The Rain Must Drain: An Engineering Perspective on a Common Pitfall of Overlay Reroofing

Rehabilitation of commercial roofs is often accomplished by overlaying existing roof coverings with new single-ply membranes. In preparation for this type of recovering, evaluation of the subject building’s existing structural system, and analysis of the new roof system’s drainage characteristics by properly licensed design professionals, are usually necessary. Often, such evaluation and analysis are also required by municipal authorities having jurisdiction over construction activities to ensure the adequacy of the new systems, their compatibility with the existing structure, and their conformance to applicable building codes. In recent years, and in separate occurrences, operators of a galleria in Oklahoma and a joint retail and service facility in North Texas retained contractors to overlay the existing roofs at the respective buildings with single-ply roof membrane systems. In the months following each reroofing, a portion of each building’s roof collapsed during rainfall events. The objective of this paper is to explain the authors’ investigation of the causes of these two collapses, with the ultimate goal of providing a memorable reminder to those involved in future recovering projects. The entire investigative process is discussed, from initial site evaluation to determination of causation, including structural analysis and the associated calculation of estimated rainwater loading through iterative flow modeling. The two investigations revealed remarkably similar failures, each characterized by accumulated rainwater on the roofs that overloaded the structural framing; both accumulations having occurred due to inadequacy of the roof drainage systems.