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4/14/2009 12:00:00 AM
Compiled by FacilitiesNet Staff
Single ply roofing products were born out of the oil crisis of the 1970s, which led to lightweight, flexible roof membranes. Today, a variety of single ply roofing products exist.
Among the common single ply roofing products are PVC, EPDM, TPO and KEE. All these changes mean that selecting single-ply roofing needs to be done with care and forethought. Here is a brief overview of the leading single ply roofing products on the market today.
Single-ply membranes are usually broken down into two subgroups — thermoplastics and synthetic rubber. (Modified bitumen membranes are virtually never installed as single-ply membranes.) Thermoplastics have a common characteristic not found in synthetic rubbers — they can be heated and reshaped or melted multiple times. Because of this, the most common method of seaming a thermoplastic is by heat-welding the membrane. Properly melting the edges together fuses the membrane into a strongly bonded seam.
There are two major thermoplastic membranes currently on the market. These are polyvinyl chloride (PVC) and thermoplastic polyolefin (TPO). A third, KEE, is manufactured by only one company. Although its use is limited, the membrane is notable for being able to pass the test of being hit with a two by four shot from an air cannon.
PVC roof membranes have the longest track record of any thermoplastic membrane, with the first PVC-based systems installed in Europe in the early 1960s. Vinyl membranes are inherently self-extinguishing, which enables them to earn fire ratings from Underwriters Laboratories and Factory Mutual and to perform reliably in real-world flame exposure.
The finished vinyl roof membrane contains polyester or fiberglass reinforcement, vinyl resins, ultra-violet light inhibitors, heat-stabilizers, biocides, pigments and plasticizers. Polyester reinforcement imparts high tearing and breaking strengths needed for mechanically fastened roofing systems. Because PVC is not naturally flexible, plasticizers are added to the formulation. In past years, the loss of the plasticizers in un-reinforced membranes caused catastrophic failures as the membrane reverted to its inflexible state and shattered during cold weather. Now, all PVC membranes are reinforced, and new formulations minimize the loss of plasticizers from the membrane.
TPO membranes have become widely used as roof membranes in the past ten years. A TPO roofing membrane is typically made from polypropylene and ethylene-propylene (EP) rubber polymerized together using state-of-the-art polymer manufacturing technology. This technology enables the production of TPO membranes that are flexible at low temperatures without the use of polymeric or liquid plasticizers.
Unlike some other popular thermoplastic roofing membranes, the TPO polymer does not contain chlorine and no chlorine-containing ingredients are added during sheet production.
The TPO resin is compounded with other components, including a weathering package, fire retardants and pigments, to create a product that can withstand the elements associated with rooftop exposure. TPO itself is not fire resistant and requires the addition of fire retardants to obtain a fire rating. The ratios of weathering material and fire retardants are still inconsistent from manufacturer to manufacturer. Because TPO membranes are inherently flexible, unlike PVC membranes, additional plasticizers are not required in the formulation so there is little danger of plasticizer migration.
Synthetic Rubber: EPDM
Virtually all synthetic rubber roof membranes are EPDM. Synthetic rubber is a thermoset, which means once it has cured, it cannot be melted and reshaped. Consequently, the rubber membrane is seamed by an adhesive — generally either a field-applied contact cement or an adhesive tape that is applied in the field or factory and activated by removing a release paper. Tapes have become the de facto standard as they are generally easier to apply, reducing the chances they will be installed incorrectly.
EPDM membrane rolls can come as wide as 10 feet. Wider rolls require fewer seams. Fewer seams means less chance of seam failure. Because of the roll sizes, EPDM is often used on buildings with very large roofs. EPDM can stretch and relax, so it can easily accommodate thermal expansion and contraction on large expanses of roof.
Like PVC, EPDM roof membranes have been in production for more than 30 years. The systems have gone through several enhancements over time — reinforcing has been added, new seaming and flashing methods and materials have been developed, and white membranes and clean sheets that minimize on-site dust contamination have been introduced — but the basic product is still the same and performs well.
The majority of thermoplastics come in white with other pastel colors available. These white membranes give an initial high reflectance value that can help keep the temperature of the roof and consequently the inside of the building cooler. As the roof becomes older and collects dirt, however, this initial value will decrease unless the roof is periodically washed. Synthetic rubber is available with a white surface, but the more durable membrane is a dark grey color. There are white acrylic coatings for EPDM membranes that can give the same reflectance as the thermoplastics and are advertised as increasing the longevity of the roof by as much as 5 to 10 years.