Low-Slope Roof Assembly Failures – GreenBuildingAdvisor – GWC Mag

It’s not particularly surprising to learn that many designers and builders don’t read Green Building Advisor. But here’s what is surprising: many American designers and architects still don’t know how to design a low-slope roof.  According to a Michigan envelope failure consultant, Matt Dupuis, hundreds of relatively new buildings, most completed within the last 3 to 10 years, have rotting low-slope roofs—roofs that are in such bad shape that they result in litigation over repairs costing hundreds of thousands of dollars per building. The designers of these buildings committed elementary design errors that any GBA reader would have easily avoided.

Dupuis is a licensed professional engineer; he’s also the president of Structural Research Incorporated (SRI), a consulting firm in Waunakee, Wisconsin. In early August 2023, at the Building Science Summer Camp in Westford, Massachusetts, Dupuis gave a fascinating (and chilling) presentation on a growing cluster of failed low-slope roofs.

Install the insulation on the exterior side of the roof sheathing

The traditional way to build a low-slope commercial roof is to install a continuous layer of rigid foam insulation on the exterior side of the roof sheathing. This insulation approach results in a so-called “hot roof”—that is, an assembly designed to keep the roof sheathing warm enough in winter to avoid moisture accumulation or condensation.

Dupuis reports that around 10 years ago, some cold-climate designers who were looking for ways to lower construction costs decided to switch to a “cold roof” approach—a cost-cutting measure that Dupuis blames on the value engineering. While the concept of value engineering—basically, using contractors’ experience to find ways to cut construction costs—makes sense in theory, it only works if designers avoid risky assemblies. In the flat-roof fiascos described by Dupuis, the designers didn’t know much about cold-climate roof assemblies, so the…