by nithya_caleb | December 10, 2018 2:43 pm
by Mike Ennis, RRC
Thermoplastic roofing membranes, one of the fastest growing forms of low-slope roofing products, come in different material families, including thermoplastic olefin (TPO), polyvinyl chloride (PVC), and PVC alloy. Long before TPO was introduced in North America, PVC membranes capitalized on several events during the 1970s to solidify their position. First, the oil embargo of 1973 inflated the price and restricted the availability of quality roofing asphalt. At the same time, high energy costs increased demand for roof insulation systems with high R-values. This began to make single-ply membranes more appealing. This attractiveness included their direct compatibility with polyisocyanurate (polyiso) insulation, also a growing product at the time.
In 1984, Ducker Research (now Ducker Worldwide) predicted single ply sheets (including PVC and PVC alloys, ethylene propylene diene monomer [EPDM], chlorosulfonated polyethylene [CSPE], and chlorinated polyethylene [CPE]), would capture 25 per cent of the low-slope market. By
the middle of that year, single plies’ share reached 35 per cent of the roofs installed and Ducker soon revised its year-end forecast.
TPO membranes were introduced in the late 1980s, and by 2006, the thermoplastic market share had grown astonishingly. PVC and TPO together represented about 30 per cent of the commercial roofing market in Canada and the United States, according to Ducker Research.
Thermoplastic materials are distinguished from thermoset materials (EPDM) in that there is no chemical crosslinking. Unlike thermoset materials, thermoplastics do not form irreversible chemical bonds during the curing process. These membranes can be repeatedly softened by heating or hardened when cooled. Due to the materials’ chemical nature, thermoplastic membranes typically are seamed by heat welding with hot air or solvent welding.
Five years ago, Single Ply Roofing Industry (SPRI) also reported the great majority of the billions of square feet of single-ply membrane cumulatively sold in North America (including TPO and PVC) performed without issue.
“Single-ply membranes, including thermoplastics, have become a great addition to the roofing industry,” says Wendy Fraser, P.Eng., technical manager of the Canadian Roofing Contractors Association (CRCA) in Ottawa, Ont.
Attributes of TPO/PVC membranes
Some attributes shared by TPO and PVC membranes include long-term weathering resistance, cold temperature flexibility, resistance to tear, puncture, and chemicals, and heat-seaming capability.
These membranes can be manufactured in a wide range of colours, do not cure after exposure to the elements, and remain capable of hot-air welding throughout their service life.
Each TPO and PVC membrane has a unique formulation. Probably the most noticeable distinguishing physical attribute among these thermoplastic sheets is their relative stiffness. Some thermoplastic membranes feel relatively soft and flexible while others are more rigid. This has no relation to cold temperature flexibility. These stiffness characteristics may affect the membrane installation process in very cold weather. However, the cold-handling properties of thermoplastic membranes in the field do not affect their cold-temperature mechanical properties. During installation in cold weather, some roofing contractors claim stiffer sheets require less effort to heat weld because the membrane is less likely to move and form voids as the automated welder creates the finished seam. A stiffer sheet can also be easier for the contractor to move around and properly position once unrolled.
Stiffness is more of an issue when installing non-reinforced flashing. Flashing membrane is used when completing details requiring the membrane to be formed around penetrations of various shapes and sizes. If the flashing is stiff, the contractor will have a more difficult time completing details. Fortunately, many thermoplastic membrane manufacturers have formulated their non-reinforced flashing material in such a way as to increase its flexibility.
Thermoplastic membranes also are highly resistant to a variety of chemicals, and both TPO and PVC materials are formulated to be fire-resistant. When designed as part of an appropriate roof assembly, both TPO and PVC roofing systems can achieve Underwriters Laboratories (UL) Class A fire-resistance listings. Beyond fire testing, thermoplastic membranes have been certified by Factory Mutual (FM) Approvals and have wind-uplift resistance classifications exceeding 2224 N (500 lbf).
TPO and PVC membranes have a reinforcement layer made of either polyester or fibreglass, incorporated at the factory using a variety of techniques. Regardless of manufacturing line configuration, the compounded material is heated to a high temperature to allow the forming process to occur.
What makes them popular?
One factor driving the growth of TPO and PVC membranes is the focus on the energy efficiency of buildings. Global warming has become a focus point for many property owners, specifiers, and government agencies. Electrical blackouts caused by the increased use of air-conditioning have energy providers looking for ways to reduce peak electrical demand while designers attempt to limit a building’s carbon footprint. Highly reflective roof membranes can help alleviate the heat load placed on a building by reflecting sunlight and maintaining a lower surface temperature than darker-coloured roof surfaces.
Additionally, the US Green Building Council (USGBC) created its Leadership in Energy and Environmental Design (LEED) certification program for buildings designed, constructed, and operated with sustainability in mind. The program assigns points for certain types of designs and provides a construction “point” for roofs with high reflectivity, such as those employing PVC and TPO membranes.
“LEED certification is prominent in both Canada and the United States,” says Fraser, “and the specification of reflective thermoplastic roofing has become more popular because of it.”
Further, increasingly stringent reflective roof standards have been continually introduced into the International Building Code (IBC), which is referenced in almost all Canadian low-slope roofing specifications. TPO roof membranes used in Canada follow material properties as per the requirements of ASTM E1980–11, Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces. ASTM E1980–11 offers manufacturers and designers guidance on calculating solar reflectance, but does not mandate minimum solar reflectance of roof membranes.
Another reason for the increasing popularity of TPO and PVC roofs is the induction welding attachment method for these membranes. Induction welding is the use of an electromechanical field to heat a pre-attached bonding plate located under the thermoplastic membrane to weld to the TPO or PVC membrane. This is considered a “non-penetrating” attachment method.
From an economic and labour standpoint, particularly on large buildings, roofing contractors often like to use the widest TPO/PVC sheet possible, which reduces the number of seams to be heat-welded. This reduces the labour required to install the roof.
However, to meet certain wind-uplift requirements for mechanically attached systems, the spacing of the fasteners may not allow a contractor to use the widest sheets available or may require fasteners be spaced closely together in the lap. A narrower width sheet can help distribute the uplift forces over more structural members, allowing it to withstand greater uplift pressures, and using more fasteners in the lap area reduces the load each fastener is subjected to in a wind event.
Consequently, with induction welding, a contractor may be able to use wider sheets, fewer fasteners, and less labour, while meeting the design pressures required for the roof.
Perhaps what contributes most to the growth of TPO and PVC membranes is the variety of attachment methods—stone ballast or pavers, mechanical fasteners, induction welding components, and bonding adhesives. Common adhesives used as bonding agents include solvent-based, waterborne, and 100 per cent solids-reactive products, including two-part and moisture cured/activated adhesives.
“As the TPO and PVC markets have increased in sales volume, roofing contractors in Canada have become more accustomed and comfortable with the different installation methods available for thermoplastic membranes,” observes Fraser.
It is true certain regions of the United States limit the maximum content of volatile organic compound (VOC) in various adhesives. Canada also limits the VOC content of construction materials, including those used in roofing applications. As such, TPO and PVC manufacturers have developed low-VOC, water-based adhesives, low-rise, and moisture-cured adhesives, and self-adhering membranes, including self-adhering flashings and details for roof system penetrations. These products can be used wherever there are stringent low-VOC requirements limiting or excluding the use of solvent-based adhesives (subject to temperature and humidity limits). It is important to note the application of water-based adhesives may be limited due to environmental conditions (low temperatures) in some Canadian locations.
Formulating chemists have engineered waterborne adhesives to sufficiently bond TPO and PVC membranes to various substrates such as roof insulation, wood, concrete, lightweight insulating concrete, and other surfaces. Again, these water-based adhesives are especially attractive when local air quality legislation restricts VOC content in bonding adhesives.
Prefabricated TPO and PVC accessories are available and also come with installation options. Accessories include curb wraps and split pipe seals that can save many hours of labour for
a typical project.
Moulded sealant pockets also are available with TPO and PVC systems to waterproof pipe clusters and other oddly shaped penetrations. Some square tubing wraps provide a split (cut) and overlap tab allowing the seals to be opened and wrapped around a square tubing penetration.
Finally, some manufacturers offer cover strips constructed of a TPO or PVC membrane laminated to a fully cured synthetic rubber pressure-sensitive adhesive. This type of product is ideal for use with metal drip edges and suitable for a variety of other applications.
TPO continues to be known as the “newest” membrane for commercial low-slope roofing, but it is far from unproven. ASTM D6878, Standard Specification for Thermoplastic Polyolefin-Based Sheet Roofing, first published in 2003, has continually updated TPO performance requirements. Separately, ASTM D4434, Standard Specification for Poly(Vinyl Chloride)Sheet Roofing, covers PVC membranes and was first published in 1985. It has seen many improvements over the past 30 years. ASTM D6754/D6754M – 15, Standard Specification for Ketone Ethylene Ester Based Sheet Roofing, is used for PVC ketone ethylene ester- (KEE) modified membranes manufactured using a hot melt vinyl coating technology with DuPont Elvaloy as the foundation for the vinyl compound. Updates to each of these product standards have included strengthening the weathering resistance requirements which have been incorporated into their respective standards and
into individual roofing manufacturers’ performance specifications.
TPO and PVC membranes are known to resist the hydrostatic pressure resulting from standing or “ponding” water.
Additionally, they are not affected by freeze and thaw cycles. However, similar to other roof membranes, TPO and PVC weather from the effects of ultraviolet (UV) light and heat. For this reason, accelerated aging is conducted to expose TPO and PVC membranes to conditions often far in excess of what is experienced during rooftop use but in shorter (accelerated) time periods.
Thermoplastic membranes are not a “cure-all”
While the popularity and performance characteristics of these membranes are clear, every thermoplastic membrane exhibits strengths and weaknesses. While thermoplastic membranes represent more than 50 per cent of the low-slope membrane roofing market in 2018, there are installations where their traditional use is not recommended.
All thermoplastic membranes rely on one layer of roofing material to waterproof the building. More traditional roofing options in Canada, such as built-up roofing (BUR) and modified bitumen, offer redundancy in the form of multiple layers of roof membrane protection. Some asphalt-based product specifications also offer greater puncture resistance and are more forgiving of rooftop abuse than standard thermoplastic membranes.
Another installation where care should be taken is one with high heat loading combined with concentrated exposure to sunlight, although this is less of a concern in northern climates. Most roofing materials degrade over time when exposed to heat and UV and their long-term performance depends on formulation and in-situ conditions, so exposure to high temperatures and high UV, such as south- and west-facing walls in extremely hot climates, may warrant either a thicker membrane or a membrane formulated for such severe conditions.
As mentioned previously, thermoplastic membranes are often specified with polyiso insulation. As one of North America’s most widely used, readily available, and cost-effective insulation products, polyiso has been cited by the U.S. Environmental Protection Agency (EPA) for its responsible impact on the environment. However, according to some sources, the insulating performance of polyiso insulation is degraded at colder temperatures. Questions about the cold-weather performance of polyiso should initially be directed to pima.org.
Regarding the environmental impact of using thermoplastic membranes, manufacturers have the opportunity to publish an environmental product declaration (EPD) in accordance with the International Organization for Standardization (ISO) 14025, Environmental labels and declarations – Type III environmental declarations – Principles and procedures. EPDs rely on life-cycle assessment (LCA) to provide information on a number of environmental impacts of products over their life cycle.
Thermoplastic membranes are also suitable for roofing upgrades on existing low-energy buildings, although this is highly dependent on building location, construction, and interior conditions.
In colder climates, cooling load is less of a concern. In fact, building scientist John Straube does not recommend using light-coloured roofing materials in colder climates, as they reduce the potential for drying of condensation that may build up within the roof assembly.
Regarding condensation, a hydrothermal analysis should be conducted using a program such as WUFI to determine the condensation potential of the specific roofing design using the expected interior and exterior conditions for the building and location.
WUFI allows realistic calculation of the transient coupled one-dimensional heat and moisture transport in multilayer building components exposed to natural weather. It is based on the newest findings regarding vapour diffusion and liquid transport in building materials and has been validated by detailed comparison with measurements obtained in the laboratory and on outdoor testing fields. In general, the building design should not show increasing levels of condensation versus time. If it does it should be revised to prevent this situation.
Thermoplastic membranes can meet a variety of different building requirements by offering a range of options in new construction, retrofit, and tear-off/reroofing situations. Single-ply membranes, including thermoplastics, are particularly suited to “big box” applications due in part to the availability of large 3 x 30 m (10 x 100 ft) sheets using some of the labour-saving techniques mentioned earlier. Thermoplastic membranes also offer excellent resistance to UV degradation and ozone and chemical exposure.
It is also important to note these high-performance thermoplastic roofing membranes are compatible with the latest rooftop technologies, including solar arrays and vegetative roofing. Given the wide variety of installation options available, it is easy to understand why these membranes have continued to gain in use and provide long-term performance for building owners.
Mike Ennis, RRC, joined the Single Ply Roofing Industry (SPRI) trade association in 1993. He has chaired various SPRI committees and task forces, and served as president from 2004 to 2006. He became the group’s technical director the following year. Ennis can be reached at email@example.com.
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