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|Published in Maintenance
Solutions (April 1995)
The Latest Generation of Roofing Options
Tougher expectations among users force changes in technology
By Jeff Evans
Fresh out of college with a degree in construction administration, I began my roofing career in 1977 with a contractor specializing in a "new revolutionary product," a single-ply ballasted roof membrane. Knowing little about roofing in general, I was moderately successful in selling the new membrane to costumers fed up with perceived performance problems of built-up roofing, the other option at the time.
Roofing has changed dramatically since then and continues to change daily. Over the past 20 years, buyers' choices have multiplied many times over. For example, single-ply roofing has progressed from a new product into a major player in the roofing market.
The built-up roofing industry, out of favor for some time because of real and perceived problems, has re-established its visibility as a capable performer by improving its base components and developing better flashing and accessory products. Built-up roofing manufacturers and contractors also have educated the roof-buying public of essential design principles, such as eliminating ponding water, to reduce the inappropriate use of built-up materials.
For maintenance managers, engineers, facility managers and others involved in purchasing roofs today, two words best describe the state of the industry: choice and change. Compared to the 1970's, when the only decision a roof buyer had to make was how many plies were best, the 1990s present a dizzying array or choices. Following is a review of some updates in each generic product line.
Modified bitumen systems
Typically, mod bit membranes are made in sheets 3 feet - 1 meter - wide and resemble rolled roofing. Two primary modifiers are used:
APP modifieds are generally torch applied, while SBS membranes are applied in hot asphalt. Cold adhesives for both are now on the market, which may help in places where installers can't use torches or hot asphalt.
Mod bit systems are usually multiple-ply systems; two plies are a recommended minimum. Again, there are many choices available to the roof designer, depending on the needs of the application. Hybrid systems have become popular, using one-three plies of fiberglass felts in hot asphalt, followed by a finish ply of modified bitumen. If these don't offer enough choices, several roofing manufacturers supply kegs of modified asphalt to allow a modified bitumen application similar to built-up roofing, alternating layers of hot-modified asphalt with reinforcing felts of fiberglass or polyester.
Modifieds have the same general sensitivity to ponding water as asphalt built-up membranes, so sloping to drain is a needed design feature. They tend to be fairly durable and generally have performed well over the past 10-15 years in the U.S.
Chemically, these membranes are made from synthetic rubber or plastic, sometimes incorporating a layer of reinforcing fabric or mesh. More commonly, they are known by their primary polymer component makeup: EPDM, PVC, CSPE, PIB, EIP, etc. Single-ply roofing membranes, adapted from other uses of elastomeric membranes materials, such as pond liners, have excellent elongation. Here are some things to look for and to avoid in selecting single-ply systems:
We've witnessed a peculiar result of wind flutter that occurs in some mechanically attached roof applications. A roof membrane that billows or flutters in winter conditions draws warm, moist interior air to the underside of the roof membrane, where it can reach dew-point temperature and condense. Infrared moisture detection scans on roofs that had not leaked still revealed wet insulation.
Our educated hunch is that the repetitive billowing of the membrane contributes to this moisture pickup, especially on air-permeable roof decks, such as steel decks. An air barrier below the roof insulation reduces wind flutter and subsequent moisture infiltration.
Metal roof systems can be used in new roofs and some reroofing situations, but they always require a minimum 1/4 inch per foot slope. When buildings are too wide to accommodate a single slope from ridge to eave, metal roofing designers must turn to using an interior common gutter, a practice we don't recommend,
We also discourage using metal roofing on facilities that expect to add new roof penetrations with some frequency, since the new units must resort to caulk and exposed fasteners, the very items that caused the first generation of metal roofs to leak.
The consequences are predictable. If maintenance managers were dissatisfied with built-up roofing, the spotty performance of some single plies has evoked similar dissatisfaction. To those fortunate enough to be working with the right unproven single-ply products and are enjoying years of watertight service, maybe it was solid investigation and sound decision making, or perhaps it was other factors. Many others have not enjoyed such success.
Over the past five years, there has been a major settling of this market, leaving fewer, more successful companies and membranes to deal with. Maybe we can be thankful for this small upside.