Every roof system needs a flashing system that works. Poor flashings have allowed water to get in and under the best of roof systems. Sometimes, water entry is attributable to an improper design detail; other times, it may be a problematic application. Often, flashing material fails because of the demands of direct weathering, differential movement, inadequate attachment, cumulative movement or other stresses. Spray polyurethane foam (SPF) roof systems are unique because of the direct use of SPF material as the flashing system.
For example, a roof mechanic applying SPF material literally applies SPF up a wall, curb or projection to a prepared elevation and stops. Flashing sheets and fasteners are not required. The use of metal as a counterflashing over SPF material and coating also may be used. Currently, The NRCA Roofing and Waterproofing Manual, Fifth Edition's construction details call for SPF roof systems to use metal counterflashings. This article addresses what is happening in the field with SPF flashing details that do not incorporate metal counterflashings.
Background
The use of SPF on roof systems began in the 1960s. But getting SPF recognized by the industry as a viable roofing material took some time. As a result, a number of independent studies of SPF roof systems had been conducted during the 1980s and early 1990s; papers about the studies can be found in the proceedings of the second and third International Symposiums on Roofing Technology (log on to shop.nrca.net to purchase the proceedings). And the Spray Polyurethane Foam Division, now known as the Spray Polyurethane Foam Alliance (SPFA), developed a contractor accreditation program that continues to publish technical bulletins about the design, use and installation of SPF roof systems.
SPF contractors began to formalize installation practices for review and possible adaptation by NRCA during the mid-1990s. Along with SPF installation practices, a host of other technical issues had to be considered because different job-site conditions could be addressed with spray foam technology that were not necessarily standard practice with other roof systems, such as built-up, single-ply, metal, tile or shingles. One primary issue was how SPF roof systems incorporate flashing details, specifically whether counterflashings were needed. In 1996, NRCA published The NRCA Sprayed Polyurethane Foam-Based Roofing Manual, its first document addressing SPF roof systems.
As discussions ensued about SPF, the SPF segment of the roofing industry felt a need to examine the basic physical characteristics of SPF roof systems in service. It sought to discover the following: How does spray foam age, and does it retain its compressive strength or suffer a loss of mass and density?
These questions led to an independent study funded by the National Roofing Foundation (NRF), a separate but related educational organization of NRCA. The study included 140 roof systems with an average age of 11.3 years; the oldest SPF roof system studied was 27 years.
I issued a report in 1998 that established the fact that SPF roof systems of varying ages all produced similar results for their retained physical properties of compressive strength and foam density. Various coating systems also were examined for mil thickness of base and top coat. In addition, moisture contents were determined from core samples; the average moisture contents ranged from 1.05 percent by weight to 6.4 percent depending on coating type.
The study did not specifically address SPF flashings nor their abilities to adhere to a variety of building materials, such as masonry, metal, wood, tile or other building components.
Current study
Flashing details often are a slighted aspect in roof system design. To truly understand each flashing detail takes time. If a flashing detail is not well-thought-out, a roofing contractor still has to provide something to complete the roof system. Many roofing contractors solve complete flashing details by installing a variety of materials, including metal, along with the bituminous or single-ply roof system they are installing.
In 2000, NRF began planning another field study of SPF as a flashing system. The objective was to study existing SPF flashing details and determine whether metal counterflashings are needed to ensure protection of SPF roof systems and coating systems. SPFA's technical details do not call for metal counterflashings; NRCA's roofing and waterproofing manual does.
The field study began in late 2001 and finished in September 2003. The study included 188 SPF roof systems of various sizes and shapes located in different climatic areas of the United States. The roof systems ranged from new to 31 years of age. About 21 percent of the SPF roof systems inspected had been recoated at the time of inspection; the average service time before recoating was 15 years.
The figure details the survey information for the 1998 and 2003 studies, including roof system age and recoating information.
SPF has the unique ability to solidly adhere to most, if not all, exterior building materials. A coating typically is applied to finish the detail; the coating may go further up the projection by 4 inches (102 mm) or more where possible. The photos show a series of SPF flashing details found in the field. Basically, the industry has learned SPF—when correctly applied to prepared surfaces and then coated—does not require metal counterflashings. SPF flashings were found to work well in the various regional climates of the United States. This includes roof-to-wall conditions, roof-to-roof transitions, penetrations and equipment supports.
Study summary
The NRF studies have shown SPF-based roof systems can provide long service lives, but maintenance is important—as it is with all roof systems. With correct procedures, SPF roof systems can be sustainable. The use of SPF as a flashing system was seen to easily accommodate many termination details found on roofs.
Specifically, it was noted SPF could adhere to the following materials:
There is a need to properly prepare, clean and prime all materials to which SPF is to be adhered. The use of SPF and appropriate coatings were seen to work quite well as a singular flashing system. The use of metal counterflashing was not seen to be required as part of an SPF roof system.
René M. Dupuis is president of Structural Research Inc., Middleton, Wis.
Photo 1: Standard metal counterflashing
Photos courtesy of Structural Research Inc., Middleton, Wis.
Photo 2: Base flashing for a nonwall supported deck
Photo 3: Roof-to-short-wall transition
Photo 4: SPF roof-to-metal-roof transition
Photo 5: Interior gutter system
Photo 6: SPF roof-and-wall transition
Photo 7: SPF wall flashing
Photo 8: SPF low-wall flashing application
Photo 9: Equipment support curbs
Photo 10: Equipment support curbs
Photo 11: Equipment support columns
Photo 12: SPF flashing
Photo 13: SPF flashing
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