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Abrasion resistance

The ability to resist being worn away by contact with another moving, abrasive surface, such as foot traffic, mechanical equipment, wind-blown particles, tree limbs, etc.


The ability of a material to accept within its body quantities of gases or liquid, such as moisture.

Accelerated weathering

The exposure of a specimen to a specified test environment for a specified time with the intent of producing in a shorter time period effects similar to actual weathering.

Acrylic resin

Polymers of acrylic or methacrylic monomers. Often used as a latex base for coating systems.


(1) The degree of attachment between two surfaces held together by interfacial forces—mechanical or chemical or both; (2) the degree of attachment or bonding between application of the same substance; (3) the combined ultimate strength of the molecular forces and the mechanical interlocking achieved between the adhesive and the surface bonded. Adhesion is measured in shear and peel modes.


A cementing substance that produces a steady and firm attachment or adhesion between two surfaces.

Aged R-value

Thermal resistance value established by using artificial conditioning procedures for a prescribed time period.


(1) The effect on materials of exposure to an environment for an interval of time; (2) the process of exposing materials to an environment for an interval of time.


American Institute of Architects.

Air barrier

The assembly of materials used in building construction to reduce or retard the uncontrolled passage of air into and out of the building.

Air infiltration

Air leakage into the building.

Air leakage

The unintended movement of air from a location where it is intended to be contained to another location.


American National Standards Institute.

Application rate

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

Application temperature

The temperature of a material, such as hot asphalt, when applied to the roof.


A person technically qualified and professionally licensed to practice architecture; that is, designing and administering the construction of buildings.

Area divider

A raised, flashed assembly, typically a single- or double-wood member attached to a wood base plate, which is anchored to the roof deck. It is used to separate large roof areas or roof systems composed of different/incompatible materials and also may be used to facilitate installation of tapered insulation.


American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Asphalt, air-blown

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


A dark brown to black cementitious material in which the predominating constituents are bitumens found in a natural state or more commonly left as a residue after evaporating or otherwise processing crude oil or petroleum. See “bitumen.” Asphalt may be further refined to conform to various roofing grade specifications:
– asphalt, dead-level: Roofing asphalt conforming to the requirements of ASTM D312, Type I.
– asphalt, flat: Roofing asphalt conforming to the requirements of ASTM D312, Type II.
– asphalt, steep: Roofing asphalt conforming to the requirements of ASTM D312, Type III.
– asphalt, special steep: Roofing asphalt conforming to the requirements of ASTM D312, Type IV.
– asphalt, waterproofing: A waterproofing asphalt conforming to the requirements of ASTM D449, Types I, II and III.

Asphalt felt

An asphalt-saturated and/or asphalt-coated felt.

ASTM International

The full designation in current use of the largest voluntary standards development organizations in the world formerly known as the American Society for Testing and Materials.
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A material, such as minimum nominal #1-1/2 inch size or #4, or alternatively, #3, #24, #2 or #1—as specified in ASTM D448—smooth river stone, crushed stone, standard precast concrete pavers or interlocking, beveled, doweled or contoured fit lightweight concrete pavers, which employs its mass and the force of gravity to hold a roof membrane system in place.

Base flashing (membrane base flashing)

Plies or strips of roof membrane material used to closeoff and/or seal a roof at the horizontal-to-vertical intersections, such as at a roof-to-wall juncture. Membrane base flashing covers the edge of the field membrane and extends up the vertical surface.

Base ply

The bottom or first ply in a built-up or polymer- modified bitumen roof system when additional plies are to be subsequently installed.

Base sheet

An impregnated, saturated or coated felt placed as the first ply in some low-slope roof systems.

Basic wind speed

Three second gust wind speed in miles per hour at 33 feet above ground in Exposure C as defined in the latest edition of the American Society of Civil Engineers standard ASCE 7, “Minimum Design Loads for Buildings and Other Structures.”


(1) A class of amorphous, black or dark-colored, (solid, semi-solid or viscous) cementitious substances, natural or manufactured, composed principally of high-molecular-weight hydrocarbons, soluble in carbon disulfide, and found in asphalts, tars, pitches and asphaltites; (2) a generic term used to denote any material composed principally of bitumen, typically asphalt or coal tar.


Containing or treated with bitumen, e.g., bituminous concrete, bituminous felts and fabrics, and bituminous pavement.

Bituminous waterproofing

A waterproofing method available for positive-side waterproofing only; cold or hot bituminous application with reinforcing material.

Blanket insulation

Fiberglass or other compressible fibrous insulation, generally available in roll form.


(1) A raised portion of a roofing membrane resulting from local internal pressure, such as an enclosed pocket of air, which may be mixed with water or solvent vapour, trapped between impermeable layers of felt or membrane or between the membrane and substrate; (2) the similarly formed surface swelling in coated prepared roofing such as asphalt shingles; (3) separation of a coating from a substrate; may be caused by water absorption and the resultant swelling or subsurface corrosion.

Blowing agent

An expanding agent used to produce a gas by chemical or thermal action or both in manufacture of hollow or cellular materials.


(1) The adhesive and/or cohesive forces holding two components in positive contact; (2) a surety; typical types are: bid, performance and payment; (3) a guarantee relating to roof system performance.


Structural elements installed to provide restraint or support or both to other members so the complete assembly forms a stable structure; may consist of knee braces, cables, rods, struts, ties, shores, diaphragms, rigid frames, etc.

British thermal unit (Btu)

The heat energy required to raise the temperature of one pound of water one degree Fahrenheit.


An upward, elongated displacement of a roof membrane frequently occurring over insulation or deck joints. A buckle may be an indication of movement within the roof assembly.

Building code

The minimum construction requirements established generally by national organizations and adopted completely or in altered form by local governing authorities. Building code controls design, construction, quality of materials, use and occupancy, location, and maintenance of buildings and structures within the area for which the code was adopted.

Building envelope

Exterior of a building.

Built-up roof (BUR)

A continuous, semi-flexible roof membrane consisting of multiple plies of saturated felts, coated felts, fabrics or mats assembled in place with alternate layers of bitumen and surfaced with mineral aggregate, bituminous materials, a liquid-applied coating or a granule-surfaced cap sheet.

Butt edge

The lower, exposed edge of a shingle, tile or shake.

Butt joint

A joint formed by adjacent, separate sections of material, such as where two neighboring pieces of insulation abut.
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Cant strip

A beveled strip used to modify the angle at the point where the roofing or waterproofing membrane meets any vertical element.

Cap flashing

(1) Usually composed of metal, used to cover or shield the upper edges of the membrane base flashing or wall flashing; (2) a flashing used to cover the top of various buildings components, such as parapets or columns.

Cap sheet

A sheet, often granule-surfaced, used as the top ply of some built-up or polymer-modified bitumen roof membranes and/or flashings.

Capillary action

(1) The action by which the surface of a liquid where it is in contact with a solid is elevated or depressed depending on the relative attraction of the molecules of the liquid for each other and for those of the solid; (2) the siphoning of liquid into a joint or void between two adjacent surfaces.

Cellular concrete

A poured-in-place roof deck material composed of Portland cement, water, a foaming agent or pregenerated foam and air. It’s oven-dry density is about 24 to 32 pounds per cubic foot.

Centimeter (cm)

A metric unit of measurement equal to one-hundredth (0.01) of a meter, or 0.393 inches.

Centipoise (cP or cPs)

A unit of measure of dynamic viscosity in the centimeter-gram-second system of units equal to one one-hundredth of a poise (1 P = 100 cP = 1 g•cm-1•s-1). (The viscosity of water at 70 F is one centipoise. The lower the number, the less viscous the material.)

Centistokes (cSt)

A unit of kinematic viscosity in the centimeter-gram-second system of units equal to one one-hundredth of a stokes (1 St = 100 cSt = 1 cm2•s-1); the ratio of a liquid’s dynamic viscosity to its density.

Chemical resistance

The ability to withstand contact with specified chemicals without a significant change in properties.


A material used as the exterior wall enclosure of a building.

Coated felt (sheet)

(1) An asphalt felt that has been coated on both sides with harder, more viscous asphalt; (2) a fiberglass felt that has been simultaneously impregnated and coated with asphalt on both sides.


A fluid material applied in the field as a film to the roof surface to provide weather protection to the original roof substrate.


A collection of laws (regulations, ordinances or statutory requirements) adopted by an authority having jurisdiction.


Designed for or capable of being applied without heating as contrasted to hot-applied. Cold-applied materials are furnished in liquid state, whereas hot-applied materials are furnished as solids that must be heated to liquefy before application.


Capable of burning.

Compressive strength

The property of a material that relates to its ability to resist compression loads.


The conversion of water vapor or other gas to liquid phase as the temperature drops or atmospheric pressure rises; the act or process of condensing. See “dew point.”

Control joint

A groove that is formed, sawed or tooled in a concrete or masonry structure to regulate the location and amount of cracking and separation resulting from the dimensional change of different parts of the structure, thereby avoiding the development of high stresses.

Cool roof

A roof system that uses products made of highly reflective and emissive materials for its top surface. Cool roof surfaces can remain at markedly lower temperatures when exposed to solar heat in service than surfaces of roofs constructed with traditional non-reflective roofing products.


(1) The term used for a row of roofing material that forms the roofing, waterproofing or flashing system; (2) one layer of a series of materials applied to a surface (e.g., a five-course wall flashing is composed of three applications of roof cement with one ply of felt or fabric sandwiched between two layers of roof cement).


The surface area uniformly covered by a specific quantity of a particular material at a specific thickness.

Cover board

An insulation board used over closed cell plastic foam insulation (e.g., polyisocyanurate) to prevent blistering when used in conjunction with hot bituminous membranes. Suitable cover-board insulations are glass-faced siliconized gypsum board, fiberglass board, perlite board, wood fiberboard or mineral fiberboard. Cover boards are also recommended between polyisocyanurate insulation and single-ply membranes to protect the polyisocyanurate.


A nonlinear separation or fracture occurring in a material.


In coatings, visible breaks in the film thickness that extend to the surface and the previously applied coating or substrate.


A relatively small area of a roof constructed to divert water from a horizontal intersection of the roof with a chimney, wall, expansion joint or other projection.


(1) A raised member used to support roof penetrations, such as skylights, mechanical equipment, hatches, etc., above the level of the roof surface; (2) a raised roof perimeter relatively low in height.


A process whereby a material is caused to form permanent molecular linkages by exposure to chemicals, heat, pressure and/or weathering.

Cure time

The time required for a material to reach its desirable long-term physical characteristics.


Solvent-thinned bitumen used in cold-process roof adhesives, roof cements and roof coatings.
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Treatment of a surface or structure to resist the passage of water in the absence of hydrostatic pressure.

Dead load

The weight of a structure itself, including the weight of fixtures or equipment permanently attached to it.


A structural component of the roof of a building. The deck must be capable of safely supporting the design dead and live loads, including the weight of the roof system, and the additional live loads required by the governing building codes and provide the substrate to which the roof or waterproofing system is applied. Decks are either noncombustible, (e.g., corrugated metal, concrete or gypsum) or combustible (e.g., wood plank or plywood).

Deflection (bowing, sagging)

(1) The deformation of a structural member as a result of loads acting on it; (2) any displacement in a body from its static position or from an established direction or plane as a result of forces acting on the body.


Any change of form, shape or dimensions produced in a body by a stress or force, without a breach of the continuity of its part.


Separation of the laminated layers of a component or system.

Dew point temperature

The temperature at which air becomes saturated with water vapor; the temperature at which air has a relative humidity of 100 percent.


(1) The movement of water vapor from regions of high concentration (high water vapor pressure) toward regions of lower concentration; (2) spreading of a constituent in a gas, liquid or solid tending to make the composition of all parts uniform; (3) the spontaneous movement of atoms or molecules to new sites within a material.

Dimensional stability

The degree to which a material maintains its original dimensions when subjected to changes in temperature and humidity.


A vertical pipe or conduit used to carry runoff water from a scupper, conductor head or gutter of a building to a lower roof level or to the ground or storm water runoff system; also called a conductor or leader.

Drainage course

A separate layer of material that provides a location for moisture to move laterally through a protected-membrane roof system. A drainage course relieves hydrostatic pressure from a material’s surface and the associated weight of water.

Drip edge

A metal flashing or other overhanging component with an outward projecting lower edge intended to control the direction of dripping water and help protect underlying building components.

Drying time

The time required for the loss of volatile components so that the material will no longer be adversely affected by weather conditions such as dew, rain or freezing.


The ability to withstand physical, chemical or environmental abuse.

Dynamic load

Any load that is nonstatic, such as a wind load or moving live load.
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An encrustation of soluble salts, commonly white, deposited on the surface of stone, brick, plaster or mortar; usually caused by free alkalies leached from mortar or adjacent concrete as moisture moves through it.


The ratio of the extension of a material to the length of the material prior to stretching. Usually, elongation is expressed as a percentage of the original length.


(1) The process of pressing/positioning a felt, aggregate, fabric, mat or panel into hot bitumen or adhesive to ensure contact at all points; (2) the process of pressing/positioning granules into coating in the manufacture of factory-prepared roofing, such as shingles.


(1) Infrared emissivity is a measure of the ability of a surface to shed some of its absorbed heat (in the form of infrared radiation) away from the surface; emissivity is expressed as a percentage or a decimal factor; (2) the ratio of radiant energy emitted from a surface under measurement to that emitted from a black body (the perfect emitter and absorber) at the same temperature.

End lap

The distance of overlap where one ply, pane or piece extends beyond the end of the immediately adjacent underlying ply, panel or piece.


A terpolymer of ethylene, propylene and diene with the residual unsaturated portion of the diene in the side chain to provide for vulcanization. It is a thermosetting synthetic elastomer. EPDM is an acronym for “ethylene propylene diene M-class rubber,” which is a name assigned to this material within the classification established in ASTM D1418. Ethylene propylene diene M-class rubber, also called ethylene propylene diene terpolymer

Equiviscous temperature (EVT)

The temperature at which a bitumen attains the proper viscosity for built-up membrane application.

Equiviscous temperature (EVT) application range

The recommended bitumen application temperature range. The range is approximately 25 F above or below the EVT, thus giving a range of approximately 50 F. The EVT range temperature is measured in the mop cart or mechanical spreader just prior to application of the bitumen to the substrate.

Equiviscous temperature (EVT) for asphalt

The recommended EVT for roofing asphalt (ASTM D312, Type I, II, III or IV) is as follows:
– mop application: the temperature at which the asphalt’s apparent viscosity is 125.
– mechanical spreader application: The temperature at which the asphalt’s apparent viscosity is 75 centipoise.
– Note: To avoid the use of two kettles if there are simultaneous mop and mechanical spreader applications, the EVT for mechanical spreader application can be used for both application techniques.

Expanded polystyrene (EPS)

A type of plastic foam insulation product having predominately closed-cell structure manufactured from expanded polystyrene beads in a molding process; boards or blocks are formed.

Expansion joint

A structural separation between two building elements that allows free movement between the elements without damage to the roof or waterproofing system.

Exterior insulation and finish system (EIFS)

A nonload-bearing outdoor wall finish system consisting of a thermal insulation board, attachment system, reinforcement system and compatible finish.

Extruded polystyrene (XPS)

A type of plastic foam insulation product manufactured by a continuous extrusion process as the resin foams. This forms a tight and complete skin on each side of the board.
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The outermost adhered top or bottom, or both, sheet (or layer) of an insulation board that is composed of a different material than the insulation itself. It is commonly composed of organic paper, glass mat or a combination thereof; generally impregnated or coated, or both, with asphalt, latex, or other like material; or metal foil.


Any of a wide variety of mechanical securement devices and assemblies, including nails, staples, screws, cleats, clips and bolts, that may be used to secure various components of a roof assembly.


A flexible sheet manufactured by the interlocking of fibers with a binder or through a combination of mechanical work, moisture and heat. Felts are manufactured principally from wood pulp and vegetable fibers (organic felts), asbestos fibers (asbestos felts), glass fibers (fiberglass felts or ply sheets) or polyester fibers.

Fiberglass base sheet

A fiberglass-reinforced base sheet for built-up roof system construction, impregnated and coated with asphalt and surfaced with mineral matter.


Materials used to limit the spread of fire.

Fire rating

Grade based on standard testing procedures of various materials.

Fire resistance

The property of materials or their assemblies that prevents or retards the passage of excessive heat, hot gases or flames under conditions of use.


(1) A half-cylindrical or half-conical shaped opening or void in a lapped edge or seam, usually caused by wrinkling or shifting of ply sheets during installation (also referred to as an edge wrinkle); (2) in shingles, a half-conical opening formed at a cut edge.

Flame spread

The propagation of a flame away from its source of ignition.

Flash point

The lowest temperature at which vapors above a combustible substance ignite in air when exposed to an ignition source.


Components used to weatherproof or seal roof system edges at perimeters, penetrations, walls, expansion joints, valleys, drains and other places where the roof covering is interrupted or terminated. For example, membrane base flashing covers the edge of the field membrane, and cap flashings or counterflashings shield the upper edges of the base flashing.

Flood (pour) coat

The surfacing layer of bitumen into which surfacing aggregate is embedded on an aggregate-surfaced built-up roof.

FM Approvals

Commonly referred to as FM, a research and testing business unit of FM Global (a commercial and industrial property insurer) that classifies roofing components and assemblies for their fire, traffic, impact (hail), weathering and wind-uplift resistance.
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A metal thickness measurement; a smaller gauge number indicates a thicker metal.


Opaque, natural or synthetically colored aggregate commonly used to surface cap sheets, shingles and other granule-surfaced roof coverings; also referred to as mineral or ceramic granules.

Gypsum board panels

Cementitious board stock with noncombustible core primarily comprised of gypsum that is commonly used as a barrier board, thermal barrier or cover board in a roof assembly.
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A horizontal framing structural member of a door, window or other framed opening.


To be installed by heating the underside of the sheet with a propane torch or other heating device, melting the polymer-modified bitumen on the bottomside and adhering the sheet in the molten material.

Heat transfer

The transmission of thermal energy from a location of higher temperature to a location of lower temperature. This can occur by conduction, convection or radiation.

Heat welding

A method of melting and fusing together the overlapping edges of separate sheets or sections of polymer-modified bitumen, thermoplastics or some uncured thermoset roofing membranes by the application of heat (in the form of hot air or open flame) and pressure.


Heating, ventilating and air-conditioning equipment.

Hydrostatic pressure

The pressure equivalent to that exerted on a surface by a column of water of a given height.
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Ice dam

A mass of ice formed at the transition from a warm to a cold roof surface, frequently formed by refreezing meltwater at the overhang of a steep roof causing ice and water to back up under roofing materials.


In roofing materials manufacture, to completely surround the fibers in a felt or mat with bitumen with the spaces between the fibers partially or completely filled without a continuous coating of bitumen on the surface.


The slope of a roof expressed in percent or in units of vertical rise per units of horizontal run.


Being or composed of materials other than hydrocarbons and their derivatives, or matter that is not of plant or animal origin.

In-service R-value

Thermal resistance value established under installed conditions and measured during the expected service life of the material.


Any of a variety of materials designed to reduce the flow of heat from or into a building.
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K or k-value

Thermal conductivity; the time rate of heat flow through a unit area of a homogeneous material in a direction perpendicular to isothermal planes induced by a unit temperature gradient. In English (inch•pound) units of measurement, it is the number of Btu that pass through a 1-inch thickness of a 1-square-foot sample of material in one hour with a temperature difference between the two surfaces of 1 degree Fahrenheit. It is expressed as Btu•in/hr•ft2•F.
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To join layers of materials together using fusion; the process of joining layers of materials together using adhesion.


That part of a roofing, waterproofing or flashing component that overlaps or covers any portion of the same or another type of adjacent component.

Lap seam

Occurs where overlapping materials are seamed, sealed or otherwise bonded.

Life-cycle assessment (LCA)

A method of evaluating a product or system by reviewing the ecological effect over its entire life. Life stages include extraction of raw materials, processing and fabrication, transportation, installation, use and maintenance, and reuse, recycling and disposal. At each stage, the product or system and its components are evaluated based on materials and energy consumed and the pollution and waste produced. LCA is an environmental review methodology.

Life-cycle cost analysis

A technique of economic evaluation that sums up the costs of a product or system during its entire life including raw materials extraction, processing and fabrication, transportation, installation, use and maintenance, and reuse, recycling or disposal. At each stage the product or system and its components are evaluated based on the costs of materials and energy consumed and pollution and waste produced.

Lightweight concrete

(1) Concrete with a density of less than 115 lb/ft3; (2) Poured deck fill composed of a combination of two or more of the following: gypsum, vermiculite, perlite, wood fibers and air-entrained concrete.

Lightweight or insulating concrete fill (LWIC)

Concrete made with or without aggregate additions to Portland cement, water and air to form a hardened material that when oven dried will have a unit weight of 50 lb/ft3 or less.


Application of bituminous cements, adhesives or coatings installed at ambient or slightly elevated temperatures.

Liquid-applied built-up roof

A continuous, semi-flexible roof membrane consisting of multiple plies of felts, mats or fabrics laminated together with alternate layers of roof cements and surfaced with a liquid-applied coating with or without aggregate surfacing.

Live loads

Temporary loads that the roof structure must be designed to support, as required by governing building codes. Live loads are generally moving and/or dynamic or environmental (e.g., people, installation equipment, snow, ice, rain).


In roofing: (1) production lot—all material produced in one eight-hour shift of the same type (and color when applicable); (2) delivery lot—all material of the same type delivered at one time by one truck or railroad car.

Low-slope roofs

A category of roofs that generally includes weatherproof membrane types of roof systems installed on slopes at or less than 3:12.

Low-temperature flexibility

The ability of a membrane or other material to resist cracking when flexed after it has been cooled to a low temperature.
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Construction, usually set in mortar, of natural building stone or manufactured units, such as brick, concrete block, adobe, glass block, tile, manufactured stone or gypsum block.


A thick adhesive material used as a cementing agent for holding waterproofing membrane in place.


A thin layer of woven, nonwoven or knitted fiber that serves as reinforcement to a material or membrane.

Material safety data sheet (MSDS)

A written description of the chemicals in a material or product and other pertinent data, including such things as safe handling and emergency procedures. In accordance with OSHA regulations, it is the manufacturer’s responsibility to produce MSDSs and the employer’s responsibility to communicate their contents to employees.

Mechanical damage

In roofing, physical damage to a roof system not caused by normal wear and tear.

Mechanically fastened membrane

Generally used to describe a membrane that has been attached to the substrate at defined intervals.


A flexible or semiflexible roof covering or waterproofing whose primary function is to exclude water.


A unit of measure, one mil is equal to 0.001 inches; often used to indicate the thickness of a roof membrane.

Mineral fiber

Insulation composed principally of fibers manufactured from rock, slag or glass, with or without binders.


The application of hot bitumen with a mop or mechanical applicator to the substrate or plies of a bituminous membrane. There are two types of mopping as follows:
– mopping, solid: A continuous coating,
– mopping, spot: Bitumen is applied roughly in circular areas, leaving a grid of unmopped perpendicular areas.
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Newton (N)

SI unit of measure for force.


National Fire Protection Association; an international nonprofit organization. Its mission is to reduce the worldwide burden of fire and other hazards on the quality of life by providing and advocating consensus codes and standards, research, training and education. NFPA publishes the NFPA 70, “National Electrical Code” (NEC).


National Institute of Standards and Technology.

Nonwoven fabric

A textile structure produced by bonding or interlocking of fibers, or both, accomplished by mechanical, chemical, thermal, or solvent means and combinations thereof.


National Roofing Contractors Association.
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Being or composed of hydrocarbons or their derivatives; matter of plant or animal origin.

Organic felt

An asphalt roofing base material manufactured from cellulose fibers.


Any material installed on top of a waterproofing assembly.


Undesirable depositions of airborne spray.


(1) A weathering mechanism of materials exposed to the elements; can be promoted by elevated temperature, ultraviolet radiation, galvanic action and other agents; (2) loss of electrons by a constituent of a chemical reaction; sometimes, more specifically the combination of oxygen with a reactant.
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Parapet wall

The part of a perimeter wall that extends above a roof.


A support or base for rooftop components such as pavers, pipes and small rooftop units.

Peel strength

The average load per unit width required to separate progressively a flexible member from a rigid member or another flexible member.


(1) Any construction (e.g., pipes, conduits, HVAC supports) passing through a roof; (2) the consistency of a bituminous material expressed as the distance, in tenths of a millimeter (0.1 mm), that a standard needle penetrates vertically into a sample of material under specified conditions of loading, time and temperature (ASTM D5 is the test method used for bituminous materials). A cone is sometimes used for special purposes instead of a needle.

Perforated felt

Bitumen-saturated felt perforated with closely spaced small holes to allow air and moisture to escape during application of built-up roofing; depending on the type of material or specific use, can be classified in accordance with ASTM D226, D2626 or D4897 requirements.


(1) The time rate of vapor transmission through unit area of flat material of unit thickness (values in reference sources are quoted for unit thicknesses) induced by unit vapor pressure difference between two specific surfaces under specified temperature and humidity conditions. The English (inch•pound) unit of measurement for permeability is (grains•in.)/(h•ft2•in. Hg), which is commonly referred to as “perm•inch” units; (2) The property of a porous material that permits a fluid (or gas) to pass through it; commonly refers to water vapor permeability of a sheet material or assembly and is defined as water vapor permeance per unit thickness.


1) The time rate of water vapor transmission through unit area of flat material or construction induced by unit vapor pressure difference between two specific surfaces (values in reference sources are quoted for specific material thicknesses) under specified temperature and humidity conditions. The English (inch•pound) unit of measurement for permeance is (grains)/(h•ft2•in. Hg), which is commonly referred to as “perm” units.

Phased application

(1) The installation of a roof or waterproofing system during two or more separate time intervals or different days. Applications of surfacings at different time intervals are typically not considered phased application. See “surfacing.” (2) A roof system not installed in a continuous operation.


The material property of being flexible or moldable.


A layer of felt or ply sheet in a built-up roof membrane or roof system.


A polymer in which the repeated structural unit in the chain is of the ester type.

Polyisocyanurate foam board

A thermal insulation composed of polyisocyanurate foam with adhered facers; commonly called iso or isoboard; classified in accordance with ASTM C1289.


A macromolecular material formed by the chemical combination of monomers having the same or different chemical composition.

Polymer-modified bitumen

(1) A bitumen modified by including one or more polymers (e.g., atactic polypropylene, styrene butadiene styrene); (2) composite sheets consisting of a polymer-modified bitumen often reinforced with various types of mats or films and sometimes surfaced with films, foils or mineral granules.

Polyvinyl chloride (PVC)

A synthetic thermoplastic polymer prepared from vinyl chloride. PVC can be compounded into flexible and rigid forms through the use of plasticizers, stabilizers, fillers and other modifiers. Rigid forms are used in pipes; flexible forms are used in the manufacture of sheeting and roof membrane materials.


A polymer prepared by the polymerization of styrene as the sole monomer.


The excessive accumulation of water at low-lying areas on a roof that remains after 48 hours after precipitation under conditions conducive to drying.

Positive drainage

The drainage condition in which consideration has been made during design 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 precipitation.


A term used to describe a category of adhesives that in dry (solvent-free) form are tacky at room temperature and adhere to a variety of dissimilar surfaces when contact is promoted by application of pressure. Pressure-sensitive adhesives do not require activation by an energy source such as heat.

Prestressed concrete

Concrete in which the reinforcing cables, wires or rods are tensioned before there is load on the structural member, holding the concrete in compression for greater strength.

Protected membrane roof (PMR)

An insulated and ballasted roof assembly in which the insulation and ballast are applied on top of the membrane (sometimes referred to as an inverted roof assembly).

Protection course

(1) A sacrificial material used to shield a waterproofing material from damaging external forces; (2) a separate layer of material installed on top of the membrane to protect a vegetative roof waterproofing membrane from damage.


Pounds per square inch.

Puncture resistance

The ability of a material to withstand the action of a penetrating or puncturing object.


A horizontal secondary structural member that transfers loads to the primary structural framing.
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The process of installing an additional roof covering over a prepared existing roof covering without removing the existing roof covering.


Solar reflectance is the fraction of the solar energy (flux) that is reflected by the surface; reflectivity is expressed as a percentage or a decimal value between 0.00 and 1.00.


Defined as the reflectance of an opaque, optically flat surface or coating of thickness sufficient to be a completely opaque. Reflectivity is a property of a material, while “reflectance” is a property of a sample of the material.

Reinforced membrane

A roof or waterproofing membrane that has been strengthened by the addition or incorporation of one or more reinforcing materials, including woven or nonwoven glass fibers, polyester mats or scrims, nylon or polyethylene sheeting.

Relative humidity (RH)

The ratio of the pressure of water vapor present in a given volume of air to the pressure of fully saturated water vapor at the same temperature, expressed as a percentage.


The process of removing the existing roof covering, repairing any damaged substrate and installing a new roof covering; also know as “tear-off and replacement.”


The process of recovering or replacing an existing roof covering.

Roll materials

A general term applied to rolls of roofing felt, ply sheet, etc., that are typically furnished in rolls.

Roll roofing

Coated felts, either smooth or mineral-surfaced.


(1) The cover of a building; (2) to cover with a roof.

Roof assembly

An assembly of interacting roof components including the roof deck, vapor retarder (if present), insulation and membrane or primary roof covering designed to weatherproof.

Roof slope

The angle a roof surface makes with the horizontal, expressed as a ratio of the units of vertical rise to the units of horizontal length (sometimes referred to as run). For English units of measurement, when dimensions are given in inches, slope may be expressed as a ratio of rise to run, such as 4:12, or as an angle in degrees.

Roof system

A system of interacting roof components generally consisting of a membrane or primary roof covering and roof insulation (not including the roof deck) designed to weatherproof and sometimes improve the building’s thermal resistance.
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A bitumen of low softening point for impregnating dry felts in the manufacture of saturated roofing felts.

Saturated felt

A felt that has been immersed in hot bitumen; the felt adsorbs as much bitumen as it can retain under the processing conditions but remains porous and contains voids.


A woven, nonwoven or knitted fabric composed of continuous strands of material used for reinforcing or strengthening membranes.


A joint formed by mating two separate sections of material. Seams can be made or sealed in a variety of ways, including adhesive bonding, hot-air welding, solvent welding, using adhesive tape and sealant.

Self-adhering membrane

A membrane that can adhere to a substrate and to itself at overlaps without the use of an additional adhesive. The undersurface of a self-adhering membrane is protected by a release paper or film, which prevents the membrane from bonding to itself during shipping and handling.


A term used to describe materials that have the ability to adhere to a variety of surfaces when contact is promoted by application of pressure but that require no substances to form the bond.


(1) An edge or edging that differs from the main part of a fabric, granule-surfaced roll roofing or cap sheet, or other material; (2) a specially defined edge of the material (lined for demarcation), which is designed for some special purpose, such as overlapping or seaming.

Service life

(1) The period of time a building component or system will function successfully without replacement or excessive repair assuming reasonable or expected periodic maintenance is performed; (2) the number of years of service a material, system or structure will provide before rehabilitation or replacement is required.

Shear strength

The resistance to forces that cause or tend to cause two contiguous parts of a body to slide relative to each other in a direction parallel to their contact plane.

Shelf life

The maximum time interval during which a material may be stored and remain in a usable condition according to the material manufacturer; usually related to storage conditions.


A decrease in one or more dimensions of an object or material.

Side lap

The continuous longitudinal overlap of neighboring like materials.

Side-lap fastener

A fastener used to connect adjacent panels together at the side lap.


A layer of reinforced concrete, generally flat and horizontal (or minimally sloped), usually of uniform thickness, placed on prepared earth or supported by beams, columns or walls.

Slab below grade

A slab on grade below the elevation of the exterior earth grade. See “slab on grade.”

Slab on grade

A horizontal placement of concrete placed directly over a prepared earth substrate.


The angle of incline, usually expressed as a ratio of rise to run, or as an angle.

Softening point

The temperature at which bitumen becomes soft enough to flow as determined using test methods defined in ASTM D36 (ring-and-ball) or D3461 (Mettler cup-and-ball).

Solids content

The percentage by weight of the nonvolatile matter in an adhesive.


A measure of the extent to which a material can be dissolved in a given solvent under specified conditions.


Any liquid used to dissolve another material.


The breaking off of plate-like pieces from a concrete, rock or masonry surface.


A precise statement of a set of requirements to be satisfied by a material, product, system or service.


Bonding or joining of overlapping materials.


(1) A unit used in measuring roof area equivalent to 100 square feet; (2) a quantity of material sufficient to cover 100 square feet of a roof deck.


(1) A blade of leather or rubber set on a handle and used for spreading, pushing or wiping liquid material on, across or off a surface; (2) to smooth, wipe or treat with a squeegee.


Temporary or permanent discoloration to the surface of a roof membrane, coating or other covering caused by foreign material on the surface.

Starter sheet

(1) Felt, ply sheet or membrane strip that is made or cut to a width narrower than the standard width of the roll material and used to start the shingling pattern at a roof edge; (2) particular width sheet designed for perimeters in some mechanically attached and fully adhered single-ply systems.

Strip flashing

Membrane flashing strips used for sealing or flashing metal flashing flanges into the roof membrane.

Stripping in

Application of membrane stripping ply or plies.

Structural panel

A metal roof panel designed to be applied over open framing rather than a continuous or closely spaced roof deck.

Styrene butadiene styrene copolymer (SBS)

High molecular weight copolymer with thermoset and thermo-plastic properties (thermoplastic elastomer), formed by the block copolymerization of styrene and butadiene monomers; used as the modifying compound in SBS polymer-modified asphalt roofing membranes to impart rubberlike qualities to the asphalt.


The surface upon which a roofing or waterproofing membrane is applied (e.g., in roofing, the structural deck or rigid board insulation).


An intentional depression around a roof drain or scupper that promotes drainage.


The top layer or layers of a roof covering specified or designed to protect the underlying roofing from direct exposure to the weather.
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Tapered insulation

A system of precut or premolded insulation boards or a poured insulation fill designed to provide slope to the roof deck before installing the roof membrane.


(1) The technique of connecting joints between insulation boards or deck panels with tape; (2) the technique of using self-adhering tape-like materials to seam or splice single-ply membranes.

Tear resistance

The load required to tear a material when the stress is concentrated on a small area of the material by the introduction of a prescribed flaw or notch.

Tear strength

The maximum force required to tear a specimen.

Tensile strength

The strength of a material under tension as distinct from torsion, compression or shear.


The treatment or method of anchoring and/or sealing the free edges of a membrane in a roof or waterproofing system.

Test method

A definitive procedure for the identification, measurement and evaluation of one or more qualities, characteristics or properties of a material, product, system or service that produces a test result.

Thermal barrier

In torched membrane applications over combustible substrates, an above-deck barrier incorporated into a roof system before torching as a fire-safety practice. Acceptable thermal barriers include: minimum 3/4-inch-thick perlite board insulation, minimum 3/4-inch-thick fiberglass or mineral wool board insulation, or minimum 1/4-inch-thick glass-faced gypsum board.

Thermal conductance (C)

The time rate of steady-state heat flow through a unit area of a material or construction induced by a unit temperature difference between the body surfaces. In English (inch•pound) units of measurement, the number of Btu that pass through a specified thickness of a one-square-foot sample of material in one hour with a temperature difference between the two surfaces is 1 degree Fahrenheit. In English (inch•pound) units, it is expressed as Btu/h•ft2•F.
Note 1: A thermal conductance (C) value applies to a specific thickness of a specific material.
Note 2: It is mathematically incorrect to multiply or divide the thermal conductance (C) value for a specific thickness of a material to determine the thermal conductance value of a different thickness of the same material.
Note 3: It is mathematically incorrect to add thermal conductance (C) values to determine overall thermal performance. If it is necessary to determine the overall thermal performance of a construction, it is appropriate to convert the individual thermal conductance (C) values to thermal resistance (R) values (i.e., R = 1/C) and then add the thermal resistance values (i.e., Rt = R1 + R2 + …).

Thermal conductivity (k)

The time rate of steady state heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area. In English (inch•pound) units of measurement, it is the number of Btu that pass through a 1-inch thickness of a 1-square-foot sample of material in one hour with a temperature difference between the two surfaces of 1 degree Fahrenheit. In English (inch•pound) units, it is expressed as Btu•inch/h•ft2•F.
Note 1: A thermal conductivity (k) value applies to 1-inch thickness of a specific material.
Note 2: It is mathematically incorrect to add, multiply or divide the thermal conductivity (k) value of a material to determine the thermal performance value of a different thickness of the same material. If it is necessary to determine the thermal performance of a specific thickness of a material, it is appropriate to convert the thermal conductivity (k) of the material to a thermal resistance (R) value (i.e., R = 1/k) and then perform the mathematical calculation.

Thermal cycling

A fluctuation in material, system component or system stress mode, such as a change from tension to compression and back to tension, and any related displacements caused by recurring temperature fluctuations.

Thermal expansion

The increase in the dimension or volume of a body because of temperature variations.

Thermal insulation

A material applied to reduce the flow of heat.

Thermal movement

Changes in dimension of a material as a result of temperature changes.

Thermal resistance (R)

The quantity determined by the temperature difference at steady state between two defined surfaces of a material or construction that induces a unit heat flow rate through a unit area. In English (inch•pound) units, it is expressed as F•ft2•h/Btu.
Note 1: A thermal resistance (R) value applies to a specific thickness of a material or construction.
Note 2: The thermal resistance (R) of a material is the reciprocal of the thermal conductance (C) of the same material (i.e., R = 1/C).
Note 3: Thermal resistance (R) values can be added, subtracted, multiplied and divided by mathematically appropriate methods.

Thermal transmittance (U or U-factor)

The heat transmission in unit time through unit area of a material or construction and the boundary air films induced by unit temperature difference between the environments on each side. In English (inch•pound) units, it is expressed as Btu/h•ft2•F.
Note 1: A thermal transmittance (U) value applies to the overall thermal performance of a system (e.g., roof assembly).
Note 2: Thermal transmittance (U) is sometimes called the overall coefficient of heat transfer.
Note 3: Thermal transmittance (U) is the reciprocal of the overall thermal resistance (Rt) of a system (i.e., U = 1/Rt).


A material that softens when heated and hardens when cooled. This process can be repeated provided the material is not heated above the point at which decomposition occurs.

Thermoplastic olefin membrane (TPO membrane)

A blend of polypropylene and ethylene-propylene polymers, colorant, flame retardants, ultraviolet radiation absorbers and other proprietary substances that may be blended with the TPO to achieve the desired physical properties. The membrane may or may not be reinforced.


A class of polymers that when cured using heat, chemical or other means change into a substantially infusible and insoluble material.

Through-wall flashing

A water-resistant membrane or material assembly extending totally through a wall and its cavities positioned to direct water within the wall to the exterior, usually through weep holes.


In roofing and waterproofing, the transitional seal used to terminate a roofing or waterproofing application at the top or bottom of flashings or by forming a watertight seal with the substrate, membrane, or adjacent roof or waterproofing system.
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UL label

An identification label or seal affixed to a roofing product or package with the authorization of Underwriters Laboratories Inc. The presence of the label indicates the product has met certain performance criteria.

Unforeseen conditions

(1) Unusual situation not reasonably anticipated based on contract documents; (2) unknown physical condition of an unusual nature that differs materially from those ordinarily encountered.

UV degradation

Damage done by the ultraviolet rays of the sun.
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Vapor migration

The movement of water vapor from a region of high vapor pressure to a region of lower vapor pressure.

Vapor pressure

The pressure exerted by a vapor of a solid or liquid when in equilibrium with the liquid or solid.

Vapor retarder

Layer(s) of material or a laminate used to appreciably reduce the flow of water vapor into a roof assembly.


(1) A single wythe of masonry for facing purposes that may not be structurally connected; (2) any of the thin layers of wood glued together to form plywood.


The resistance of a material to flow under stress. For bitumen, viscosity measurements are reported in centipoise (cP or cPs) at a specific temperature; as viscosity increases, flow rate decreases.


An open space or break in consistency.
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(1) High traffic and high-service-frequency areas on a rooftop, particularly those leading and adjacent to vents, hatches and heavy duty air-conditioning units; (2) materials or accessories such as traffic mats or pavers installed on rooftop surfaces to provide wearing surfaces for traffic and/or protect a roof system from damage resulting from rooftop traffic.

Water table

The level within the ground below which the soil is saturated with water.

Water vapor pressure

The pressure of water vapor at a given temperature; the component of atmospheric pressure contributed by the presence of water vapor. See “vapor pressure.”

Water vapor transmission

A measure of the rate of transmission of water vapor through a material under controlled laboratory conditions of temperature and humidity. Customary units are grains/h•ft2.


The quality of a membrane, membrane material or other component to prevent water entry.


Treatment of a surface or structure to prevent the passage of water under hydrostatic pressure.

Waterproofing assembly

An assembly of interacting waterproofing components including the deck, membrane and protection/drainage/insulation course.

Waterproofing membrane

The part of a waterproofing system that has the primary function of excluding water; it does not include accessories such as drainage materials or protection boards.

Waterproofing system

A system of interacting waterproofing components consisting at a minimum of a membrane but may also include protection, drainage and insulation courses, as well as waterstops, expansion and control joints, various flashings and counterflashings, and overburden such as pavers, cast concrete and wire mesh or rebars. It does not include the substrate.

Wet film gauge

A gauge for measuring the thickness of wet coating as applied to a flat smooth surface.

Wet film thickness

The thickness, expressed in mils, of a coating or mastic as applied but not cured.

Wind uplift

The force caused by the deflection of wind at roof edges, roof peaks or obstructions causing a drop in air pressure immediately above the roof surface.
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Want to learn more about residential roofing systems? Read our Residential Roofing Glossary.