feet, also performed as an excellent vapor retarder, reducing the moisture emissions from the concrete into the roofing materials. In addition, typical insulation products of the past had moisture storage capacities higher than those of the foam insulations currently used. Moisture contents of water-sensitive roofing materials remained below levels that caused them to degrade. As a result, older roof systems did not succumb to concrete moisture-related deterioration even though vapor retarder performance may not have been part of the design.
Times have changed
Although many modern roof membranes and insulations are moisture-resistant, modern insulation board often is faced with moisture-sensitive paper facers and is frequently adhered to substrates with moisture-sensitive adhesives. The change to moisture-sensitive adhesives is a result of recent volatile organic compound (VOC) regulations that caused manufacturers to convert to either 100 percent solids, low-VOC or water-based formulations.