NRCA Thermoplastic key terms

A single-ply roofing sheet made from polymers that soften when heated and harden on cooling, allowing seams to be fused by heat, solvent, or hot-air welding rather than glued.

Thermoplastic polyolefin, a flexible single-ply membrane formulated from polypropylene and ethylene-propylene rubber, commonly installed in white or light colors for reflectivity and heat-welded at seams.

Polyvinyl chloride, a thermoplastic single-ply membrane plasticized for flexibility, joined by solvent or heat welding and available in a range of colors and thicknesses.

A seaming method in which a hand-held or automatic welder directs heated air between overlapping membrane plies, softening them so they fuse under roller pressure into a continuous bond.

A joining technique for PVC membrane in which a solvent or solvent-based adhesive temporarily softens the sheet surfaces so overlapping plies bond as the solvent evaporates.

The fused overlap joint between two membrane sheets, produced by heat or solvent, whose continuity and strength are critical to the watertightness of the roof assembly.

A destructive seam check in which a rounded probe is worked along the seam edge to detect unbonded channels or voids, with samples later patched using approved membrane and seaming methods.

A field or laboratory seam evaluation in which the bonded plies are pulled apart at a controlled rate to measure the force required to separate them and to reveal the failure mode.

The load a finished weld or solvent joint can carry before separating, verified through peel or shear testing and dependent on temperature, speed, pressure, and surface cleanliness.

A hand or powered tool with a weighted roller that presses freshly welded membrane plies together while the material is still soft, consolidating the bond and expelling trapped air.

The distance one membrane sheet extends over an adjacent sheet, measured perpendicular to the seam edge, which provides the material width needed for a properly fused joint.

The point where the roof membrane ends and is anchored, typically at a wall, curb, or edge, using a termination bar, fastener, or reglet to secure and seal the edge.

A metal strip fastened over the top edge of membrane flashing to clamp it to the substrate, usually sealed with a compatible sealant and often covered by a counterflashing.

Membrane or accessory components used to waterproof transitions, penetrations, walls, curbs, and edges where the field membrane cannot simply continue across a plane.

The lower flashing component that extends from the roof surface up a vertical substrate, bonded to the field membrane at its base and secured near its top edge.

A metal or membrane component installed over the top of base flashing and secured to the wall or curb, shedding water away from the flashing termination.

Any pipe, duct, post, or other object passing through the roof surface, requiring a curb, boot, or prefabricated flashing detail to maintain watertightness.

A prefabricated metal or membrane pan set around a penetration and filled with sealant or grout, used to protect and seal the flashing around the object.

A prefabricated flexible flashing sleeve, often conical or cylindrical, that fits over a pipe penetration and is welded or clamped to the field membrane to seal the joint.

A roof attachment method in which membrane or insulation is secured to the deck with fasteners, plates, or battens rather than being adhered or weighted down.

A mechanized attachment technique in which an electromagnetic field heats a coated metal plate beneath the membrane, fusing the sheet to the plate without penetrating it.

A round or rectangular metal or plastic plate used with fasteners to spread holding force over a wider area of insulation or membrane and resist wind uplift.

A screw, nail, or anchor used to secure membrane, insulation, or accessories to the roof deck or structural substrate, selected for length, corrosion resistance, and pullout capacity.

A roof attachment method in which the membrane is bonded continuously to the substrate with an approved adhesive or by heat fusion across its entire underside.

A bonding agent, such as a solvent-based, water-based, or low-rise foam product, applied between membrane and substrate or between plies to create a adhered assembly.

An adhesive formulated specifically to bond thermoplastic membrane to substrates or insulation, requiring compatible chemistry and proper open time to develop full strength.

A roof attachment approach in which the membrane is held down by gravel, pavers, or other weights instead of fasteners or adhesive, relying on mass to resist wind uplift.

The upward pressure exerted on a roof assembly by wind flowing over and around the building, resisted by attachment method, edge securement, and perimeter detailing.

The structural substrate, such as steel, concrete, wood, or cementitious board, that supports insulation and membrane and receives fasteners or adhesive.

Rigid board installed over the deck to provide thermal resistance, also affecting attachment strategy, fastener length, and the flatness of the finished membrane surface.

A rigid board placed between insulation and membrane to improve load distribution, protect the membrane from substrate irregularities, and enhance fire or impact performance.

A low-permeance layer installed in the roof assembly to limit moisture vapor movement from the building interior into insulation, reducing condensation risk.

A survey using infrared thermography, capacitance, or nuclear methods to locate wet insulation or trapped moisture within an existing roof assembly before or after installation.

The fraction of incoming solar energy reflected by a roof surface, a key cool-roof property often expressed with initial and aged values for thermoplastic membranes.

The ability of a roof surface to radiate absorbed heat back to the sky, paired with solar reflectance in cool-roof ratings and codes.

A roof surface designed with high solar reflectance and high thermal emittance to lower surface temperature, reduce cooling load, and mitigate urban heat island effects.

The movement of plasticizers out of PVC membrane into adjacent materials such as insulation or bitumen, which can stiffen the sheet or degrade incompatible components.

Standing water on a roof that remains more than a set period after rain, indicating insufficient slope or deflection and increasing load and membrane stress.

The incline of the roof surface, expressed as a ratio or percentage, which governs drainage and influences membrane, insulation, and attachment detailing requirements.

Rigid board cut to varying thicknesses and laid out to create slope toward drains, used to improve drainage on otherwise flat roof decks.