Thermoplastic materials do not cross-link, or cure, during manufacturing or during their service life. Field-fabricated seams are typically welded with robotic hot-air welders. Hand-held, hot-air welders are used to weld seams at flashings and penetrations. Thermoplastic membrane seams are typically extremely reliable when properly installed, resulting in a very low incidence of seam failures. These sheets are normally around 5 to 12 feet wide [1.5 to 3.6 m]. Some manufacturers weld the sheets together in the factory to form large sheets that are then welded together on the roof.

Primary membrane types in this category are:

Polyvinyl chloride (PVC): PVC membranes are among the oldest single-plies still available. If in contact with polystyrene insulation, the polystyrene will cause the plasticizers in the membrane to leach out. To avoid such membrane embrittlement, a separator sheet needs to be installed between the membrane and the polystyrene. To avoid membrane damage, a separator is also needed to isolate PVC from asphalt and coal tar products. The ballasted attachment method is not recommended because fine dust particles from the ballast or particulate fall-out from the atmosphere may leach plasticizers from the membrane. PVC membranes are available in a wide variety of colors and can even be printed with building names and logos. This membrane is sometimes selected for steep-slope roofs where a strong or unique color is desired. PVC is naturally a brittle material and must be modified with plasticizers to be suitable for roofing. Some early formulations of PVC suffered from plasticizer leaching out over time and experienced catastrophic failures. Select PVC membranes that have been manufactured for many years to verify their stability.

PVC and PVC alloys are resistant to animal fats and grease and are a good choice for roofs with kitchen exhausts.

PVC Alloys or Compounded Thermoplastics (also referred to as PVC blends): These membranes are related to PVC membranes. They are primarily compounded from PVC, but they have additional polymers that provide somewhat different physical properties. Only a very small number of manufacturers make these products. The primary types of membranes in this category are: copolymer alloy (CPA), ethylene interpolymer (EIP) and nitrile alloy (NBP).

Thermoplastic Polyolefin (TPO): TPO membranes are a relatively new roof membrane in the commercial roofing market and have seen several reformulations in the past decade. They are typically white in color and, as a thermoplastic, the seams are heat welded. Since they are new, long-term performance is unknown at this time.

TPO is the latest thermoplastic membrane introduced into the marketplace. It was commercialized in North America in the early 1990s. It is formulated from polypropylene, polyethylene or other olefins. Unlike PVC and PVC blends, TPO membrane do not rely upon plasticizers for flexibility, so embrittlement due to plasticizer loss is of no concern. TPO membranes are typically white, and are available in sheet widths up to 12′ [3.6 m]. NRCA suggests designers specify 60-mil-thick or thicker TPO membranes.

Ketone ethylene ester (KEE): This membrane is also referred to as a tripolymer alloy (TPA), and the polymer is known by the trade name of Elvaloy. KEE sheets are similar to PVC.