What You Can’t See Can Hurt You: Moisture in Concrete Roof Decks Can Result in Premature Roof System Failure
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.