b'floor coverings. The research has shown normal-weightNRCA maintains its longstanding position that a structural concrete floor slabs may reach acceptable lev-els in about 90 days and lightweight structural concreteroofing contractor should not decide when a newly floor slabs may take about six months to reach the 75% relative humidity level. This research was conducted in-doors, with test specimens not subjected to rewetting.placed normal-weight or lightweight structural concrete The longer drying time for lightweight structural con-crete floor slabs is an indication of the greater amount of water contained in lightweight structural concretesubstrate is ready to be covered with a new roof system.design mixes.Unlike the flooring industrys 75% relative humid-ity threshold, the roofing industry has no established guidelineforconcretesdrynessbeforeroofsystem application. However, because the flooring industrys thresholdconsidersmoisture-sensitivematerialsand blistering potentialboth of which also are consid-erations for roof systems installed over concrete roof decksthe flooring industrys guideline can serve as a practical basis.Also, a membrane roof system on a concrete roof deck, such as a vinyl floor covering, for example, func-tions as a vapor retarder on the top surface of a concrete slab preventing upward moisture release.That said, concrete roof decks and concrete floor slabs differ in two fundamental aspects. First, concrete floor slabs typically are protected from weather during curing and drying. Concrete roof decks, on the other hand, are exposed to weather and rewetting by precipi-tation. Rewetting significantly can lengthen the drying time of concrete roof decks.Second, the moisture vapor transport characteristics are different between concrete floor slabs and concrete roof decks. With floor slabs, there usually is little to no difference in the ambient conditions between a con-crete slabs bottom and top sides, which means there usually is little to no temperature-derived vapor pres-sure differential across the slab that would move any moisture vapor from within the slab.Conversely, with concrete roof decks, the bottom side of a roof deck typically is exposed to conditioned space while the top side experiences changing tempera-tures, humidity and wind, all of which can affect mois-ture vapor transport. Temperature differentials across the cross-section of a roof assembly can result in net va-por pressure drive in the direction from warm to cold.Therefore, during periods of cool or cold outside temperatures, residual moisture vapor from the con-cretes free water can be forced outward into the roof system. Excessive moisture accumulation in a roof sys-tem can result in damage to moisture-sensitive roof sys-tem components and affect roof system performance.During periods of hot outside temperatures, the con-cretes residual moisture vapor can be forced downward into a buildings occupied space provided a concreteViana Roong & Sheetmetal Ltd. is a progressive roong, roof deck was not poured on a metal form deck. Withwaterproong and restoration company who has successfully concrete roof decks poured in metal form decks thatcompleted thousands ofprojects over the past37 years. remain in-place, the metal form deck can act as a vapor retarderrestrictinginwardmoisturevaportransport.Our services include all types of roong, sheetmetal and siding,When conditions result in an inward direction of vaporcaulking work, waterproong and more.drive, a buildings HVAC system typically helps man-age the driven moisture. Mechanical cooling processes, such as air conditioning, can remove large amounts of moisture from air.Why Now?A question frequently raised when discussing mois-ture migration problems associated with concrete roof decks is why is this an issue now when the industry has successfully installed roof systems over structural concrete roof decks for more than 100 years? There are74 Advance Road, Toronto|416.763.2664|vianaroong.comseveral answers.ORN THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ONTARIO ROOFING NEWSISSUE 1 202213'