Tip of the iceberg

slaton patterson FAILURES
Deborah Slaton, David S. Patterson, AIA, and Kenneth M. Itle
Evidence of water leakage was reported in a recently constructed three-story building located in a moderate climate, including staining observed at a few locations on interior finishes. The exterior walls of the building were wood-framed construction with 13-mm (½-in.) oriented strand board (OSB) sheathing and building wrap as the exterior air- and water-resistive barrier (WRB). The interior finish consisted of 16-mm (5/8-in.) gypsum wallboard (GWB). Closed-cell sprayed polyurethane foam (SPF) insulation was installed between wood framing members. The exterior façade was clad with stucco in some areas, and with stone veneer 
in others.

Inspection openings at selected locations revealed widespread water infiltration much more severe than initially thought. Leakage was found to be occurring around the perimeter of the roof, through flashing deficiencies and problematic details at roof gables, parapets, and the interface between the roof and hanging gutters. Conditions contributing to water leakage included lack of proper kick-out flashing and associated voids in the gable and parapet wall flashings, as well as voids, gaps, and penetrations in the flashing and at the interface between the roof system, gutter, and stucco wall cladding.

In some areas of the project discussed, the wood decay had advanced to the point that the oriented strand board (OSB) has completely disintegrated, exposing the building wrap.
Photo courtesy WJE

This leakage at the roof level had severe consequences for the wall construction below. Examination of selected exterior wall locations revealed elevated moisture levels and deterioration of the OSB sheathing, wood framing, and/or gypsum board—especially at the top floor of the building, but also at lower levels where water had migrated downward within the exterior wall assembly. Where water had penetrated behind the exterior cladding and wet the moisture-sensitive OSB sheathing, wood framing, and GWB interior finish. These materials were stained and in varying degrees of deterioration; in some cases, the damage within the wall was severe, but concealed. Additionally, biological growth was present in the exterior wall assembly, supported by organic materials including the OSB, wood framing, and paper facing on the GWB. The fact that leakage was relatively severe and widespread was not immediately obvious because of the use of SPF. In this installation, the insulation deflected much of the bulk water into the OSB sheathing layer, somewhat protecting the interior GWB from water and resulting in only limited visible water staining.

With continued water infiltration, the OSB sheathing and wood framing would be expected to lose structural integrity over time. The wood framing is the primary loadbearing structure of the building, and the OSB sheathing provides structural bracing for the framing members and serves as the substrate for the support and the securement of the stucco cladding. Therefore, evidence of water leakage in the form of surface staining at a few areas of gypsum board was a small indication of more severe deterioration and a portent of potentially significant future structural problems.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of The Construction Specifier or CSI.      

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com.

David S. Patterson, AIA, is an architect and senior principal with the Princeton, New Jersey, office of WJE, specializing in investigation and repair of the building envelope. He can be reached at dpatterson@wje.com.

Kenneth M. Itle, AIA, is an architect and associate principal with the Northbrook office of WJE, specializing in historic preservation. He can be reached at kitle@wje.com.

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