Beefing Up Low Slope Roofing Installation

With commercial construction at low ebb, innovation has generally focused on gathering financial incentives for installing PV systems or vegetated roofs.

Though the low slope roofing industry has essentially stabilized, the methods of installation have evolved. Reasons for innovation include compliance with more restrictive environmental issues, such as volatile organic compounds (VOC), the ability to recycle, enhanced reflectivity (albedo) and/or emissivity, and cleaner and quieter application techniques.

Much credit for the changes should go to the Roofing Committee on Weather Issues (RICOWI), whose field teams traveled to the sites of hurricanes and reported on needed improvements in wind resistance. Credit also goes to work directed by Dr. Bas A. Baskaran, group leader of the Performance of Roofing Systems and Insulation subprogram at the Institute for Research in Construction, part of the National Research Council of Canada in Ottawa.

The State of the Industry
For now, the low slope roofing industry is holding steady at five categories:

1. Conventional bituminous build-up roofing (BUR)
2. Polymer-modified bituminous roofing (MB)
3. Single-ply polymeric roofing, especially PVC and TPO
4. Hydrostatic (low slope) metal roofing
5. Sprayed in place polyurethane foam and associated surfacing

As for the beef, requirements for thermal insulation have grown from low thermal resistance products such as wood fiber, perlite, glass fiber, and foamed glass with R values of less than 3 per inch, to Rs of 5 per inch or more. (R values are reported in resistance units, e.g., hours that it takes a Btu to travel from the warm side of a construction to the cold side.) These early products were fine for bridging the rib openings of a steel deck and were efficient enough while energy was cheap. By the time of the oil embargo of 1973-1974, concerns were raised that R values of 3 were inadequate. The crisis called for serious energy conservation.

In the next (and current) generation, cellular plastic foams began to replace the low R materials. A number of composite products hit the market to beef up deficiencies in an all-foam material. One issue was fire resistance. Foam plastics applied directly to a steel deck could not meet requirements for resistance to an under-deck fire. One composite consisted of a base layer of fire-resistant perlite-board, a core of urethane foam, and a top facer or asphalt-saturated roofing felt. Another used a fiberglass base. Both complied with requirements. Another successful innovation was to mechanically attach gypsum board (usually 1/2 inch thick) to a steel deck, followed by the urethane or isoboards.

PageBreakThe blowing agents in those urethane foams were early members of the Freon family, and the issue of ozone depletion required more environmentally acceptable alternatives. Fortunately, eliminating chlorofluorocarbons (CFCs) and substituting pentane gas as a blowing agent resolved the fate of factory-formed urethane foams. Innovative fire retardants and renamed polyisocyanurate foam (Isoboards) beefed up the fire resistance of urethane. A fire-protecting base layer was no longer needed to meet FM requirements.

With the advent of single-ply roofing, polystyrene roof insulation gained new importance. Whereas a BUR or MB system was typically installed in hot asphalt, solvent-based adhesive or torch application, all of which would melt the styrene foam, loosely laid and ballasted single ply systems were a perfect match.

ASTM Specification C-578 covers the current standards for both types of polystyrene foam (extruded and molded—XEPS and MEPS respectively). Densities of styrene insulation vary from 0.7psf to 3.0psf and compressive strengths as much as 100 psi.

Meanwhile, the oil embargo of the 1970s caused a huge ripple in the availability of petroleum-based asphalt. Up to that time, bituminous built up roof membranes consisted of 3-5 plies of asphalt saturated roofing felt mopped together with hot asphalt and sometimes surfaced with more asphalt to retain the roofing aggregate.

Now, just two or three plies of beefed up glass or polyester mat, factory-coated with polymer-modified bitumen (MB) provided performance equal to or better than the multiple-ply built-up systems. Factory-embedded granules in the cap sheet replaced the material and labor-intensive flood coat and gravel, capturing a significant portion of the low-slope roofing market. Torch application of MB sheets eliminated the logistics of kettles and tankers, providing heat as needed only at the torch itself.

While torching is widely used in Europe, there is not as much danger of fire there because most European roof decks are non-combustible concrete. In the U.S., where steel decks with combustible insulation and cant strips are widely used along with OSB and plywood, several serious rooftop fires have occurred. To address this hazard, the Midwest Roofing Contractors and later the National Roofing Contractors Association developed the Certified Roof Torch Applicator (CERTA) accreditation course.

Roofing Evolution
Innovation is also moving into cold-process application systems for MB, where the adhesive (a mixture of asphalt, petroleum solvent, fibers and fillers, and perhaps polymeric material) is sprayed, rolled on or spread with a squeegee, eliminating the need for hazardous torches. We're moving toward self-adhering membranes to eliminate the odors of solvent-based systems and meet more restrictive VOC regulations. Where just the mineral granules alone on MB cap sheets may not be reflective enough to meet stringent cool roof requirements, factory or post-application of reflective coatings over the granules may comply.

Single-ply roof membranes were already available in the early 1960s, especially PVCs in Europe. The oil embargo jumpstarted the use of these systems as they became competitive with the traditional BUR systems. Virtually all of these first generation materials, such as non-reinforced PVC, chlorinated polyethylene, polyisobutylene, Tedlar, Korad, neoprene, butyl, and chlorobutyl have disappeared.

EPDM rubber, reinforced PVCs, and TPOs now dominate the market. Several generations of TPO have already appeared, and there is some concern that newly appearing problems may cause further reformulation.

The flexible single-ply systems need not be fully adhered. Alternatives include ballast in the form of large river-rounded stones, pavers, vacuum adhered, or mechanically fastened systems. The proper use of ballast and fastened systems was greatly enhanced by the publication of ANSI/SPRI RP-4.

PageBreakEPDM roofing has been beefed up over the years by replacing the earlier uncured neoprene flashings with uncured EPDM, the use of tape sealants replacing solvent-based neoprene adhesives, and the innovation of incorporating stronger steel roof decks (80ksi steel for 33ksi), which in turn permitted wind ratings on sheets as wide as 10 feet with fasteners only in the lap areas. While 45-mil (0.045 in) thickness is still used, 60-, 75-, and 90-mil sheets are being used more because of the anticipated heavy traffic on our current roof designs. Generally, these systems may also qualify for longer warranties, i.e. 30 years instead of 20. (It has not yet been confirmed that longer life will be achieved through increased mil thickness, although puncture resistance is certainly enhanced.)

PVC membranes have also been beefed up. Non-reinforced and poorly formulated PVC products have left the roofing market, mainly due to premature shrinkage and embrittlement. As with EPDM, thicker sheets are regularly promoted. Both PVC and TPO are sealed by heat-welding the seams of the sheets. The use of double welds, one on each side of the row of fasteners, has greatly improved wind uplift resistance of the mechanically fastened systems.

Facers on isoboards and gypsum boards continue to get beefed up. Originally faced with paper, gypsum boards used either for underlayment or overlay have evolved into glass facers and or factory-primed surfaces. Isoboards are subjected to a new rolling and crushing test called the rolling load emulator, detailed by NRCA in their report Study of Polyisocyanurate Rigid Foam Board Facer Behavior Using the Rolling Load Emulator. The device simulates the effect of construction traffic that could cause fracturing and delaminating of the interface between foam and facer. Much like gypsum boards, facers on Iso are evolving to glass – and in some cases, very heavily coated glass – which provides the boards with better resistance to moisture, rooftop traffic, hail, etc. The most current development is the introduction of thin, very high density Iso boards as cover boards.

Not to be left out, metal systems are also being beefed up. New heat-resistant underlayments, generally of a non-woven or plastic film and self-adhesive, are used not just at eaves and roof valleys, but instead over the entire roof deck. To meet the cool roof requirements, a number of "cool" pigments were developed. While they look just like the old pigments as seen in visible light, they have higher reflectivity in the infrared range, as detailed at coolmetalroofing.org. Stainless steel-based non-penetrating clamps are used not only for their customary function to hold snow guards in place, but as an efficient means to anchor photoelectric cell panels to metal roofs.

Closed cell sprayed-in-place urethane foam roofing (ccSPF) has also evolved. The Spray Polyurethane Foam Alliance (SPFA) has assisted ASTM in developing ASTM D7425 standards for SPF, including increased compressive strength and density. The SPFA has also implemented certification programs not only for their applicators, but for roofing consultants and material suppliers as well. As mentioned above, the foam urethane business was forced to eliminate blowing agents that affected the ozone layer. While pentane is okay under factory conditions, the danger of static electricity requires a non-flammable blowing agent for field application. A new blowing agent called Enovate (HFC245fa) meets that need.

A scorecard for where the beef is:

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