Roofing Dictionary

Roofing terms you should know

It’s important to make an educated decision when it comes to repairing or replacing your roof. The following is a list of roofing terms posted for your knowledge and benefit. Below are common terms, phrases and definitions associated with residential and commercial roofing. Please don’t hesitate to contact us if you have any questions, comments or concerns.


Crushed stone, crushed slag, gravel or granules used as a surfacing on a built-up or modified bitumen roof systems.


The cracking of the surfacing bitumen on a built-up roof, producing a pattern of cracks similar to an alligator’s hide; the cracks may or may not extend through the surfacing bitumen.

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 no expansion joints have been provided.


A group of natural, fibrous, impure silicate materials.


Asphalt is also known as bitumen, and is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It is a key ingredient in many roofing materials.

Asphalt, Air Blown

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

Asphalt Felt

An 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 cold.

Asphalt, Steam Blown

An asphalt produced by blowing steam through molten asphalt to modify its properties.


A high molecular weight hydrocarbon fraction precipitated from asphalt by a designated paraffinic naphthasolvent at a specified temperature and solvent-asphalt ratio.

Asphaltic Roof Fill

A blend of asphalt and pearlite aggregate typically installed at precise drainage slopes.


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

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 membrane.


A black viscous mixture of hydrocarbons obtained naturally or as a residue from petroleum.  Also used as a generic term to denote materials composed of mainly asphalt or coal-tar pitch.  Typically used directly or in the manufacture of products for roofing and waterproofing.

Bituminous coating

A fluid material consisting mainly of asphalt or coal-tar pitch applied in the field to the roof surface to provide weather protection to the original roof substrate.  Also referred to as an asphalt coating.

Bituminous Emulsion

(1) a suspension of minute globules of bituminous material in water or in an aqueous solution; (2) a suspension of minute globules of water or an aqueous solution in a liquid bituminous material (invert emulsion).

Bituminous Grout

A mixture of bituminous material and fine sand that will flow into place without mechanical manipulation when heated.


A small bubble or blister in the flood coating of a gravel-surfaced roof membrane.

Blind Nailing

The practice of nailing the back portion of a roofing ply in a manner that the fasteners are not exposed to the weather in the finished product.


An enclosed pocket of air mixed with water or solvent vapor, trapped between impermeable layers of felt, or between the felt and substrate.


Wood built into a roofing system above the deck and below the membrane and flashing to stiffen the deck around an opening, act as a stop for insulation, or to serve as a nailer for attachment of the membrane or flashing.


The adhesive and cohesive forces holding two roofing components in intimate contact.


Embedding a ply of roofing material by using a broom to smooth out the ply and ensure contact with the adhesive under the ply.

British Thermal Unit (BTU)

The heat energy required to raise the temperature of 1 pound of water 1 degree Fahrenheit.

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 surface with mineral aggregate, bituminous materials, or a granule-surfaced roofing sheet.

Cant Strip

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


The action by which the surface of a liquid (where it is in contact with a solid) is elevated or depressed, depending upon the relative attraction of the molecules of the liquid for each other and for those of the solid.

Cap Sheet

A granule-surfaced coated sheet used as the top ply of a built-up roof membrane or flashing.


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

Coal Tar

A dark brown to black, semi-solid hydrocarbon obtained as residue from the partial evaporation or distillation of coal tar.

Coal-Tar Felts

A felt that has been saturated with refined coal tar.

Coated Sheet Felts

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

Cold-Processing Roofing

A continuous, semi-flexible roof membrane, consisting of plies of felts, mats or fabrics that are laminated on a roof with alternate layers of cold-applied roof cement and surfaced with a cold-applied coating.


The conversion of water vapor or other gas to liquid as the temperature drops or the atmospheric pressure rises.


The covering piece on top of a wall exposed to the weather, usually sloped to shed water.


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.


(1) the term used for each application of material that forms the waterproofing system or the flashing; (2) one layer of a series of materials applied to a surface (i.e., a five-course wall flashing is composed of three applications of mastic with one ply of felt sandwiched between each layer of mastic).


The surface area continuously covered by a specific quantity of a particular roofing material.


A separation or fracture occurring in a roof membrane or roof deck, generally caused by thermal induced stress or substrate movement.


The permanent deformation of a roofing material or roof system caused by the movement of the roof membrane that results from continuous thermal stress or loading.


A relatively small, elevated area of a roof constructed to divert water around a chimney, curb or other projection.


Solvent-thinned bitumen used in cold process roofing adhesives, flashing cements and roof coatings.


A detail designed to prevent lateral water movement into the insulation where the membrane terminates at the end of a day’s work, or used to isolate sections of the roofing system. It is usually removed before the continuation of the work.


Treatment of a surface or structure to resist the passage of water in the absence of hydorstatic pressure.

Dead Level

Absolutely horizontal, or zero slope.

Dead Loads

Non-moving rooftop loads, such as mechanical equipment, air conditioning units, and the roof deck itself.


The structural surface to which the roofing or waterproofing system is applied.


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

Dew Point

The temperature at which water vapor starts to condense in cooling air at the existing atmospheric pressure and vapor content.


The process of applying two layers of aggregate and bitumen to a built-up roof.


A device that allows for the flow of water from a roof area.


A reduction in the softening point of bitumen that occurs when bitumen is heated in the absence of air.

Edge Sheets

Felt strips that are cut to widths narrower than the standard width of the full felt roll, used to start the felt shingling pattern at a roof edge.

Edge Stripping

Application of felt strips cut to narrower widths than the normal felt roll width to cover a joint between flashing and built-up roofing.

Edge Venting

The practice of providing regularly spaced protected openings along a roof perimeter to relieve moisture vapor pressure.


A macromolecular material that returns rapidly to its approximate initial dimensions and shape after substantial deformation by a weak stress and the subsequent release of that stress.


A rubber like synthetic polymer that will stretch when pulled and will return quickly to its original shape when released.


(1) the process of pressing a felt, aggregate, fabric, mat, or panel uniformly and completely into hot bitumen or adhesive; (2) the process of pressing granules into coating in the manufacture of factory prepared roofing.


The intimate dispersion of an organic material and water achieved by using a chemical or clay emulsifying agent.


A continuous membrane edge seal formed at the perimeter and at penetrations by folding the base sheet or ply over the plies above and securing it to the top of the membrane. The envelope prevents bitumen seepage from the edge of the membrane.

Equilibrium Moisture

(1) the moisture content of a material stabilized at a given temperature and relative humidity, expressed as percent moisture by weight; (2) the typical moisture content of a material in any given geographical area.

Equiviscous Temperature (EVT)

The temperature at which the viscosity is 75 centipoise for asphalt and 25 centipoise for coal tar products; the recommended temperature plus or minus 25º F at the time of application.

Expansion Joint

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


(1) the traverse dimension of a roofing element not overlapped by an adjacent element in any roof system. The exposure of any ply in a membrane may be computed by dividing the felt width minus 2 inches by the number of shingled plies; thus, the exposure of 36 inch-wide felt in a shingled, four-ply membrane should be 8 1/2 inches; (2) the time during which a portion of a roofing element is exposed to the weather.


A woven cloth of organic or inorganic filaments, threads or yarns.

Factory Mutual (FM)

An organization that classifies roof assemblies for their fire characteristics and wind uplift resistance for insurance companies in the United States.

Factory Square

108 square feet of roofing material.


A flexible sheet manufactured by the interlocking of fibers through a combination of mechanical work, moisture and heat. Felts are manufactured principally from vegetable fibers (organic felts), asbestos fibers (asbestos felts) or glass fibers (glass fiber felts); other fibers may be present in each type.

Felt Layer

A machine used for applying bitumen and built-up roofing felts.

Felt Mill Ream

The mass in pounds of 480 square feet of dry, unsaturated felt; also termed “point weight”.

Fine Mineral Surfacing

Water-insoluble, inorganic material, more than 50 percent of which passes the no. 35 sieve, used on the surface of roofing.


(1) a half-cylindrical or half-conical opening formed by an edge wrinkle; (2) in shingles, a half-conical opening formed at a cut edge.


An impervious material or components used to seal the roof system at perimeters, penetrations, walls, expansion joints, valleys, drains or other places where the roof covering is either interrupted or terminated and prevent water from entering the structure.

Flashing cement

A trowelable mixture of cutback bitumen and mineral stabilizers, including asbestos or other inorganic fibers.

Flood Coat

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

Fluid Applied

An elastomeric material, fluid at ambient temperature, that dries or cures after application to form a continuous membrane. Such systems normally do not incorporate reinforcement.

Glass Felt

Glass fibers bonded into a sheet with resin and suitable for impregnation in the manufacture of bituminous waterproofing materials, roof membranes, and shingles.

Glass Mat

A thin mat composed of glass fibers with or without a binder.

Glaze Coat

(1) the top layer of asphalt in a smooth surfaced built-up roof assembly; (2) a thin protective coating of bitumen applied to the lower plies or top ply of a built-up roof membrane when application of additional felts or the flood coat and aggregate surfacing are delayed.


Course, granular aggregate, with pieces larger than sand grains, resulting from the natural erosion of rock.

Gravel Spot

A flanged device, frequently metallic, designed to provide a continuous finished edge for roofing material and to prevent loose aggregate from washing off of the roof.


The minimum distance, measured at 90 degrees to the eaves along the face of a shingle or felt, from the upper edge of the shingle or felt to the nearest exposed surface.


An area where a liquid-applied material is missing.

“Hot Stuff” or “Hot”

The roofer’s term for hot bitumen.


Attracting, absorbing and retaining atmospheric moisture.

Hydrostatic pressure

According to the NRCA, hydrostatic pressure is defined as “the pressure equivalent to that exerted on a surface by a column of water of a given height.” The pressure increases in proportion to the depth measured from the surface because of the increasing weight of the water exerting downward force.

Ice Dam

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


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


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

Job-Average Basis

A technique for determining the average dimensions or quantities of materials, by analysis of roof test cuts. The technique requires a minimum of three test cuts per roof area, plus one cut for each additional 10,000 square feet of roof area. Job-average basis is computed by dividing the sum of all measurements taken by the number of measurements taken. The results would describe the job-average for the quantity or dimension. It’s generally not considered a good idea to evaluate roofs on this basis as sample size is small relative to job size. The NCRA recommends competent visual examination.


An imperfection or non-homogeneity in materials used in fabric construction, the presence of which causes surface irregularities.

Live Loads

Moving roof installation equipment, wind, snow, ice or rain.


A thick material used as an adhesive or sealant for holding roofing and waterproofing membranes in place and preventing water from entering where they terminate.


A flexible or semi-flexible roof covering or waterproofing layer, whose primary function is the exclusion of water.


The square opening of a sieve.

Metal Flashing

Metal flashing is frequently used as through-wall flashing, cap flashing, counterflashing or gravel stops.

Mineral Fiber Felt

A felt with mineral wood as its principal component.

Mineral Granules

Opaque, natural, or synthetically colored aggregate commonly used to surface cap sheets, granule-surfaced sheets, and roofing shingles.

Mineral Stabilizer

A fine, water-insoluble inorganic material, used in a mixture with solid or semi-solid bituminous materials.

Mineral-Surfaced Roofing

Built-up roofing materials whose top ply consists of a granule-surfaced sheet.

Mineral-Surfaced Sheet

A felt that is coated on one or both sides with asphalt and surfaced with mineral granules.

Modified Bitumen

Are composite sheets consisting of a copolymer modified bitumen often reinforced and sometimes surfaced with various types of films, foils and mats.

Mole Run

A meandering ridge in a roof membrane not associated with insulation or deck joints.


An application procedure in which roofing elements (insulation boards, felt plies, cap sheets, etc.) are initially placed upside down adjacent to their ultimate locations, are coated with adhesive, and are then turned over and applied to the substrate.


The application of hot bitumen with a mop or mechanical applicator to the substrate or to the felts of a built-up roof membrane.


National Roofing Contractor Association. Professional trade group for the roofing industry.


A synthetic rubber (polychloroprene) used in liquid-applied and sheet-applied elastomeric roof membranes or flashings.

Nineteen-Inch Selvage

A prepared roofing sheet with a 17-inch granule surfaced exposure and a nongranule-surfaced 19-inch selvage edge. This material is sometimes referred to as SIS or as Wide Selvage Asphalt Roll Roofing Material Surfaced with Mineral Granules.


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


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

Parapet Wall

That part of any wall entirely above the roof.


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


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).


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.

Picture Framing

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

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.


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.


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


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.


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


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


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.


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.

Relative Humidity

The ratio of the weight of moisture in a given volume of air-vapor mixture to the saturated (maximum) weight of water vapor at the same temperature, expressed as a percentage. For example, if the weight of the moist air is 1 pound and if the air could hold 2 pounds of water vapor at a given temperature, the relative humidity (RH) is 50 percent.


The practice of removing an existing roof system and replacing it with a new roofing system.


The process of re-covering or replacing an existing roofing system.


Resaturants are carefully prepared blends of selected materials designed to help rejuvenate the old roof. The type of restaurant will vary depending on the existing roof material.


An upward, tenting displacement of a roof membrane frequently occurring over insulation joints, deck joints and base sheet edges.

Roll Roofing

Smooth-surfaced or mineral-surfaced coated felts.

Roof Assembly

An assembly of interacting roof components (including the roof deck) designed to weatherproof and, normally, to insulate a building’s top surface.


The trade name for the workman who applies roofing material.

Roofing felt

Also known as felt paper, roofing underlayment or rolled roofing, roofing felt is composed of natural or synthetic materials that is saturated or coated with a protective coating, typically bitumen, which allows the product to breathe. It helps to repel water, provides backup protection from ice damage, provides extra weather protection and can help meet Class A fire ratings.

Roof System

A system of interacting roof components (not including the roof deck) designed to weather proof and, normally, to insulate a building’s top surface.


A small structure that helps channel surface water to drains, frequently located in a valley, and often contracted like a small hip roof or like a pyramid with a diamond shape base.

Saturated Felt

A felt that has been partially saturated with low softening point bitumen.


An apparatus with circular apertures from separating sizes of materials.


A hatch that provides access to the roof from the interior of the building.


(1) a narrow closure strip made of bituminous materials; (2) to secure a roof from the entry of moisture.


A mixture of polymers, fillers, and pigments used to fill and seal joints where moderate movement is expected; it cures to a resilient solid.


An edge or edging that differs from the main part of (1) a fabric, or (2) granule-surfaced roll roofing material.

Selvage Joint

A lapped joint designed for mineral-surfaced cap sheets. The mineral surfacing is omitted over a small portion of the longitudinal edge of the sheet below in order to obtain better adhesion of the lapped sheet surface with the bituminous adhesive.

Shark Fin

An upward-curled felt side lap or end lap.


(1) a small unit of prepared roofing material designed for installation with similar units in overlapping rows on inclines normally exceeding 25 percent; (2) to cover with shingles; (3) to apply any sheet material in overlapping rows like shingles.


(1) the procedure of laying parallel felts so that one longitudinal edge of each felt overlaps and the other longitudinal edge underlaps, the adjacent felt. Normally, felts are shingled on a slope so that the water flows over rather than against each lap; (2) the application of shingles to a sloped roof.


An apparatus with apertures for separating sizes of material.


A hard, air-cooled aggregate that is left as a residue from blast furnaces, used as a surfacing aggregate.


Relative lateral movement of adjacent components of a built-up membrane. It occurs mainly in roofing membranes on a slope, sometimes exposing the lower lies or even the base sheet to the weather.

Smooth-Surfaced Roof

A built-up roof membrane surfaced with a layer of hot-mopped asphalt, cold-applied asphalt clay emulsion, cold-applied, asphalt cutback, or sometimes with an unmopped inorganic felt.

Softening Point

The temperature at which bitumen becomes soft enough to flow, as determined by an arbitrary, closely defined method.

Softening Point Drift

A change in the softening point of bitumen during storage or application.

Solid Mopping

A continuous mopping of a surface, leaving no unmopped areas.


Spalling happens when water enters brick, stone or concrete and causes the surface to flake off, peel or pop out. Eventually it can cause crumbling and destruction of the structure.


A membrane tear resulting from tensile strength.

Spot Mopping

A mopping pattern in which hot bitumen is applied in roughly circular areas, leaving a grid of unmopped, perpendicular bands on the roof.

Sprinkle Mopping

A random mopping pattern in which heated bitumen beads are strewn onto the substrate with a brush or mop.


The process of removing the roofing aggregate and most of the bituminous top coating by scraping and chipping.


The term used to describe 100 square feet of roof area.

Stack Vent

A vertical outlet in a built-up roof system designed to relieve the pressure exerted by moisture vapor between the roof membrane and the vapor retarder or deck.

Strip Mopping

A mopping pattern in which hot bitumen is applied in parallel bands.

Stripping or Strip-Flashing

(1) the technique of sealing a joint between metal and the built-up roof membrane with one or two plies of felt or fabric and hot-applied or cold-applied bitumen; (2) the technique of taping joints between insulation boards or deck panels.


The surface upon which the roofing or waterproofing membrane is applied (i.e., the structural deck or insulation).


An intentional depression around a drain.

Superimposed Loads

Loads that are added to existing loads. For example, a large stack of insulation boards placed on top of a structural steel deck.

Tapered Edge Strip

A tapered insulation strip used to (1) elevate the roof at the perimeter and at curbs that extend through a roof; (2) provide a gradual transition from one layer of insulation to another.


A brown or black bituminous material, liquid or semi-solid in consistency, in which the predominating constituents are bitumens obtained as condensates in the processing of coal, petroleum, oil shale, wood, or other organic materials.

Test Cut

A sample of the roof membrane that is cut from a roof membrane to: (a) determine the weight of the average interply bitumen moppings; (b) diagnose the condition of the exiting membrane (e.g., to detect leaks or blisters).

Thermal Conductance (C)

A unit of heat flow that is used for specific thicknesses of material or for materials of combination construction, such as laminated insulation.

Thermal Conductivity (k)

The heat energy that will be transmitted by conduction through one square foot of one inch thick homogeneous material in one hour when there is a difference of one degree Fahrenheit perpendicularly across the two surfaces of the material.

Thermal Insulation

A material applied to reduce the flow of heat.

Thermal Resistance (R)

An index of a material’s resistance to heat flow; it is the reciprocal of thermal conductivity (k) or thermal conductance (C).

Thermal Shock

The stress-producing phenomenon resulting from sudden temperature changes in a roof membrane when, for example, a rain shower follows brilliant sunshine.

Through-Wall Flashing

A water-resistant membrane or material assembly extending through a wall and its cavities, positioned to direct water entering the top of the wall to the exterior.

Tuck Pointing

(1) troweling mortar into a joint after masonry units are laid; (2) final treatment of joints in cut stonework. Mortar or a putty-like filler is forced into the joint after the stone is set.

Underwriters Laboratories (UL)

An organization that classifies roof assemblies for their fire characteristics and wind uplift resistance.

Vapor Migration

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

Vapor Retarder

A material designed to restrict the passage of water vapor through a roof or wall.


An opening designed to convey water vapor or other gases from inside a building or a building component to the atmosphere, thereby relieving vapor pressure.


An aggregate used in lightweight insulating concrete, formed by the heating and consequent expansion of a micaceous mineral.


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


A masonry wall, one masonry unit, a minimum of two inches thick.