Roofing Dictionary


Abrasion Resistance: The ability of a membrane to resist being worn away by contact with a moving, abrasive surface, such as foot traffic, mechanical equipment, wind-blown particles, etc.

Accelerated Weathering: The process in which materials are exposed to a controlled environment where various phenomena, such as heat, water, condensation and light are altered to magnify their effects, thereby accelerating the weathering process.

Adhere: To cause two surfaces to be held together by adhesion. Flexible membranes are often partially or totally adhered to a substrate with the use of contact cement or other similar adhesives.

Adhesion: The combined ultimate strength of the molecular forces and the mechanical interlocking achieved between the adhesive and the surfaces bonded.

Adhesive: A substance capable of holding materials together by surface attachment. These substances may be used to adhere or attach various roofing materials and components, such as membranes, flashings insulation boards, etc.

Aggregate: Crushed stone, crushed slag or water worn gravel used for surfacing a built-up roof; any granular mineral material.

Albedo: The fraction of solar radiation reflected by a surface.

Alligatoring: Alligatoring is term used to describe the cracking of surfacing bitumen on a built-up roof. These cracks are the result of the limited tolerance of asphaltto thermal expansion or contraction, and produce a pattern that resembles analligator’s hide.

Alloy: Any combination of two or more chemically different polymers which have been reformed through processing into a new material from which the original materials cannot be separated.

Application Rate: The quantity (mass, volume or thickness) of material applied per unit area.

Area Divider: A raised, double wood member attached to a properly flashed wood base plate that is anchored to the roof deck. It is used to relieve thermal stresses in a roof system where expansion joints have been provided.

Aromatic Hydrocarbon: A hydrocarbon compound characterized by a molecular structure involving one or more of six carbon atom rings (benzene rings).

Asbestos: A group of natural, fibrous, impure silicate materials.

Asphalt: A dark brown to black material in which the predominating constituents are bitumens, which occur in nature or are obtained in petroleum processing.

Asphalt, Air Blown: Asphalt produced by blowing air through molten asphalt at an elevated temperature to raise its softening point and modify other properties.

Asphalt, Felt: Asphalt saturated felt or an asphalt coated felt.

Asphalt, Mastic: A mixture of asphaltic material and graded mineral aggregate that can be poured when heated but requires mechanical manipulation to apply when cool.

Asphalt, Steam Blown: asphalt produced by blowing steam through molten asphalt to modify its properties.

Asphaltene: A high molecular weight hydrocarbon fraction precipitated from asphalt by a designated paraffinic naphtha solvent at a specified temperature and solvent-asphalt ratio. Note: The asphaltene fraction should be identified by the temperature and solvent asphalt ratio used.

ASTM: American Society for Testing Materials. ASTM provides a management system in which voluntary consensus standards may be developed

Atatic Polypropylene: A high molecular weight polymer formed by the polymerization of polypropylene. Typically used as a modifier of asphalt in modified bitumen membranes.


Backnailing: The practice of blind nailing roofing felts to a substrate in addition to hot-mopping to prevent slippage.

Ballast: An anchoring material, such as aggregate or precast concrete pavers, which employ the force of gravity to hold (or assist in holding) sheet membranes in place.

Base Ply: The lowermost ply of roofing material in a roof membrane assembly.

Base Sheet: A saturated or coated felt placed as the first ply in some multi-ply built-up roof membranes.

Bitumen: The generic term for an amorphous, semi-solid mixture of complex hydrocarbons derived from any organic source. Asphalt and coal tar are the two used in the roofing industry.

Bituminous: Containing or treated with bitumen. Examples: bituminous concrete, bituminous felts and fabrics, and bituminous pavement.

Blister: A spongy raised portion of a roof membrane, ranging in area from 1 inch in diameter and of barely detectable height upward to large areas of several square feet. Blisters result from the pressure build up of gases entrapped in the membrane system. These gases most commonly are air and/or water vapor but may also be from solvent vapors. Blisters usually involve delamination of the underlying membrane plies and are usually deleterious when at a lap.

Bond: The adhesion of a membrane to itself or its substrate achieved through the use of an adhesive or other bonding agent.

Bonding Agents: Essentially synonymous with adhesives.

Built-up Roof Membrane: A continuous, semi-flexible roof membrane assembly, consisting of plies of saturated felts, coated felts, fabrics or mats between which alternate layers of bitumen are applied, generally surfaced with mineral aggregate, bituminous materials, or a granule-surfaced roofing sheet. (Abbreviation: BUR.)

Butt Joint (Butt Splice): A joint or seam formed by joining separate sections of membrane at the edges without overlap (i.e. edge to edge). Once the edges have been joined, the seam is usually covered and sealed with a narrow strip of membrane material or compatible tape.

Butyl: A rubber material produced by copolymerizing isobutylene with a small amount of isoprene. Butyl is variously manufactured into sheet goods, blended with other rubber materials and is often used to make sealants and adhesives.


Calendering: A manufacturing process by which polymeric membranes and sheeting are produced. The finished material is formed by passing it between the nips of a series of large counter-rotating steel rollers which produce a film or sheet of uniform thickness.

Cant Strip: A beveled strip of wood, or wood fiber or perlite board that fits into the angle formed by the intersection of a horizontal surface and a vertical surface. The 45-degree slope of the exposed surface of the cant strip provides a gradual angular transition from the horizontal surface to the vertical surface.

Cast Sheeting: A manufacturing process in which a liquid is poured into a mold, cured and removed from the mold. Cast films are also made by depositing the material, either by solution or in a hot melt form, against a highly polished supporting surface.

Caulking: A composition of vehicle and pigment, used at ambient temperatures for filling joints, that remains plastic for an extended time after application.

Chlorinated Polyethylene (CPE): A thermoplastic material used for flexible membranes which is composed of high molecular weight polyethylene which has been chlorinated – a process which yields a flexible rubber-like material.

Chlorosulfonated Polyethylene (CSPE): A synthetic, rubber-like thermoset based on high molecular weight polyethylene with pendant sulphonyl chloride groups, usually formulated to produce self-vulcanizing membrane.

Coal Tar Pitch: A dark brown to black, semi-solid hydrocarbon formed as a residue from the partial evaporation or distillation of coal tar. It is used as the waterproofing agent in dead-level or low-slope built-up roofs. (For specification properties, see ASTM Standard D 450, Types 1and II.)

Coeffivient of Thermal Expansion: The change in length of a material as a function of temperature. A dimensionless coefficient, it is usually reported in inches per inch x °F or millimeter x °C.

Cold Adhesive: A material, often modified asphalt, used to bond modified bitumen sheets together or to a substrate.

Color Stability: The ability of a material to retain its original color, even after long exposure to strong sunlight and/or other harsh environmental conditions, including air pollutants, acid rain, extremes of temperatures, etc. Color stability may be especially important for white specifically pigmented materials which may have been deliberately selected for their high degree of reflectivity or aesthetic effect.

Compression: The squeezing or pressing together by the application of pressure, as in the pressure applied to roofing materials and components by the weight of mechanical loads, foot traffic, etc.

Compressive Strength: The ability of roofing materials and components to resist deformation or other damage caused by the weight or compression of either “live” or “dead” loads. High compression strength may be especially important in insulation boards.

Condensation: As it relates to water vapor, and the build-up of water on the surface of roofing and building components, condensation is the change from water vapor to liquid water, resulting from a drop in the temperature of an air/water vapor mixture below the “dew point” of that mixture. There are basically two types of condensation to consider:

1) concealed condensation – that which takes place within a roof (i.e. between the components of the roofing system) and is largely unseen, and 2) surface condensation – that which appears in the colder exposed surface of a roof, and is easily seen. Contact Elements: Adhesives which may be used to adhere or bond together various roofing components. The adhesive is applied to the last surfaces to be joined in a liquid state, and then allowed to dry before the surfaces are mated. The bond is formed immediately as the surfaces touch. Because contact cement forms a bond immediately upon mating the surfaces, great care must be taken to assure that the membrane is positioned properly. Any attempt to lift or reposition a misaligned and cemented membrane could result in damage to the membrane or in poor adhesion.

Cool Roofs: A generic term applied to roofs that reflect sunlight. These roofs have high albedo or reflectivity greater than 50% for low slope roofs and 25% for steep roofs.

Cool Roof Rating Council: The Cool Roof Rating Council (CRRC) is a volunteer organization dedicated to the development of test standards for reflective roofs and fair equitable and consistent reporting of reflective and emissive v data for roofing products.

Cool Roof Surfaces: A roof system that substantially reduces the heat gain caused by the sun’s energy radiated at the building. This may be accomplished by the following methods, but is not limited to these:                 – A highly reflective surface                 – Moisture transpiration through plants               – Mass with high emissivity

Use of highly reflective surfaces is recognized as one of the key methods for substantially reducing the heat gain in a building. These highly reflective surfaces consist of the following types, but are not limited to them, and may use other possible methods:                 Membrane sheet products designed to be highly reflective                 – Coating applied in the field to produce the highly reflective surface                 – Coats that are factory applied to membranes or metal panels                 – Special infrared (IR) reflective materials added to the formula to allow non-white colors to be highly reflective

Coping: The covering piece placed on top of a wall that is exposed to the weather. It is usually sloped to shed water.

Copolymerization: A chemical reaction that results in the bonding of two or more dissimilar monomers to produce large, long-chain molecules which are co-polymers.

Counter flashing: Formed metal or elastomeric sheeting secured on or into a wall, curb, pipe, rooftop unit or other surface to cover and protect the upper edge of a base flashing and its associated fasteners.

CPA: Abbreviation for copolymer alloy.

CPE: Abbreviation for chlorinated polyethylene.

Creep: The gradual movement (i.e. deformination or stretching) of a roofing membrane or other roofing material caused by a mechanical loading or gravity. Excessive creep, especially in improperly anchored membranes, can result in the permanent deformation of the membrane material.

Crosslink: A chemical phenomenon by which polymers are cured or vulcanized. A crosslink is a chemical bond formed between the long chain molecules in the block polymer. This bond connects adjacent molecules and prevents their relative displacement (molecular slippage) when the material is stressed.

CSM: The designated nomenclature for chlorosulfonated polyethylene by ASTM*D-1418.

CSPE: Abbreviation for chlorosulfonated polyethylene.

Curb: A raised member used to support roof penetrations, such as skylights, mechanical equipment, hatches, etc. above the level of the roof surface.

Cure: A process whereby a material is caused to form a permanent molecular linkage by exposure to chemicals, heat or pressure.


Dead Level Asphalt: A roofing asphalt that has a softening point of 140F (60C) and that conforms to the requirements of ASTM Standard D 312, Type 1.

Dead Level: The term used to describe an absolutely horizontal roof. Zero slope. (See SLOPE.)

Dead Loads: Permanent, non-moving rooftop loads that result from the weight of structural components, mechanical equipment, heating, ventilation, and air-conditioning exhausts, sprinklers, etc. as well as the various materials and components of the roof system itself. Essentially the same as “dead weight” or dead weight loads.”

Deck: The structural surface to which the roofing or waterproofing system (including insulation) is applied.

Delamination: Separation of the plies in a roof membrane system or separation of laminated layers or facers of insulation.

Department of Energy: The Department of Energy’s (DOE) overarching mission is to advance the national, economic and energy security of the United States; to promote scientific and technological innovation in support of that mission; and to ensure the environmental cleanup of the national nuclear weapons complex.

Dewpoint: The temperature at which an air mixture is saturated with water vapor. If the temperature drops below the dew point, condensation will occur.

Dimensional Stability: The change in length and/or width of a material that results from exposure to elevated temperatures over time. Expressed as a percentage.

Disc Fasteners: A wide variety of devices of mechanical assemblies used to attach membranes and/or insulation boards to a substrate or deck. Disc attachments generally consist of a square or circular shaped plate with a hole in the center through which a screw or nail-like clip may be inserted. They are generally set in place with a drill-like device.

DOE: Abbreviation for the Department of Energy.

Downspout: A pipe for draining water from roof gutters. A downspout is also called a leader.

Drain: An outlet or other device designed to capture and/or direct the flow of water from a roof. Without effective roof drainage, rainwater and/or water from refrigeration units and storage tanks, etc. could pond adding considerable weight to the roof system. In cold weather, the trapped water could freeze, potentially causing a loss of adhesion or damage to the roofing materials and components.


Elasticity: The property of matter by which it ends to return to its original size and shape after removal of the stress which causes a deformation.

Elastomer: Any natural or synthetic macromolecular material which, at room temperature, can be stretched under low stress and, upon immediate release of the stress, will return with force to its approximate original length.

Elastomeric: A term generally used to describe the elastic rubber-like properties of a material.

Elastomeric Membranes: A broad group of sheet materials which possess elastomeric or elastic rubber-like properties. Elastomeric materials and membranes may be manufactured from a variety of polymers.

Elongation: The ability of a roofing material to be stretched or elongated by the application of a force.

Embrittlement: The loss of flexibility, elasticity or ductility of a material. The transition of a flexible material into a brittle material.

Environmental Protection Agency (EPA): An agency of the federal government of the United States charged with protecting human health and with safeguarding the natural environment.

Energy Star®: Energy Star is a government-backed program helping businesses and individuals protect the environment through energy efficiency.

EP: Abbreviation for Ethylene Propylene.

EPA: Abbreviation for Environmental Protection Agency.

EPDM: Abbreviation for Ethylene Propylene Diene Terpolymer Monomer.

EPDM Lap Sealant: A material used to caulk the exposed edges of field seams of EPDM membranes.

Ethylene Interpolyers (EIP): A group of thermoplastic compounds generally based on PVC from which flexible roofing membranes can be formulated.

Ethylene/Propylene: A copolymer of polyethylene and polypropylene. See also TPO.

Ethylene Propylene Diene Terpolymer (EPDM): An elastomeric material synthesized from ethylene, propylene and small amounts of diene monomer. EPDM is widely used in flexible roofing membranes. It may also be used alone or in EPDM/butyl blends.

Expansion Joint: A structural separation between two building elements designed to minimize the effect of the stresses and movements of a building’s components and to prevent these stresses from splitting or ridging the roof membrane.

Extrusion: A manufacturing process which consists of forcing molten polymers through an orifice called a “die.” The shape and dimensions of the die orifice determine the shape and dimensions of the finished product. Extrusion is one method by which flexible roofing membranes may be manufactured.


Fabric: Different kinds of woven or knitted cloths of treated or non-treated organic or inorganic fibers used for reinforcement in certain membranes, assemblies and flashing materials.

Factory Mutual (FM) Global: An insurance company organization that tests and approves classifies roof assemblies for their fire, hail, foot traffic resistance characteristics as well as wind-uplift resistance for insurance companies in the United States.

Factory Seam: A splice made by the manufacturer during the assembling of narrow width material into large sheets.

Factory Seam Strength: The force required to cause a seam (created by the membrane supplier) to fail in peel or shear. Expressed in units of force or force per unit area, or as a percentage of the strength of the sheet itself.

Fasteners: Any of a wide variety of mechanical fastening devices and assemblies, including clips, screws or bolts, which may be used to secure battens, discs, termination bars and wood nailers to the deck or other suitable substrate.

Felt: A fabric manufactured from cellulosevegetable fibers (organic felts), asbestos fibers (asbestos felts), or glass fibers (glass fiber felts). The manufacturer process involves mechanically interlocking the fibers of the particular felt material in the presence of moisture and heat.

Ferrule: A small metal sleeve placed inside a gutter at the top. A spike is nailed through the gutter into the fastening board. The ferrule acts as a spacer in the gutter to maintain its original shape.

Field Seam: A splice made in the field which joins two sheets together using an adhesive, splicing tape or heat or solvent welding.

Fishmouth: Membrane defect consisting of an opening in the edge lap of a felt in a built up membrancemembrane; a consequence of an edge wrinkle.

Flammability: The characteristics to burn or support combustion. Although some flexible membranes possess a considerable degree of inherent fire resistance, all organic materials will burn under the right condition of heat and oxygen supply. They should not therefore be exposed to direct flame or extreme heat.

Flashing: Components used to weatherproof or seal the roof system edges at the perimeters, penetrations, walls and other places where the r oof covering is interrupted or terminated.

Fleece: Mats of felt composed of fibers (usually non-woven polyester fibers) used as a membrane backer or separator sheet.

Flexible Membranes: Roofing membranes that are field applied using prefabricated sheets of membrane material (either homogeneous or composite).

Flexible Membrane Roofing: A roofing system in which the principal roofing component is a flexible membrane of thermoset, thermoplastic or modified bituminous compounds.

FM: Abbreviation for Factory Mutual.


Glass Fiber Mat: A thin mat of glass fiber bonded into a sheet with a resinous binder. This mat serves as reinforcement to the membrane.

Gravel: Coarse, granular aggregate containing pieces approximately 5/8 inch to 1/2 inch in size and suitable for use as a surfacing embedded into a flood coat of bitumen in aggregate on built- up roofs.

Gravel Stop: A flange device, frequently metallic, designed to provide a continuous finished edge for roofing materials and to prevent loose aggregate form washing off of the roof.


Hardness: The ability of a membrane or other material to resist indentation resulting from pressure or impact.

Heat Aging: Controlled exposure to elevated temperatures over time.

Heat Welding: Method of melting or fusing together the overlapping edges of separate sheets of thermoplastics and polymer modified bitumens.

HVAC: Abbreviation for Heating, Ventilation and Air Conditioning.

Hydrocarbon: Any of a large number of organic compounds which are based primarily on carbon and hydrogen atoms, such as ethylene, benzene, etc. Petroleum and petroleum products are mixtures of numerous hydrocarbons.

Hygroscopic: A term which describes a membrane or other roofing material or component which attracts, absorbs or retains moisture from the air.

Hypalon: A registered trademark of E.I. Dupont de Nemours, Inc. for chlorosulfonated polyethylene.


ICBO: Abbreviation for International Conference of Building Officials.

ICC: Abbreviation for International Code Council.

ICC-ES: Abbreviation for International Code Council Evaluation Service.

Impact Resistance: The ability of a roofing material to resist damage (e.g. puncturing) from falling objects, application equipment, foot traffic, etc.

Impregnate: A term used to describe the process of coating, saturating or surround the fibers of a reinforcing mat or fabric with an elastomeric or other enveloping material.

Infrared Thermography: A practice of roof analysis where an infrared camera is used to measure the temperature differential of a roof surface to locate areas of underlying moisture.

Inorganic: Any chemical or compound which is derived from minerals, does not contain carbon and is not classified as organic; being or composed of materials other than hydrocarbons and their derivatives; not of plant or animal origin.

Insulation: Any of a variety of materials designed to reduce the flow of heat, either from or into a building. Insulating materials are generally installed either just below or immediately above the roof membrane, depending on the roofing system employed. Currently, rigid or semi-rigid boards or panels of extruded or expanded polystyrene, polyisocyanurate, fiberboard and various composite insulations are among the most popular and widely used insulating materials in a flexible membrane roofing system.

International Code Council: The International Code Council (ICC) is a membership organization dedicated to building safety and fire prevention, develops the codes used to construct residential and commercial buildings, including homes and schools.

International Code Council – Evaluation Service (ICC-ES): is the United States’ leader in evaluating building products for compliance with code. A nonprofit, public-benefit corporation, ICC-ES does technical evaluations of building products, components, methods and materials. The evaluation process culminates with the issuance of reports on code compliance, which are made available free of charge to code officials, contractors, specifiers, architects, engineers and anyone else with an interest in building industry and construction. ICC-ES evaluation reports provide evidence that products and systems meet code requirements.

Inverted Roof Membrane Assembly (IRMA™): A patented variation of the Protected Membrane Roof Assembly, in which Styrofoarm® Brand Insulation and ballast are placed over the membrane.

IRMA™ and Styrofoam® : are registered trademarks of the Dow Chemical Company.

Isocyanurate Foam: Thermoset plastic expanded with blowing agents to form a foam insulation. Typically used as a board stock with facers to provide thermal insulation and a substrate for roof membrane materials.


Joists: Any of the small timbers or metal beams ranged parallel from wall to wall in a structure to support a floor or ceiling.



Laminate: To bond together, usually with the application of heat and/or pressure, two or more layers of plies of a material to make a finished product. Scrim, fibers or mats may be introduced between the two components being laminated to serve as reinforcement in the finished sheets.

Lap: That part of a roofing membrane which overlaps or covers any portion of another section of membrane which is then sealed to form a watertight connection.

Leadership in Energy and Environmental Design (LEED): The Leadership in Energy and Environmental design Green Building Rating Systemâ„¢ is the nationally accepted benchmark for the design, construction and operation of high performance green buildings. LEED gives building owners and operations the tools they need to have an immediate and measurable impact on their buildings’ performance. LEED promotes a whole-building approach to sustainability by recognizing performance in five key areas of human and environmental health: sustainable site development, water savings, energy efficiency, materials selection and indoor environmental quality.

LEED: Abbreviation for Leadership in Energy and Environmental Design

Live Loads: Temporary loads which the roof system must be designed to support, as required by governing building codes. Live loads are generally moving and/or temporary, such as people, installation equipment, wind, snow, ice or rain.

Loosely Laid Membranes: Membranes which are not attached to the substrate except at the perimeter of the roof. Loosely laid membranes are held in place with appropriate and adequate ballast, such as round river washed stone, gravel, pavers, etc. This assembly may only be used on roof struts able to support the added weight of the ballast, which is generally applied at the rate of 10 pounds per square foot of roof area.

Low Temperature Flexibility: The ability of a membrane or other material to remain flexible (resist cracking when flexed), after it has been cooled to a low temperature. Low temperature flexibility is important, especially in a membrane which is to be installed during the winter and in cold climate.

Low Temperature Resistance: The lowest temperature at which a material does not fracture or crack under prescribed impact and flexing conditions. Expressed in °F or °C.


Mastic: A sealant that has a “non-sag” consistency to prevent the material from flowing away from the join or surface to which it is applied. Mastics are usually applied using a standard caulking gun, trowel or knife.

Mat: A thin layer of woven, non-woven or knitted fiber which serves as reinforcement to the membrane.

MB: Abbreviation for Modified Bitumen.

Mechanically Fastened Membranes: Generally used to describe roofing membranes which have been positively attached at intervals to the substrate, usually with various fasteners and other mechanical devices such as battens. Mechanical fastening makes it possible to install membranes over certain substrates such gypsum or lightweight concrete fills, which may not accept adhesive or heavy ballasting. Mechanical fastening permits the membrane to float free between the fasteners, and allows greater movement between the membrane and the substrate than in adhered systems.

Membrane: A continuous flexible sheet of thermoset, thermoplastic or modified bituminous material, which functions as the weather- and water-proofing element of a roof assembly.

Metal Film: A layer of foil made from a single metallic substance or from an alloy. This foil, when used in a modified bitumen roofing membrane is laminated to the membrane at the factory. It serves as the weathering surface of the membrane, providing strength, reflectivity and ultraviolet protection.

Microbiological Resistance: The ability of a roofing membrane or other material to resist attach and degradation by various air- and soil-borne microorganisms. Typically, fungicides are added to certain compounds to render them not susceptible to microbiological decay.

Micron: A unit of linear measure equal to one millionth of a meter, or one thousandth of a millimeter. Often used to indicate the thickness of a very thin sheet or film (25,400 microns = 1/100 inch = 1 mil).

Mil: A unit of measure used to indicate the thickness of a roofing membrane. One mil is equal to .001 inches or 25,400 microns.

Millimeter: A unit of measure equal to one thousandth of a meter or 0.03937 inches.

Modified Bitumen: Composite sheets consisting of a polymer, e.g., atactic polypropylene (APP) or styrene butadiene styrene (SBS), often reinforced and sometimes surfaced with various types of mats, films, foils and mineral granules.

Modulus: A measure of a material’s stiffness. Since polymeric materials do not exhibit traditional elastic behavior, the modulus is not a constant. For a polymeric material, the modulus as the tensile stress required at a given elongation. Express as force per unit area at a given percent elongation.

MSDS: Material Safety Data Sheets.


Nailer: A piece of dimension lumber or plywood secured to the structural deck or walls, which provides a receiving medium for the fasteners. Nailers must be the same thickness as the insulation and should be treated with a non-oil borne preservative.

Night Seal: A material used to temporarily seal a membrane edge during construction to protect the roofing assembly in place from water penetration.

Ninety-Pound: A prepared organic felt roll roofing with a granule surfaced exposure that has a mass of approximately 90 pounds per 100 square feet.

Nitrile Alloy: An elastomeric material of synthenic non-vulcanizing polymers. These alloys are generally compounded from butadiene-acrylonitrile copolymers (NBP), PVC, plasticizers and other proprietary ingredients.

Nitrile Rubber: A membrane whose predominant resinous ingredient is a synthetic rubber made by the polymerization of acrylonitrile with butadiene; also known as acrylonitrile rubber, acrylonitrile-butadiene rubber, butadiene-acrylonitrile copolymer elastomer, nitrile-butadiene rubber.

Non-vulcanized Membrane: A membrane manufactured from thermoplastic compounds that retains its thermoplastic properties throughout the life of the membrane.

Nonwoven: A term used to describe the random arrangement of reinforcing fibers (glass, polyester, etc.) in a mat or scrim.

NRCA: Abbreviation for National Roofing Contractors Association.


Open Time: After a contact adhesive has been applied and allowed to dry, the period of time during which an effective bond can be achieved by joining the two surfaces.

Organic: Being or composed of hydrocarbons or their derivatives, or matter of plant or animal origin

Ozone Resistance: The ability of a flexible membrane to resist the deteriorating effects of ozone exposure.


Parapet Wall: Perimeter wall, which extends above the roof surface.

Partially Attached: A roofing assembly in which the membrane has been spot affixed to a substrate usually with an adhesive such as contact cement or a mechanical device

Peel Strength: The average force (or force per unit width) required to peel a membrane from the substrate to which it has been bonded.

Perlite: An aggregate used in lightweight insulating concrete and in preformed perlitic insulation boards, formed by heating and expanding siliceous volcanic glass.

Perm: A unit of water vapor transmission defined as 1 grain of water vapor per square foot per hour per inch of mercury pressure difference (1 inch of mercury = 0.49 psi).

Permeance: An index of a material’s resistance to water vapor transmission.

Phased Application: The installation of a roof system or water proofing system during two or more separate time intervals.

PIB: Abbreviation for polyisobutylene

Picture Framing: A rectangular pattern of ridges in a roof membrane over insulation or deck joints.

Pipe Boot: Covering of flexible material which may be preformed to a particular shape, used to seal around a pipe penetration.

Pitch Pocket: A flange, open-bottomed, metal container placed around columns or other roof penetrations that is filled with hot bitumen or flashing cement to seal the joint. The use of pitch pockets is not recommended by NRCA.

Plastic Film: A flexible sheet made by the extrusion of thermoplastic resins. When used in a modified bitumen roofing membrane, the plastic film may be used on the outer surfaces of the membrane to prevent the membrane from sticking to itself when in the roll; or the film can be used as an integral layer within the membrane serving as reinforcement.

Plasticizer: A chemical substance (e.g. an organic compound) added to natural or synthetic resins for the purpose of increasing flexibility and facilitating processing and workability.

Plastic: Any of a large group of synthetic materials, usually produced by the polymerization of various organic compounds, which can be formed (i.e. molded, cast, extruded, etc.) into flexible sheets or membranes.

Pipe Boot: Covering of flexible material which may be preformed to a particular shape, used to seal around a pipe penetration. Plastomeric: A plastic like polymer consisting of any of various complex organic compounds produced by polymerization which are capable of being molded, extruded or cast into various shapes or films. Generally, they are thermo plastic in nature, i.e., they will soften when heated and harden when cooled.

Plastomeric Membranes: A broad group of plastic-based materials in sheet form, which possess elastomeric properties. Plastomeric materials may be manufactured from a variety of polymers including various compounds and/or blends or alloy.

Ply: A layer of felt in a built-up roof membrane system. A four-ply membrane system has four plies of felt.

Ply Laminate Strength: In a laminated sheet, the force required to separate the coating from the reinforcing or non-reinforcing fabric when peeled in a 180° plane. Expressed as pounds force per prescribed width of sample.

PMRA: Abbreviation for Protected Membrane Roof Assembly.

Polyester: A polymeric resin which is generally crosslinked or cured and made into a variety of plastic materials and products. Polyester fibers are widely used as the reinforcing medium in reinforced flexible membranes as they provide high tensile strength and tear resistance.

Polyisobutylene (PIB): A thermoplastic compound produced by the copolymerization of isobutylene and isoprene. PIB roofing membranes are composed of polyisobutylene and various other reinforcing fillers and stabilizers.

Polymer: A natural or synthetic chemical compound of high molecular weight, or a mixture of such compounds, formed when small individual molecules called monomers are combined and linked together to form long-chain molecules, called polymers.

Polymeric Alloys: A physical blend of two or more polymers combined to modify a given physical property, e.g. tensile strength.

Polymerization: The process whereby single molecules, called monomers, are combined to form large, chainlike molecules, called polymers. Heat, pressure and/or chemicals may be used to trigger this process, which depending on the raw materials used, can produce a wide variety of plastics and synthetic rubbers, a number of which are used in the fabrication of flexible roofing membranes.

Polyvinyl Chloride (PVC): A thermoplastic polymer synthesized from vinyl chloride monomer. Membranes containing polyvinyl chloride are used in a flexible membrane roofing systems

Pond: A roof surface that is incompletely drained.

Positive Drainage: The drainage condition in which consideration has been made for all loading deflections of the deck, and additional roof slope has been provided to ensure drainage of the roof area within 48 hours of rainfall.

Pourable Sealer: A specific type of sealant used at difficult-to-flash penetrations, typically in conjunction with pitch pockets to form a weather-tight seal.

Primer: A thin, liquid bitumen applied to a surface to improve the adhesion of subsequent applications of bitumen.

Protected Membrane Roof Assembly (PMRA): A loosely-laid, insulated and ballasted single-ply roofing assembly in which the insulation and ballast are applied on top of the membrane. Sometimes called an inverted assembly or an upside down roof.

PVC: Abbreviation for Polyvinyl Chloride – A polymer that is uUsually associated with a thermoplastic single- ply roof membrane system.


Rake: The slope edge of a roof at the first or last rafter.

RCI: Abbreviation for Roof Consultant Institute.

Re-covering: The process of covering an existing roofing system with a new roofing system.

Re-entrant Corner: An inside corner of a surface, producing stress concentrations in the roofing or waterproofing membrane.

Reflectivity: The ability of material to reflect or throw back light, heat, etc. In an air-conditioned building, the reflectivity or a membrane may provide an energy savings.

Reglet: A groove in a wall or other surface adjoining a roof surface for use in the attachment of counterflashing.

Reinforced Membrane: A roofing or waterproofing membrane reinforced with felts, mats, fabrics or chopped fibers.

RICOWI: Abbreviation for Roofing Industry Committee on Weather Issues.

Ridging: An upward, “tenting” displacement of a roof membrane, frequently occurring over insulation joints, deck joints and base sheet edges. Generally associated with improper application.

Roof Consulatant Institute (RCI) Incorporated: An international association of professional consultants, architects and engineers who specialize in the specification and design of roofing, waterproofing and building envelope systems.

Roof System Assemblies: There are six major types of flexible membrane roofing systems assemblies:

1. Loosely Laid                 2. Self-Adhesive                 3. Partially Adhered                 4. Totally Adhered                 5. Mechanically Fastened                 6. Protected Membrane Roof Assembly

Roofing Industry Committee on Weather Issues: Established in 1990 as a non-profit organization to identify and address important technical issues related to the cause of wind damage.

Rubber: A polymeric material which, at room temperature, is capable of recovering substantially in shape and size after removal of a force. Refers to both synthetic and natural rubber.


SBS: Styrene Butadiene styrene rubber used as a modifier for bitumen membranes.

Scrim: A woven, non-woven or knitted fabric composed of continuous strands of material used for reinforcing or strengthening flexible membranes. Scrim may be incorporated into the membrane by laminating or coating.

Sealant: A single or multi-component polymeric or asphalt-based material used to weatherproof many types of construction joints. The materials come in various grades; pourable, self-leveling, non-sag, gun-applied, and cured or uncured tapes. Sealants are used in membrane roofing systems for lap seam sealers, pitch pocket fillers and water cut-off mastics

Seam: A joint formed by mating together two separate sections of roofing membrane. Seams may be sealed in a variety of ways, including hot-air welding, solvent welding and liquid or tape adhesive bonding.

Seam Strength: The force or stress required to separate or rupture a seam in the membrane material.

Seam Tape: Polymeric material supplied in rolls, used to bond overlapping membrane seams together.

Self-Adhesive Membranes: Flexible membranes which can adhere to a substrate and to itself at overlaps without the use of an adhesive. The undersurface of a self-adhesive membrane is protected by a release paper, which prevents the membrane from bonding to itself during shipping and handling. Later, as the membrane is unrolled, the release paper is peeled away and the self-adhering undersurface is applied to the substrate. Successful application of a self-adhesive membrane requires a clean and dry substrate and the application of firm, uniform pressure.

Self-Vulcanized Membrane: A membrane manufactured from compounds that are thermoplastic during manufacture and installation. The base materials are compounded so that the polymers eventually form crosslinks within their bulk structure and become cured and lose their thermoplastic properties.

Selvage: A specifically defined edge of a membrane, which is designed for some special purpose, such as overlapping.

Shear Strength: The stress (pounds or pounds per inch width) required to disrupt a lap seam or bonded joint or attachment by forcing the substrates to slide over each other.

Sheet Membrane: A roofing membrane fabricated in a controlled factory environment. It is waterproof and weather resistant. It may be a laminate or one or more materials and may or may not contain reinforcing fabrics.

Single-Ply: A term often applied to sheet membranes.

Slip Sheet: Sheet material, such as reinforced kraft paper, polyester scrim or polyethylene sheeting, placed between two components of a roofing system (such as membrane and insulation) to ensure that no adhesion occurs between them and to prevent possible damage from chemical incompatibility.

Slope: The angle of inline of a roof surface as measured in degrees, in a ration of fall to run, or in inches of fall to run. Slope is a factor in roof system design with regard to the ability of the roof to retain gravel and shed water.

Solvent Cleaners: Heptain, hexane, white gas and unleaded gas, used to clean the membrane prior to applying the splicing adhesive in some flexible membrane roofing systems

Solvent Welding: A process used to chemically weld or join together two or more layers of certain membrane materials (usually thermoplastic), by applying a solvent, such as tetrahydrofuran or THF, to the overlapping surfaces and mating them when the bonding surfaces become tacky. Used most often in welding or sealing seams.

Spray Polyurethane Foam (SPF): a foamed plastic material, formed by spraying two components, PMDI ([A] component) and a resin ([B] component) to form a rigid, fully adhered, water resistant, and insulating membrane.

Spread Coating: A manufactured process in which the compound (plastisol) is prepared in mixers and then fed to individual coaters. The mixture is cold until after it is spread onto the supporting base (reinforcement). After coating, the membrane passes through a heating channel which causes it to change from a paste to a solid.

Spunbound Polyester Mat: Continuous filament, uniformly dispersed polyethylene terepthalate fiber mater. A binder is used to stabilize the mat, which serves as reinforcement to the membrane.

Stabilizer: An ingredient in the formulation of flexible membranes added to improve certain physical properties which are important for processing, storage, workability and performance.

Standing Seam: a metal roof system that consists of an overlapping or interlocking seam that occurs at an upturned rib.

Strain: The dimensionless expression for the elongation of a material under stress. Strain is expressed as the ratio of elongation per unit length.

Stress: The internal resistance of a material to a force, measured as a force per unit area.

Styrene-Butadiene Rubber: High molecular weight polymer having rubber-like properties, formed by the random copolymerization of styrene and butadiene monomers. Polymers of this type are often crosslinked to give maximum rubber-like properties in service. These polymers are sometimes used as the modifying compound in certain modified bitumen roofing membranes.

Styrene-Butadiene-Styrene Copolymer (SBS): High molecular weight polymers which have both thermoset and thermoplastic properties, formed by the block copolymerization of styrene and butadiene monomers. The three block copolymer formed has a center block of butadiene with end blocs of styrene. These polymers are sometimes used as the modifying compound in certain modified bitumen roofing membranes.

Square: An area of roof surface equal to 100 square feet.

Substrate: The upper surface of the roof deck, insulation or other roofing structure upon which a roofing membrane or other component of the roofing system is placed or to which is attached. Both the successful attachment of the roofing membrane to the substrate and the ability of the membrane to remain in place throughout its service life are critical to the performance of the entire roof system. For this reason, most flexible membranes are designed to accommodate a variety of substrate surfaces and materials, including structural concrete, plywood, insulation boards, etc. Compatibility between the substrate and the immediate overlying component must always be ensured.

Sump: A depression in the surface of a roof around the opening to a drain, which serves to promote drainage.

Synthetic Rubber: Any of several elastic substances resembling natural rubber, prepared by the polymerization of butadiene, isoprene, and other unsaturated hydrocarbons. Synthetic rubber is widely used in the fabrication of flexible roofing membranes.


Talc: The white powder present on the surface of vulcanized EPDM membranes, used to prevent adhesion of the membrane to itself, necessary in the manufacturing process of some sheets. The same purpose can also be served by the use of mica dust.

Tear Resistance: The load required to tear a material when the stress is concentrated on a small area of the specimen by the introduction of a prescribed flaw. Expressed in psi or pounds-force.

Tensile Fatigue Resistance: The ability of a given membrane material to resist fatigue and/or other damage (such as loss of elasticity) caused by the alternate stretching and relaxing of the material over a long period of time.

Tensile Strength: The maximum force or stress required to break a membrane sample during a tensile test. For non-reinforced membranes, strength is reported as a stress (pounds per square inch or psi); for reinforced membranes, strength is reported as a force (pounds or pound-force/inch).

Termination: The treatment or method of anchoring the free edges of the membrane in a roofing system.

Thermal Shock: The dynamic stress imposed on a membrane due to a sudden or very rapid change in the temperature of the membrane, as for example, when a cold rain follows a period of bright sunshine. In some climates, some degree of thermal stress may be caused by the simple setting of the sun, especially where the temperature differential between daytime and nighttime may be as great as 45° to 50° or more.

Thermoplastic Olefin Membrane (TPO): A blend of polypropylene and ethylene-propylene polymers. Colorant, flame retardants, UV absorbers, and other proprietary substances, which may be blended with the TPO to achieve the desired physical properties.

Thermoplastic: Materials that soften when heated and harden when cooled.

Thermoplastic Elastomer: Compounds formulated from materials traditionally used for vulcanized rubber. Curing agents are controlled in the compound so crosslinking does not occur, and the final product exhibits the properties of a thermoplastic material

Thermoplastic Polyolefin: A membrane made from a copolymer or blend of polyethylene and polypropylene polymers.

Thermoset: A material that solidifies or “sets” irreversibly when heated. This property is usually associated with cross-linking of the molecules induced by heat or radiation.

Totally Adhered: A roofing assembly in which a membrane has been full adhered to a substrate usually with the aid of an appropriate contact or water-based adhesive or emulsion. Totally adhered membranes are often installed on roofs which, because of their slope, construction or other factors, cannot support adequate ballast or into which mechanical attachment is difficult.

TPO: Abbreviation for Thermoplastic Polyolefin.


UL: Underwriters Laboratories, Inc.

UL Label: An identification label or seal attached to a roofing component with the authorization of Underwriters Laboratories. The presence of the label indicates that the product has a given rating based on performance tests for such products.

Ultimate Elongation: The amount a membrane sample stretches during tensile testing before it ruptures. Usually expressed as a percentage of the original length.

Ultraviolet (UV): Ultraviolet radiation from the sun is known to be potentially damaging to certain chemical compounds such as those used in roofing membranes. Formulations with stabilizers and UV absorbers effectively inhibit the potentially deleterious effects of UV exposure .

Underwriters Laboratories (UL): An independent, non-profit agency which functions as the testing arm of the National Board of Fire Underwriters. It maintains laboratories for the examination and testing of various devices, systems and materials to determine their safety against the hazards of fire and accident. UL is organized around a number of separate departments, including Chemical Hazards, Electrical Hazards and Fire Protection.

UV: Abbreviation for ultraviolet.


Vapor Retarder: Sheet material installed to impede or restrict the passage of water vapor through a roofing assembly. Normally, a vapor retarder has a perm rating of 0.5 or less.

Vegetative Roof: A roof system designed to grow and nurture plants. Types of integrated vegetative roofs systems include:                 Conventional – Growth media spread over the roof surface into which plants are placed or seeds are spread. Depths range from 2 to 8 inches for extensive systems and 8 inches and above for intensive systems.                 Vegetative Mats – Pregrown growth media in a matrix that allows the vegetation to cut into sections and to be handled in strips similar to sod to cover the roof.                 Module Systems – Systems that incorporate trays that may be formed to have the drainage system and moisture retention methods molded into the tray which carries the growth media and plants. These assembled trays, termed modules, are placed on the roof generating a continuous vegetative cover.                 Vulcanization: Any of various processes by which natural or synthetic rubber or other polymeric materials may be cured or otherwise treated (i.e. exposed to chemicals, heat or pressure) to render them non-thermoplastic and which improves their elastic properties through this chemical change.


Water Absorption: The amount of water absorbed by a material after immersion for a prescribed period of time. Expressed as a percentage of the original weight of the material.

Water Cut-Off Mastic: A theromset material used to form a seal between membrane sheets at indicated termination points.

Water Vapor Transmission: A measure of the rate of transmission of water vapor through the membrane material under controlled laboratory conditions of temperature and humidity.

Waterproof: The quality of a membrane, membrane material or other component which prevents water from entering the roofing system.

Wind Uplift: Wind that is deflected at roof edges, roof peaks or obstructions can cause a drop in air pressure immediately above the roof surface. The resultant force is transmitted to the roof surface and is called wind uplift. The force can lift roof membranes from the roof deck if the membrane is not adequately secured. A source of air beneath the membrane is required to provide pressure which can result in uplift. Uplift may also occur because of the introduction of wind underneath the membrane and roof edges, where it can cause the membrane to balloon and pull away from the deck. Roof loss by wind can be avoided by proper installation and adequate adhesion, attachment or ballasting.