Design and Construction Considerations
A vapor retarder limits the passage of water vapor and might be required in order to separate the roofing assembly from substrates with excessive residual moisture or other sources of moisture that might enter the roofing assembly. Similarly, an air barrier might also be required in order to limit the infiltration of moisture-laden air into the roofing assembly. The roofing industry (including roofing system and roofing adhesive manufacturers, roofing designers, and roofing contractors) often does not provide sufficient project-specific direction regarding the requirement for a vapor retarder and/or an air barrier. Roofing assembly and roofing adhesive manufacturers’ literature generally requires installation over a “dry" substrate, but manufacturers do not provide specific or practical criteria for evaluating concrete roof deck moisture content. Without defined and measureable criteria, the requirement for a “dry" substrate provides little guidance to control the risk of roofing assembly damage from residual moisture contained in the concrete roof deck. Common methods of concrete roof deck moisture testing, such as the “rubber mat test" (ASTM D4263 ), can falsely indicate a “dry" concrete substrate under some ambient test conditions and are unreliable as the sole evaluation of concrete moisture.
The issue of moisture in concrete roof decks is similar to moisture issues that must be considered when installing floor coverings on concrete substrates.
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The flooring industry continues to refine maximum moisture threshold criteria and test protocols in order to help prevent catastrophic flooring failures resulting from moisture in concrete substrates that leads to adhesive failure. The roofing and roofing adhesive industries provide less definitive moisture criteria and test protocols for concrete roof decks. We suspect that this is driven, in part, by the impact of weather conditions, variable concrete mixes, and scheduling pressure for the contractor to quickly provide a weather-tight facility, limiting the amount of time that a roof substrate can be left exposed, and affecting the practicality of implementing more sophisticated and reliable test protocols. Flooring industry substrate moisture guidelines and standards for installation might provide guidance for the development of similar roofing guidelines and standards, but future roofing substrate moisture testing protocols must consider different field conditions and installation constraints in order for the protocols to be useful and practical. In our practice, we have attempted to correlate more sophisticated and reliable concrete substrate moisture testing with less invasive moisture testing techniques during roofing installation, but data collection is limited, because our field work is also constrained by the need to install or re-install roofing assemblies in order to maintain a weather-tight facility. We have found that we can relate relative humidity measurements on specific decks to surface moisture meters to establish a quick field test. However, even when re-roofing a concrete deck after a few years of service, a vapor retarder is often still required.
The roofing and roofing adhesive industries must develop guidelines or standards that provide more reliable and practical roof substrate moisture testing protocols to help roofing designers, roofing contractors, and building owners reduce the risk of installing roofing assemblies on wet substrates that lead to adhesive bond ineffectiveness and roofing assembly failure. If an acceptable substrate moisture content cannot be achieved, then the protocol should recommend the installation of a vapor retarder (and possibly an air barrier) directly above the roof substrate.