The oil crisis of the 1970s clearly showed the need for alternatives to petroleum-based roofing. To answer this need, a plethora of lightweight flexible roofing membranes came on the market. They were easy and inexpensive to install and came with an alphabet soup of acronyms — PVC, EPDM, CPE, CPA, PIB, CSPE, NBP, EIP, etc.
Fast forward to the twenty-first century. After decades of revised formulations, changing environmental regulations and stiff competition, the alphabet soup of single-ply acronyms has been pared down to the most commonly heard: PVC, EPDM, TPO and KEE. To add to the mix, new methods of installation have been developed. All these changes mean that selecting single-ply roofing needs to be done with care and forethought.
Before a facility executive begins weighing the question of which single-ply system to use, it is important to ask whether the building is suited for a single-ply membrane. “One size fits all” is as misleading for roofing as it is for clothing. Here are some situations where a single-ply membrane is appropriate:
- When weight is a consideration.
- When some movement may be expected in the building structure.
- When re-covering an existing membrane.
- When the building is wide open and large panels can be used.
- When reroofing structural standing seam metal roof assemblies.
- When roofing in cold weather (with some precautions).
- When installing a green roof.
- When a light-colored system is desired.
Times when a facility executive may want to consider other options include:
- When vandalism or puncture-abuse is a factor.
- When there is a limited number of local approved applicators.
- When there is a chemical incompatibility between the existing roof and the proposed re-cover membrane.
- When climatic or environmental conditions are not appropriate.
- Where insurance considerations favor other types of systems.
- Where installation is over lightweight insulating concrete decks.
- Where local building code officials prohibit use of particular types of membranes.
Except for the last item, none of the above are hard and fast rules. They are issues that need to be addressed before choosing a particular type of roof system, regardless of whether it is asphalt-based, thermoplastic or synthetic rubber.
Thermoplastic Membranes: PVC and TPO
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 and therefore are beyond the scope of this article.) 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.
Both types of single-ply membranes use the same installation methods. They are either mechanically attached, fully adhered using some type of adhesive, or laid out loose and held in place by the weight of something installed over them. Each installation method has advantages in certain situations.
Mechanically attached systems are best used when weight is a consideration, when the system is to be installed during cooler weather or when a roof is being re-covered. Structural concrete decks are not a good choice for mechanically attached systems. Installing fasteners into the concrete can create problems in the future as the roof ages and allows water to penetrate to the deck. The fasteners can rust, spalling the concrete.
Steel is an excellent choice for mechanically attached systems, especially when installing a new roofing system over a structural standing seam metal building, because the mechanical attachment is light weight and less likely to overload the highly engineered weight limits of the building. Wood and cementitious wood fiber are also very good decks for mechanically fastened systems.
Mechanically attaching a single-ply membrane to a lightweight insulating concrete deck can mean drilling through the lightweight into the steel deck, which negates the use of the lightweight insulating concrete. There are modified bitumen systems with base sheets and base sheet fasteners designed specifically for lightweight insulating concrete that do not require drilling into the steel deck and therefore are much less expensive to install.
When selecting a mechanically attached system, one must also be aware that the membrane may show some fluttering in the wind. This is normal.
Thermoplastic mechanically attached systems lend themselves to cool-weather installation as there are few solvents and no hot asphalt to be concerned with. However, as in all cases of thermoplastic membranes, the temperature of the heat welding of the seams is critical to getting a good bond and cool weather must be taken into consideration when welding.
Mechanically attached EPDM is a good candidate for cool-weather installation if tape is used as the seam adhesive. Contact-adhesive seams can be used if the adhesive is kept warm until just before it is applied and the longer set-up time for the contact adhesive is taken into consideration.
Fully adhered systems are best used when the roof cannot be easily attached by means of screws or other types of fasteners, such as over a structural concrete deck. In this case, adhering the insulation with an adhesive and adhering the membrane to the insulation with adhesive minimizes damage to the deck.
Fully adhering the membrane also helps keep the roof in place in high winds because wind cannot get under an adhered membrane as easily as a mechanically attached one. When wind gets between the deck and the membrane, it adds a positive pressure to the negative pressure of the wind flowing over the membrane, increasing the risk that the roof will be pulled off the building. Fully adhering a single-ply membrane will also eliminate the flutter that can occur in a mechanically attached system.
Adhered membranes are usually installed with proprietary adhesives for the thermoplastic membranes and with contact adhesive for EPDM systems. If a fully adhered system is chosen, the adhesive type needs to be evaluated to ensure it is suitable for the specific type of system used.
Some single-ply membranes may be specified with a fleece backing. The fleece may allow for application of the membrane in hot asphalt. This can assure compatibility with the underlying roof system in a re-cover over a built-up or modified bitumen system. It is also useful when the only way of adhering insulation is hot mopping it to the deck.
The fleece backing also makes the membrane thicker and more stable. This is especially helpful when specifying applied ribs when installing a PVC roof that looks like a standing seam metal roof. The fleece will create fewer waves in the membrane when the ribs are heat-welded on.
Ballasted systems are a good choice when the installation is to be a protected membrane roof where the membrane is installed under the insulation. Ballasting is also a good choice for green roofs as the growing medium — usually dirt — can help keep the roof in place if it is thick enough. A ballasted system is also a good method of providing a plaza deck walking surface as the pavers provide the weight to keep the membrane in place. Care must be taken with ballasted systems to be sure ahead of time that the structural system of the building is capable of holding the overburden (pavers, stone, soil) without collapsing. Structural concrete is usually able to do so. All buildings should be thoroughly evaluated by a structural engineer prior to installing this kind of a system.
Although single-ply membranes can be less costly than other systems, a roof system should never be chosen on cost alone. The choice should be guided by a determination of which roof best fits the needs of the building.
For all types of roofing systems, a poor choice can lead to a costly disaster. With all the choices, it can be difficult to decide which single-ply to use. Building codes, regulations and insurance requirements make the choice even more difficult. But if roofing needs are evaluated carefully and products selected thoughtfully, single-ply membranes can provide a long lasting roof system at a reasonable cost.
Karen L. Warseck, AIA, is a contributing editor of Building Operating Management. She is president of Building Diagnostics Associates, a Hollywood, Fla., architecture firm that specializes in analysis of problems and design of repairs for roofing and exterior wall systems. Warseck has more than 25 years of experience in the roofing industry.