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Roofing Gains Come at Steady Pace
In the roofing industry, where durability is of primary concern, claims for “new and improved” may well be a cause for concern. It is the slow and cautious approach that builds confidence. Therefore, reviewing the last decade to identify trends should help facility executives predict where the roofing industry is headed.
First, there has been a gradual evolution of products, rather than the dramatic introduction of the plastic and rubber membranes of the ’70s and ’80s. On the commercial roofing side, other than the introduction of TPO single-ply membranes, few totally new products have appeared. Built up, modified bitumen and single-ply systems have roughly equal shares of the low-slope commercial roofing market. Spray foam still has a share, and structural metal has gone beyond the metal building image to cover all types of structures.
Problems with environmental, safety and health issues continue to be of more concern than technological issues, with mold and mildew perhaps displacing asbestos and lead as the current concerns for facility executives. High reflectivity as a way to address urban heat islands is also of interest; see www.energystar.gov for more information.
For commercial roofing, no-dollar-limit warranties have been widely accepted.
Glass fiber has almost displaced organic felt in the conventional hot built-up roofing (BUR) market. Asbestos was withdrawn in the early ’80s, and polyester mats remain a specialty except as reinforcements in polymer-modified bitumens.
Most BUR systems are laid over thermal insulation, typically a heat-resistant, felt-faced polyisocyanurate foam board (isoboard), which is first attached to the deck. The isoboards are very thermally efficient, with R-values of as much as 7 to an inch, compared to only 2.75 for wood fiber and perlite. Mechanical fasteners, such as corrosion-resistant screws and stress plates, are used following recommended installation patterns to ensure adequate wind resistance at corners and perimeters. Two layers of thermal insulation — the first mechanically attached and the second hot-mopped to the first with joints offset from the first — provide optimum thermal performance. If the top layer is isoboard, an overlay of nonfoam material such as perlite or gypsum board may be used to help avoid blistering.
Coated base sheets, common with the earlier organic felt systems, are rarely used with glass-fiber BUR. The only exceptions are in nailed constructions — where the base sheet might be nailed to a wood deck, followed by mopped ply sheets — and in lightweight insulating-concrete roof decks, where a coated venting base sheet is used. (Glass-ply sheets are vulnerable to alkalai attack, so they should never be in direct contact with lightweight insulating concrete.)
Flashings have probably evolved the most. Reinforced asbestos flashings, when they were available, had been extremely durable. Glass-fiber flashing proved to not be equal in conformability, durability and puncture resistance. Polymer-modified sheets are dominant in conventional BUR applications. While many installations use just a single layer of modified-bitumen flashing, a backer-sheet of fiberglass felt is still recommended for added durability and puncture resistance.
Gravel embedded in a “flood coat” of hot bitumen remains the most durable surfacing for BUR. It also provides Class A fire resistance and superior hail resistance. Alternates include black “glaze coats,” black emulsion coats, mineral surfaced glass-fiber cap sheets (generally in the West and Southwest), and reflective coatings of aluminum or white latex. Latex paints are not recommended for roofs where water ponds.
Modified-bitumen products and specifications have proliferated. The primary advantage over conventional BUR is the use of fewer plies, which translates into substantial savings in labor and smaller savings in materials. They resemble coated base sheets, but the reinforcement is polyester, glass or a combination of the two, and the asphalt is blended with a polymer for greater durability and toughness. When used as the top layer, they are frequently factory surfaced with granules or bitumen, eliminating the need for field surfacing later on.
ASTM standards are now available for most of the products, and both BUR and single-ply manufacturers have added modified bitumen to product lines. Modified-bitumen products are compatible with bitumen, a concern in reroofing projects where bitumen-contaminated surfaces may be present. They are also useful for repairing BUR and bituminous flashings. A trained in-house maintenance crew familiar with modified-bitumen systems will only have to learn a few new skills and techniques.
Reinforcements for modified bitumen are generally glass fiber or polyester. In many cases a hybrid reinforcement is now used because glass has better fire resistance and heat stability while polyester has greater puncture and work-to-break resistance.
Mineral-granule surfacing has become the surfacing of choice. It makes little sense to use gravel, as the flood coat and gravel are labor intensive and modified bitumen would lose its cost advantage. Metal foils are available, with embossed aluminum most popular. For special needs, stainless steel and copper foil may be specified. Anodized aluminum is also available if coloration is required. Uncoated modified-bitumen sheets have proven to be less durable. Colored coatings are also possible but need to be limited to roofs with positive drainage and periodically recoated.
Fire resistance of mineral-surfaced modified bitumen is achieved through the addition of fire retardants to the modified-bitumen compound rather than relying on the protection of the gravel layer as in BUR.
There are two types of modified-bitumen systems: SBS and APP. Both are polymeric modifiers that improve the toughness and flow properties of asphalt. APP is the meltable, wax-like phase of polypropylene; its low crystallinity renders it compatible with an asphalt matrix. SBS consists of two polymers: flexible butadiene rubber chains grafted to polystyrene crystalline “domains.” S-B-S represents the sequence of polymers. When SBS polymers are heated and subjected to high-energy mixing, the polystyrene liquifies and allows the rubber chains to disperse in the asphalt; upon cooling, the SBS-modified asphalt mass behaves like a true rubber.
There are three different application techniques for modified bitumen: torching, hot-mopping and cold-applied.
Torching (heat fusion) is generally used with APP-modified sheets. The advantage is that the heat is only directed to the surfaces to be fused, avoiding the necessity for roofing kettles, pumps and mops. Concerns for fire safety should not be underestimated. Certification of torch applicators is strongly recommended.
SBS-modified sheets may be either torched or hot-mopped. They are generally more flexible at low temperatures than APP sheets but may be more tender in hot weather. They are never left unsurfaced, with granule and foil being the surfacings of choice.
Both SBS and APP sheets may be installed in cold (solvent-based) adhesives. This may be preferred where the fumes of a kettle are objectionable, where there are many changes of elevation or where access is difficult. In some cases, the sheets are laid in cold adhesives while side and end laps are torched for quicker sealing. Cold adhesives take a while to set up, so traffic from other trades must be avoided.
The rush into single-ply roofing of the 1980’s has established single ply as a durable, reliable alternative to bituminous roofing. Two categories of single ply exist: the vulcanized, unweldable elastomerics, almost 100 percent based upon EPDM rubber, and the nonvulcanized, weldable thermoplastics.
EPDM Over the past several decades, there has been no need to reformulate EPDM. Black is dominant, with attempts to compound durable light-colored sheets not successful. Many EPDM roofs are installed loose-laid and ballasted by the application of large-diameter aggregate (11/2 inches or greater) or pavers; in those cases, color is of no importance. Since the rubber is not adhered to the thermal insulation below, inexpensive expanded polystyrene (MEPS) can be used. This combination of MEPS and EPDM is very cost effective. The ballast provides Class A fire resistance without the need for fire retardants.
Problems of EPDM roof systems in the 1990s were occasional failure of the seam adhesive and membrane shrinking away from wall flashings. Seam problems were resolved by the introduction of butyl seam tapes as a replacement for earlier liquid-applied neoprene adhesive. Reliability was validated by comprehensive studies at the National Institute of Standards and Technology. Shrinkage was controlled by the introduction of a mechanically fastened reinforced edge strip of EPDM. This is installed under the membrane and bonded to the bottom side of the membrane, avoiding penetration of the membrane with mechanical fasteners.
Another trend with EPDM has been to upgrade membrane thickness from 45 mils — the ASTM minimum — to 60 and 90 mils. The cost of labor is virtually the same as for the thinner membrane, but puncture resistance is greatly enhanced.
Mechanical attachment of EPDM is also possible, as is full adhesion. Use of thicker sheets in fully adhered membranes helps resist puckering and wrinkling. Most mechanically fastened membranes use reinforced sheeting to better resist stress concentrations.
Fire resistance of nonballasted membranes is achieved through the use of fire retardants added to the compound. Reflective coatings have been used to achieve Energy Star® ratings, but owners need to realize that all coatings require periodic maintenance. Concerns with durability and adhesion persist. In addition, the coating supplier and membrane manufacturer need to be consulted as to proper procedures for preparing the membrane for patching, as the coatings may make it more difficult to repair the membrane.
Weldable Single-ply Membranes
In the 1960s, hot-air weldable single-ply membranes were plasticized, non-reinforced PVC sheets. Many suffered rapid loss of the plasticizer, especially in the ballasted versions then in vogue. Shattering and uncontrolled shrinkage of these first-generation materials gave PVC a bad name, and attempts were made to produce plasticizer-free materials instead.
These included chlorinated polyethylene (CPE) and chlorosulphonated polyethylene (Hypalon® or CSPE). Both were moderately successful, with CPE giving way to improved, second-generation PVCs. Hypalon still holds a small niche market, but it is difficult to patch when aged, and the major producer of Hypalon sheeting is now promoting thermoplastic polyolefins (TPO) and PVC whenever possible. Blends of PVC with other polymers, called “copolymer alloys” or “interpolymers,” have been successful and are available. Meanwhile, reinforced, durable PVC sheets have had more than 20 years of success and the market for these sheets is now increasing, especially because they are inherently fire resistant, weld reliably, repair easily and are available in reflective colors.
The TPOs have been introduced and marketed by virtually every major material supplier. They are weldable, available in light colors and fabric-reinforced. Mechanical attachment is the most popular installation method, and sheets 10- and 12-feet wide have been produced to reduce application labor. However, many chemical versions are available, which can confuse specifiers, and ASTM standards have not yet been published. (It is not unusual for ASTM to take a cautious approach to new materials.) Contractors have found that different products handle differently, but this too is not uncommon with a newer material. Facility executives should research products that have been on the market for a while, checking a list of projects installed in a similar climate and in a similar construction. The specifier should ensure that a technical representative is present at the start of a new TPO project to train both the crew and the consultant/inspector.
Spray-in-place Polyurethane Foam
Spray foam and the companion top-coatings have changed little in the past decade. The foam is light in weight and highly energy efficient, and it adheres well to clean and dry substrates. It resists high winds, having performed remarkably well in typhoons and hurricanes, even where flying projectiles have penetrated the surface, because the closed cells resist moisture penetration until repairs can be made.
Silicone coatings provide excellent durability and have required no change in the past two decades. Acrylic coatings have improved greatly, with elastomeric acrylics demonstrating improved flexibility and adhesion. The third category of coatings is polyurethane materials, and it includes catalyzed and moisture-curing types. These may have superior hail resistance to the other types where this is of concern.
The major changes in the spray polyurethane foam industry have been improvements in training, with accredited contractors, inspectors and manufacturers now available through the Spray Polyurethane Foam Alliance (SPFA), and the issuance of ASTM standards for the materials and systems. In general, spray foam now utilizes densities nearer to 3 pounds per cubic foot than the previous 2 pounds per cubic foot, and higher compressive strengths as well. This improves impact and traffic resistance. ASTM- and SPFA-recommended practices address substrate preparation, moisture and wind concerns, and application techniques. Robotic spray machines provide uniform application of both foam and coating. SPF is most popular as a recoating application over existing BUR and metal roof systems.
Metal Systems and Accessories
Metal roofing is available in many forms. The introduction of zinc-aluminum alloy coatings made coil-coated steel a truly durable product, as compared to the previous galvanized steel. Floating clips allow long metal panels to perform without buckling as a result of thermal expansion. The use of factory-installed sealant in the female side of the panel and field-applied sealant at end laps, flashings and penetrations has permitted systems to be installed at slopes as low as 1&Mac218;4 inch per foot, or a 2 percent grade. These systems are sometimes referred to as hydrostatic because they are capable of resisting ponded water for periods of time. Architectural panels or hydrokinetic panels are used at steeper slopes and rely on water shedding and water-resisting underlayments for adequate performance.
Crafted metal panels of copper, lead and even gold have changed little over the centuries. These require highly skilled labor and are labor intensive. Some require field soldering, especially at low slopes. They are generally found on institutional structures, while foil-faced modified bitumens provide a lightweight look-alike for modern domes and arenas.
Among metal accessories, prefabricated snap-together gravel stops have solved problems for both the single ply and BUR industries. Simulated wind-load tests for edge metal are available from the Single-Ply Roofing Institute (SPRI) and ANSI, as are tests for fastener withdrawal.
The Next Decade
As with the past decade, all the major types of roofing systems are still viable. Had one system clearly been superior, market forces would have driven the others down if not out. The problems of the 1980s and 1990s have been addressed, with blistering and splitting of BUR now virtually gone, the lap and shrinkage problems of EPDM resolved through reformulation and redesign, and the brittleness of PVC eliminated by reformulation and use of reinforced sheets.
Polyisocyanurate insulation boards are meeting tighter ASTM standards and overlaid with foam-free boards to minimize blister potential in hot BUR systems. Polystyrene is very cost effective when used with single-ply systems or as embedded insulation in lightweight insulating concrete constructions. Perlite, wood fiber and gypsum board are useful as reroofing and overlay boards when high thermal resistivity is not required. Tapered insulation can improve drainage, keeping reflective roofs cleaner as well as improving durability.
For roofing to continue to be successful, not only must good materials and systems be available, but good design and quality installation are necessary. The use of roof consultants and full-time inspection helps address these last two requirements.
Dick Fricklas is an author, journalist and educator. He is the founder of and technical director emeritus of the Roofing Industry Educational Institute. He is the coauthor of The Manual of Low Slope Roof Systems.
Changes in Code and Insurance Requirements
The latest revisions to ASCE-7 (Minimum Design Loads for Buildings and Other Structures) have revised the methods of estimating wind and snow-drift loads for structures. These revisions have been incorporated into model building codes and Factory Mutual Global data sheets. This becomes important when reroofing, as the model codes now require that when more than 25 percent of a structure’s roof is replaced in a single year, the entire roof must be brought up to the current code.
For the plumbing code, this will generally require that each roof area have two independent methods of drainage. The overflow drains, if used, must be separately plumbed and not connected to the primary drain leaders. They are supposed to dump the overflow water in an observable manner so that the occupants will notice that the primary drains are malfunctioning. Codes now require a minimum 1/4-inch-per-foot slope for design. This can be achieved through the use of tapered insulation if the slope is not provided in the structure itself.