| SPF roofing: Overcoming aesthetic barriers | |||||
Research helps clarify common misconceptions about SPF systems by Thomas L. Smith, AIA, RRC and Rene M, Dupuis, Ph.D., PE During the late 1960s, sprayed polyurethane foam (SPF) roof systems were introduced to the industry At that time, a number of SPF jobs experienced serious problems (e.g., extensive blistering and problems with bonding) because the required physical properties of the foam (e.g., density) and coatings were not established, application equipment was primitive, and design principles and application parameters were not well-understood. But during the past three decades, improvements have been made to application equipment, as well as to the standards, criteria and guidelines related to SPF materials, design and application. Despite these changes, the authors believe that because some roofing professionals lack knowledge of SPF, a negative image of the system has perpetuated. Aesthetic barriers and general misconceptions about SPF may have precluded designers, contractors and building owners from considering it as a roofing option.
The SPF look Most SPF roof systems have unique aesthetic aspects when compared with built-up (BUR), modified bitumen, single-ply and metal roof systems. These qualities may not be understood by roofing professionals outside the SPF industry - minor maintenance or cosmetic problems often are viewed as serious performance concerns. Although appearance and problem misdiagnosis are separate issues, together they constitute the aesthetic barriers many indirectly associate with SPF. A majority of roofing professionals are at least somewhat familiar with BUR, modified bitumen, single-ply and metal roof systems. These systems exhibit diverse appearance characteristics. However, most SPF roof systems have an entirely different look (the exception being aggregate surfaced and, to a lesser degree, granule-surfaced SPF roof systems). The primary characteristics that make up the distinct look of most SPF systems include in-plane and surface flatness, construction details and surface coloration, as described: Flatness. SPF telegraphs substrate irregularities to the foam's surface, and the coating telegraphs these irregularities to its surface. In addition, during foam application, there is some unavoidable variation in the foam's thickness. (This variation is minimized when robotic sprayers are used.) These factors result in surfaces that typically lad in-plane flatness (i.e., the surfaces are irregular and lumpy), as shown in Photo 1. The foam's surface can be quite textured, which primarily is a function of equipment adjustment, environmental effect; (e.g., wind) and applicator skill (see Photo 2). Within prescribed limits, rough surface textures are acceptable. Construction details. Flashings at parapets, curbs and other penetrations typically do not have metal counterflashings (see Photo 1). And, as in the roof's field, the surfaces of the foamed-in-place cants are irregular. Also, particularly for reroofing projects, it is common to spray the foam directly onto mechanical units if the unit is integrated into the roof system. Surface coloration. Many SPF roof systems have light-colored coatings with blotchy appearances. These blotches result from small dark-colored areas caused by dirt accumulation at numerous small birdbaths. (Birdbaths occur because of surface irregularities.) Discoloration at birdbaths appears on other types of roof systems (e.g.,light-colored single-ply and BUR systems), but it is more pronounced on light-colored ones.
Aesthetic remedies It is possible to overcome or minimize some of SPF's appearance issues by the type and color of surfacing used: Aggregate. In addition to or in place of a roof coating, an aggregate surfacing may be specified. Except for the exposed coating at parapets, curbs and perimeter edges, this surfacing option provides the look of an aggregate-surfaced BUR system.
Color. In place of a white-colored coating, other coating colors or granules may be applied. Although white coatings are more advantageous for reducing building heat gain, other colors may be more aesthetically pleasing on some roof systems. Because of the solar heat gain that is experienced by extremely dark colors, it is suggested they be avoided to enhance coating performance. Problem misdiagnosis Even if various aesthetic remedies are made, designers and contractors not familiar with SPF tend to misinterpret minor maintenance and cosmetic problems. Commonly misdiagnosed problems are: 1. Exposed foam. Many mistakenly believe exposed foam is a major problem. It has been reported incorrectly that exposed foam will degrade quickly and allow water to enter the foam. If the foam is not protected from ultraviolet (UV) radiation by coating or aggregate, it will degrade (i.e., weather and erode) with time (see Photo 4). However, based on limited data from the NRF project, the rate of loss of quality foam (i.e., foam that meets current physical property standards) from the roofs field is believed to be less than 0.1 inches (2.5 mm) per year of exposure. After two or three years, the rate of loss often is reduced greatly because of dirt accumulation and degraded foam, which provide some UV protection. However, the more important issue is moisture gain. It has been reported that upon loss of the foam's skin (which forms on the foam's top surface), water will penetrate into the foam. But in reality, based on data from the NRF research project, when foam exposure occurs, there is little migration of liquid moisture into the closed cells of quality foam below the exposed surface. Although it is important to protect the foam, exposed foam typically is not indicative of a watersoaked roof system. 2. Mechanical damage. The coating and foam can be damaged by flying debris, large hail, bird pecking and abuse. The susceptibility to mechanical damage is related to surfacing type (i.e., coating type, presence of granules or aggregate). However, SPF roof systems are not in imminent danger of leaking, provided the penetration does not extend all the way through the foam.
3. Blisters. As with several other types of roof coverings, blisters can occur in SPF systems. If the blisters have not ruptured, they simply are a visual issue. If they are cracked or ruptured, as in Photo 5, they should be repaired. However as with mechanical damage, upon rupturing, an SPF roof system typically is not vulnerable to leakage for a substantial length of time. Blisters generally occur between lifts (i.e., applications) of foam. So if a blister ruptures, leakage protection usually is provided by the lower lift(s) of foam. Protecting SPF systems With roof systems other than protected membrane types, significant water infiltration can he expected when the insulation is exposed (i.e., the membrane is torn, punctured or blown off). The rate of infiltration is a function of insulation type and membrane attachment, exposure location (e.g., over a boardjoint or in the field of a board) and amount of water flow over the exposed area. However, with SPF systems, exposed quality foam does nor absorb water readily. Leakage was not reported on the roof systems depicted in Photos 4 and 5. Rather than thinking of the coating as the roof membrane, roofing professionals can regard it as a UV protector and initial line of defense against water intrusion, foot traffic and mechanical damage. By understanding the role of the foam and surfacing, it is possible to differentiate between problems that are cosmetic or easily reparable (which is the typical scenario with SPF roof systems) vs. problems that require major repair or roof system replacement. Although exposed foam is not immediately detrimental, it is important to protect the foam from UV degradation to minimize recoating costs and avoid weathering away. Coating or aggregate surfacing also is needed for a Class A exterior fireresistance listing. In situations where there is a strong downward vapor drive (e.g., cold-storage buildings), a low-permeance coating also acts as a vapor retarder. In closing Although appearance usually is not a concern for low-slope roof systems, the appearance of most SPF roof systems often presents a subconscious barrier. People outside the SPF industry may not understand the unique visual and performance characteristics of these systems. Editor's note: This article was based on a paper presented at the Polyurethanes Expo '96 in Las Vegas, Nev., October 1996. Thomas L. Smith is NRCA's research director and Rene M. Dupuis is president of Structural Research Inc., Middleton, Wis. Reprinted with permission from PROFESSIONAL ROOFING magazine, |
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Photo 5: These two blisters have ruptured and need to be repaired. However, this is a minor repair---the foam below the blistered area will provide water resistance.