Historically, moisture control in low-slope roofing has been limited to the selection of a membrane material to prevent moisture from entering the roofing system from the climate side and a simplified assessment to determine whether the roofing system requires a vapor retarder to limit moisture ingress from the building interior. Although most roofing systems fail due to leaks, the existing moisture control strategy does not include any requirements that deal with the removal of moisture once it has entered the roofing system.
Prescriptive requirements for a new moisture control strategy are proposed. The new moisture control strategy not only critically reviews the need for vapor retarders but also addresses the issue of moisture removal from low-slope roofs. Based on a large database of finite difference modeling results, algorithms are presented that allow the roofing practitioner, with the help of a little algebra, to determine if his (her) roofing system design requires a vapor retarder or if the system can be modified to enhance its tolerance for small leaks.
This paper will review existing moisture control guidelines, compare these guidelines with the proposed moisture control strategy, illustrate by example how modeling results were obtained, describe the process employed to develop the algorithms, and demonstrate how these algorithms can be used to design a moisture tolerant low-slope roof.
André O. Desjarlais