PVC’s proven track record of performance for the past 15 years together with an increasing emphasis on energy conservation, make the lighter-colored PVC sheets a natural choice for today’s owners.

Single-ply membranes, and polyvinyl chloride (PVC) specifically, have come into their own as viable Energy Star-rated roofing system choices. The latest National Roofing Contractors Association data indicate that while the single-ply market overall has been declining, thermoplastic membranes have continued to increase market share. PVC’s proven track record of performance for the past 15 years together with an increasing emphasis on energy conservation, make the lighter-colored PVC sheets a natural choice for today’s owners. Thicker PVC membranes are performing beyond the length of their warranties, which can be as long as 20 years.

What led to PVC entering the low-slope roofing market?

Single-ply membranes were so designated in order to differentiate them from all other types of low-slope roofing systems classified as built-up roofing. This was a natural difference since built-up roofs are manufactured onsite, using multiple plies of membrane, usually four, adhered with hot asphalt and surfacing. Single-ply membranes are factory-produced under controlled conditions, arriving at the job as a watertight sheet. There are two major types of thermoplastic membranes: TPO (thermoplastic polyolefin) and PVC. This article will focus on PVC and TPO will be covered in a later issue. In the mid-1970s, one of the major contributors to the dramatic increase in single- ply systems was the oil embargo. The overnight doubling of the price of a barrel of oil and the need to extract the maximum amount of gasoline from the barrel led to a further reduction in the available crude flux used in the manufacture of asphalt. While PVC single-ply systems also use products derived from oil, the quantities are much less. The resulting economic impact led to the growth of PVC and other single-ply systems. Another reason for the growth of PVC single-ply was the dissatisfaction with the durability of organic and inorganic built-up roofing, the quality of workmanship in BUR applications and the reduction of guarantees to five years. In addition, PVC sheets are easy to apply and installed costs have dropped considerably due to lower membrane costs and contractor efficiencies. These factors, together with the additional equipment and knowledge necessary to apply a built-up roof, led to more roofing companies installing PVC roofing systems.

What exactly are thermoplastic membranes?

Thermoplastic membranes are based on plastic polymers, the most common being PVC . A number of different products in this category are available, each having its own unique formula. PVC membranes are identifiable by their seams, which are formed using either heat or chemical welding, and their light-colored, heat-reflective sheets. The seams are as strong or stronger than the membrane itself. Key physical properties of a PVC single-ply are its flexibility, strength and durability. PVC single-ply systems are not as sensitive to inclement weather during application as are built-up roofs, because they are essentially a finished product, needing only to be mechanically attached or fully adhered, joined at the seams and flashed.

How are PVC single-ply systems manufactured?

PVC membranes are produced by using a variety of chemicals that are first preblended in a compounding process. These various ingredients include PVC resin, stabilizers, pigments, fillers, plasticizers, biocides and various processing aids. After this step, the plastic compound is calendared to form two separate sheets. Plasticizers must be able to be processed at the correct temperature and then be able to withstand rooftop temperatures for the life of the sheet. Stabilizers are added to protect the sheet from ultraviolet rays and heat during the manufacturing process. PVC sheets are usually produced by calandering or by the use of an extruder into a finished sheet. The top and bottom sheets are often different in composition with the top sheet being generally thicker and lighter in color, with added ingredients to protect against the various elements of weather. Both sheets are then laminated together around a reinforcement scrim to form the finished membrane. The reinforcements are generally woven polyester or fiberglass. The yarns are typically multiple fibers, spun or twisted into bundles of continuous filaments. With polyester, the yarn is woven, using weft-insertion of two primary sets of yarn, interlaced by a third yarn to tie or knit the weave together. Surface treatments are added to the reinforcements to improve wetting of the polymer and to control moisture wicking that could cause membrane damage. These membranes are produced to meet ASTM D-4434 as a minimum standard. The characteristics of a properly produced sheet are resistance to water, weather and fire, and flexibility.

What are the advantages of a PVC sheet?

One advantage of a PVC single-ply system is its ability to be heat welded in the field. These field-welded seams can be stronger than the sheet itself. The experienced mechanic can make a permanent weld using either the motorized seam welder or the hand-held heat gun used for smaller detail work. Motorized seam welders come with adjustable weights, variable speed control and digital temperature control systems to maintain consistent welding temperatures regardless of the changes in ambient temperature or voltage fluctuations. Another advantage of a PVC membrane is that the flashing may be heat welded. A similar material, sometimes thicker, or coated metal may be used. PVC coated metal is used quite often for flashing details with PVC systems. This method allows for a positive attachment of the membrane to the metal flashing. When joining metal to metal, allow for its expansion and contraction by using a bond-breaker patch over this area so that the metal may move freely without stressing the PVC membrane. PVC membranes are able to handle a variety of flashing details. Heat welding allows for flashing in hard-to-reach areas. It is strongly recommended that whenever a sheet is cut, the edges of the cut area should be coated or sealed to minimize possible wicking of moisture. This is important to all cut edges. Always reinforce each corner detail with pre-formed inside or outside patches, since these are high stress locations. A third advantage is that PVC membranes are generally light in color with smooth, reflective surfacing that can be used as a cooling strategy for the building. Many light-colored PVC membranes are certified and labeled to carry the EPA’s Energy Star rating for reflectivity. With the higher cost of electricity vs. natural gas, the more energy efficient the building, the less expensive will be the AC operating costs to the owner.

What does an Energy Star rating mean?

Energy Star is a measure of the solar reflectance of a membrane’s surface. To obtain the Energy Star rating, a membrane must have an initial Solar Reflectance Index of 0.65 or better and 0.50 after three years of weathering. During the three-year period, scheduled roof maintenance, such as the cleaning of the sheet, may be performed. Not all light-colored roof systems are sufficiently reflective to earn the Energy Star rating. Recent studies by such organizations as the Lawrence Berkley National Laboratories have shown the negative impact on cities caused by the increase in heat levels vs. the surrounding rural areas. Primary causes are waste heat (from AC, power generation, industry and transportation), pollutants, altered airflow and retained heat. The addition of roofs, roads and parking lots coupled with the reduction of trees and vegetation have led to dramatic increases in heat levels. The increase in heat compounds the smog problem, which in turn increases the heat levels. This increase also triggers other problems such as changes in weather patterns.

How can a PVC system handle harmful HVAC Discharge?

One of the most overlooked and easily corrected maintenance items is the discharge from the HVAC unit. This discharge of AC condensate can be harmful to the unprotected membrane. The contaminants often contain various metals, coil-cleaning detergents, algaecides and fungicides that can chemically attack the membrane. The condensate, if allowed to stay on the roof, will degrade the membrane by taking a constant path that will harm the area in contact with the discharge. In addition, in areas of persistent ponding of the AC condensate, airborne contaminants such as salts, acids, dirt and vegetation can collect and grow. On PVC and other membranes, this contamination can cause the membrane to deteriorate prematurely. These problems are easily corrected through the installation of a low-profile drainage channel, which can be run from the HVAC unit to the gutter or drainage system. Such a channel, constructed of a chemically resistant PVC-blended material that is laminated to a spun-polyester fleece backing, is adhered to the surface of the membrane. The channel is designed to carry away the condensate and is easily cleaned by sweeping and by rainfall, making it a low-maintenance item.