be, but are not always, concurrent, and should be verified separately. Achieving continuity of the drainage plane and air barrier across other areas of the building enclosure can be challenging. For parapet construction, detailing should support continuity of sheet or applied waterproofing from the edge of the roofing system up and over the parapet to overlap the wall moisture barrier. Again – items like parapet caps are environmental protection and are not intended as the primary moisture
An important test of the successful detailing of the “drainage plane” is the ability to trace its “line” throughout all the details of the building enclosure assemblies—without lifting the pencil. barrier. Continuity becomes more challenging when there are overhangs or special conditions such as concealed gutters. Here, strategies for water control are easily achieved, but the continuity of the air barriers can be difficult to integrate into or across other building systems.
Design Factors for Detailing
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As a detailer contemplates the desired aesthetic relationships between the various building elements, it is easy to assume that these relationships are “static and stable.” Depending on the location and exposure within the exterior enclosure, transitions between moisture and air barrier systems can be subject to dynamic movement (wind and live load), long-term building movement (structural creep and building settlement) and, though these are smaller in magnitude, to thermal expansion and contraction. Industry design guidelines used by structural engineers allow for minimum structural stiffness of the building structure. Allowable deflections can be as much as a half inch to one inch on long spans. Similarly, wind load on a building can create measurable joint movement in vertical joints between structure and the exterior wall construction. Failure of the architect and engineer to coordinate on the allowable design deflections in building detailing may result in joint failure, compromising moisture and air infiltration protection. A separate class of
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