Roofing Dictionary

The following is a dictionary of basic roofing terms from the NRCA Roofing and Waterproofing Manual.
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z


R-value: see Thermal resistance.

Racking: a method of asphalt shingle application, also referred to as the straight-up method, whereby shingle courses are applied vertically, up the roof rather than laterally or across and up. The NRCA Roofing and Waterproofing Manual—Fifth Edition Glossary 997

Rafter: one of a series of sloped structural members, that extend from the ridge or hip to the downslope perimeter or eave, designed to support the roof deck and its associated loads.

Raggle: a groove or slot, often cut in a masonry wall or other vertical surface adjoining a roof, for inserting an inset flashing component such as a reglet.

Rake: the sloped edge of a roof at or adjacent to the first or last rafter.

Rake-starter (Bleeder strip): starter-strip used along rake edges in conjunction with asphalt shingle roofing.

Re-cover: the addition of a new roof membrane or steep-slope roof covering over a major portion of an existing roof assembly. This process does not involve removal of the existing roofing.

Reflectivity: see Light reflectance.

a sheet metal receiver for the attachment of counterflashing. A reglet may be surface-mounted, inset into a raggle or embedded behind cladding.

Reinforced membrane: a roofing 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.

Release tape (or Strip): a plastic film or paper strip that is applied to the back of self-sealing shingles and other materials. The strip prevents the material from sticking together in the roll or bundle. With asphalt shingles, the strip need not be removed for application of the shingles.

the practice of removing an existing roof system down to the roof deck and replacing it with a new roofing system.

Reroofing: the process of re-covering, or tearing-off and replacing an existing roof system.

component B in SPF. This component contains a catalyst, blowing agent, fire retardants, surfactants and polyol. It is mixed with the A component to form polyurethane.

Ridge: highest point on the roof, represented by a horizontal line where two roof areas intersect, running the length of the area.

Ridge cap: a material or covering applied over the ridge of a roof.

Ridge course: the last or top course of roofing materials, such as tile, roll roofing, shingles, etc., that covers the ridge and overlaps the intersecting field roofing.

Ridge vent: a ventilator located at the ridge that allows the escape of warm and/or moist air from the attic area or rafter cavity.

Ridging: see Buckle.

Roll materials: a general term applied to rolls of roofing felt, ply sheet, etc., which are typically furnished in rolls.

Roll roofing: coated felts, either smooth or mineral-surfaced.

Roof: (1) the cover of a building; (2) to cover with a roof.

Roof area divider: refer to area divider. Roof area expansion Joint: see expansion joint. The NRCA Roofing and Waterproofing Manual—Fifth Edition 998 Glossary

Roof assembly: an assembly of interacting roof components including the roof deck, vapor retarder (if present), insulation and roof covering.

Roof cement: see Asphalt roof cement or Coal tar roof cement.

Roof covering: the exterior roof cover or skin of the roof assembly, consisting of membrane, panels, sheets, shingles, tiles, etc.

Roof curb: raised frame used to mount mechanical units (such as air conditioning or exhaust fans), skylights, etc. on a roof.

Roof jack: a metal or wood bracket used to support toe-boards on steep-slope roofs. (also see Flashing Collar.)

Roof overhang: a roof extension beyond the exterior wall of a building.

Roof seamer: (1) machine that crimps neighboring metal roof panels together; (2) machine that welds laps of membrane sheets together using heat, solvent, or dielectric energy.

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.

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, to improve the
building’s thermal resistance.

Rosin paper (specifically Rosin-sized sheathing paper): a nonasphaltic paper used as a sheathing paper or slip sheet in some roof systems.

Rubber: a material that is capable of recovering from large deformations quickly and forcibly.

Run: horizontal dimension of a slope.


Saddle: a small tapered/sloped roof area structure that helps to channel surface water to drains. Frequently located in a valley. A saddle is often constructed like a small hip roof or pyramid with a diamond-shaped base. (see Cricket.)

Sag: undesirable excessive flow in material after application to a surface.

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.

SBCCI: Southern Building Code Congress International, Inc.

SBS: see Styrene butadiene styrene.

Scarfed: shaped by grinding.

Screeding: the process of striking off excess concrete to bring the top surface of the concrete to the proper finish and elevation.

Screen wall: a nonstructural wall erected around units or curbs on a roof. Typically the framing consists of girts with a wood or metal covering attached to the frame.

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

Scupper: drainage device in the form of an outlet through a wall, parapet wall or raised roof edge lined with a soldered sheet metal sleeve.

Scuttle: a hatch that provides access to the roof from the interior of the building. The NRCA Roofing and Waterproofing Manual—Fifth Edition
Glossary 999

SDI: Steel Deck Institute.

Sealant: (1) a material that has the adhesive and cohesive properties to form a seal; (2) a mixture of polymers, fillers, and pigments used to fill and seal joints where moderate movements is expected; unlike caulking, it cures to a resilient solid.

Sealant backing: a compressible material placed in a joint before applying a sealant.

Sealer: a coating designed to prevent excessive absorption of finish coats into porous surfaces; a coating designed to prevent bleeding.

Sealing washer: a rubber or neoprene washer, sometimes metal-backed, typically placed on a fastener to prevent water from migrating into and through the fastener hole.

Seam: 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, sealant, etc.

Seam sample: in single-ply and sometimes modified bitumen membrane roofing, a sample from the membrane
that extends through the side lap of adjacent rolls of membrane, taken for the purpose of assessing the quality of the seam.

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.

Self-drilling screw: a fastener that taps and drills its own hole during application.

Self-sealing shingle: an asphalt shingle containing a factory-applied strip or spots of heat sensitive adhesive intended to adhere the overlying shingle once installed on the roof and warmed by the sun.

Self-tapping screw: a fastener that forms receiving threads when turned in a previously drilled hole.

Selvage: (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.

Separator layer: refer to Slip sheet.

Service temperature limits: the minimum or maximum temperature at which a coating, SPF or other material will perform satisfactorily.

Set: to convert into a fixed or hardened state by chemical or physical action.

Shading: slight differences in surfacing color, such as shingle granule coloring, that may occur as a result of manufacturing operations.

Shark fin: an upward-curled felt side lap or end lap.

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 contrast.

Shed roof: a roof having only one sloping plane and no hips, ridges or valleys.

Shelf life: the maximum time a packaged material can be stored under specified conditions and still meet the performance requirements specified.

Shingle: (1) a small unit of prepared roofing designed for installation with similar units in overlapping rows or courses on inclines normally exceeding 3:12 slope (14°); (2) to cover with shingles; (3) to apply any sheet material in succeeding overlapping rows like shingles.The NRCA Roofing and Waterproofing Manual—Fifth Edition 1000 Glossary

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

Shrinkage: a decrease in one or more dimensions of an object or material.

Shrinkage crack: in waterproofing, a separation in a material, such as a concrete substrate, caused by the inability of the material to resist a reduction in size which occurs during its hardening or curing process or both.

SI: an abbreviation for the International System of Units (Le Systeme International d’Unites).

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.

Siding: the finish covering of an exterior wall of a frame building; the siding may be a cladding material such as wood, aluminum or vinyl (but not masonry).

Sieve: an apparatus with square apertures for separating sizes of material.

Sill: the bottom horizontal framing member of an opening, such as below a window or door.

Sill flashing: a flashing of the bottom horizontal framing member of an opening, such as below a window or door.

Single-lock standing seam: a standing seam that uses one overlapping interlock between two seam panels, in contrast with the double interlocking used in a double standing seam.

Single-ply membranes: roofing membranes that are field applied using just one layer of membrane material (either homogeneous or composite) rather than multiple layers.

Single-ply roofing: a roofing system in which the principal roof covering is a single layer flexible membrane often thermoset or thermoplastic membrane.

Skinning: the formation of a dense film on the surface of a liquid coating or mastic.

Skirt flashing: a formed metal counterflashing secured under a mechanical unit or skylight to cover and protect the upper edge of a base flashing and its associated fasteners.

Skylight: an opening in a roof that is glazed with a transparent or translucent material; used to admit diffused light to the space below.
Slab on grade: a horizontal placement of concrete placed directly over a prepared earth substrate.

Slag: a hard aggregate that is left as a residue from blast furnaces, which may be used as a surfacing material on certain (typically bituminous) roof membrane systems.

Slate: a hard, brittle metamorphic rock consisting mainly of clay minerals, used extensively as dimensional stone for steep roofing and in granular form as surfacing on some other roofing materials.

Slating hook: a steep-slope roofing attachment device, shaped like a hook, that can be used for fastening roofing slate.

Slip sheet: sheet material, such as reinforced kraft paper, rosin-sized paper, polyester scrim or polyethylene sheeting, placed between two components of a roof assembly (such as between membrane and insulation or deck) to ensure that no adhesion occurs between them and to prevent possible damage from chemical incompatibility, wearing or abrasion of the membrane.

Slit sample: in SPF roofing, a small cut about 1 inch x 1/2 inch x 1/2 inch (25 mm x 13 mm x 13 mm), in a half-moon shape, used to measure coating film thickness.

Slope: the angle of incline, usually expressed as a ratio of rise to run, or as an angle. (See Roof Slope.) The NRCA Roofing and Waterproofing Manual—Fifth Edition Glossary 1001

SMACNA: Sheet Metal and Air Conditioning Contractors National Association.

Smooth surface texture: in SPF roofing, the condition of the foam in which the surface shows spray undulation and is ideal for receiving a protective coating.
Smooth-surfaced roof: a roof membrane without mineral granule or aggregate surfacing.

Snap-on cap: a separate cap that snaps on over the vertical legs of some single standing or batten seam metal roof systems.

Snow guard: a series of devices attached to the roof in a pattern that attempts to hold snow in place, thus preventing sudden snow or ice slides from the roof; any device intended to prevent snow from sliding off a roof.

Snow load: the live load due to the weight of snow on a roof; included in design calculations.

Soffit: the exposed undersurface of any exterior overhanging section of a roof eave.

Soffit vent: a premanufactured or custom built air inlet source located at the downslope eave or in the soffit of a roof assembly.

Softening point: the temperature at which bitumen becomes soft enough to flow, as determined by an arbitrary, closely defined method (ASTM Standard test method D 36 or D 3461).

Softening point drift: a change in the softening point of bitumen during storage or application. (see Fallback.)

Soil stack:
a sanitation pipe that penetrates the roof; used to vent plumbing fixtures.

Solder: a lead/tin mixture that is melted and used to bond two pieces of some types of metals together.

Solid mopping: see Mopping.

Solids content: the percentage by weight of the nonvolatile matter in an adhesive.

Solvent: any liquid used to dissolve another material.

Solvent cleaners: used to clean some single-ply roofing membranes prior to splicing, typically including heptane, hexane, white gasoline, and unleaded gasoline.

Solvent welding: a process where a liquid solvent is used to chemically weld or join together two or more layers of certain membrane materials (usually thermoplastic).

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

Special steep asphalt: asphalt complying with ASTM D 312, Type IV. (see Asphalt.)

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

SPF: spray polyurethane foam.

SPFA: Sprayed Polyurethane Foam Alliance (a business unit of the American Plastics Council).

SPF compound: a term used to describe the raw materials (isocyanate and resin) used to make polyurethane foam.

Splash block: a small masonry or polymeric block laid on the ground or lower roof below the opening of a downspout used to help prevent soil erosion and aggregate scour in front of the downspout.

Splice: bonding or joining of overlapping materials. (see Seam.)

Splice plate: a metal plate placed underneath the joint between two pieces of metal. The NRCA Roofing and Waterproofing Manual—Fifth Edition
1002 Glossary

Splice-tape: cured or uncured synthetic rubber tape used for splicing membrane materials.

Split: a membrane tear resulting from tensile stresses.

Split slab: a term used to describe two separate concrete slabs. The first is placed as a slab on grade or suspendedslab, and covered with waterproofing and a drainage system. The second slab, also referred to as a topping slab, is then placed over the underlying slab and waterproofing.

Spot mopping: see Mopping.

Sprayed 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 manufacturing process in which membranes are formed using a liquid compound that is spread onto a supporting reinforcement base layer and then dried to its finished condition.Sprinkle mopping: see Mopping.

Spunlaced: a nonwoven fabric made by mechanically bonding a dry-laid staple fabric by water jet, which entangles the individual fibers.

Spud: to remove the roofing aggregate and most of the bituminous top coating by scraping and chipping.

Square: a unit used in measuring roof area equivalent to 100 square feet (9.29 m2) of roof area.

Square-tab shingles: shingles with tabs that are all the same size and exposure.

Squeegee: (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 squegee.

Stainless steel: an alloy of steel that contains chromium and also may contain nickel or copper. Generally, has very good resistance to corrosion.

Standing seam: in metal roofing, a type of seam between adjacent sheets of material made by turning up the edges of two adjacent metal panels and then folding or interlocking them in a variety of ways.

Starter course: the first layer of roofing, applied along a line adjacent to the downslope perimeter of the roof area. With steep-slope watershedding roof coverings, the starter course is covered by the first course.

Starter sheets: (1) felt, ply sheet or membrane strips that are made or cut to widths narrower than the standard width of the roll and used to start the shingling pattern at an edge of the roof; (2) particular width sheets designed
for perimeters in some mechanically attached and fully adhered single-ply systems.

Starter strip: roll roofing or shingle strips applied along the downslope eave line before the first course of roofing and intended to fill spaces between cutouts and joints of the first course.

Static load: any load, as on a structure, that does not change in magnitude or position with time.

Steel: a malleable alloy of iron and carbon produced by melting and refining pig iron and/or scrap steel; graded according to the carbon content (in a range from 0.02 to 1.7%); other elements, such as manganese and silicon, may be included to provide special properties.

Steel joist (open web steel joist): normally used as a horizontal supporting member between beams or other structural members, suitable for the support of some roof decks.

Steep asphalt: asphalt complying with ASTM D 312, Type III. (see Asphalt.)

Steep-slope roofs: a category of roofing that generally include water-shedding types of roof coverings installed on slopes exceeding 3:12 (14 degrees). The NRCA Roofing and Waterproofing Manual—Fifth Edition
Glossary 1003

Step flashing: individual pieces of sheet metal material used to flash walls, around chimneys, dormers and such projections along the slope of a roof. Individual pieces are overlapped and stepped up the vertical surface.

Stick clip: in waterproofing, a non-penetrating fastener that is adhered to the waterproofing surface; typically used to retain insulation, drainage panels, prefabricated protection materials, etc., against the waterproofing to prevent sliding and displacement.

Stiffener rib: small intermediate bends in a metal pan used to strengthen the panel.

Storm anchor: see Wind clip.

Strapping (felts): a method of installing roofing rolls or sheet good materials parallel with the slope of the roof.

Straw nail: a long-shanked nail. Sometimes used for fastening over tile at hips and ridges.

the internal resistance of a material to a force, measured as a force per unit area.

Striations: a parallel series of small grooves, channels, or impressions typically within a metal roof panel used to help reduce the potential for oil-canning.

Strip mopping: see Mopping.

Strip shingles: asphalt shingles that are manufactured in strips, approximately three times as long as they are wide.

Strippable films: (for metal) added protection of plastic films sometimes applied to coated or finished metals after the coil coating process.

Applied after prime and top coats to resist damage to the finish prior to and during shipping, fabrication and installation.

Stripping or 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 rubber: high molecular weight polymers having rubber-like properties, formed by the random copolymerization of styrene and butadiene monomers.

Styrene butadiene styrene copolymer (SBS): high molecular weight polymers that have both thermoset and thermoplastic properties, formed by the block copolymerization of styrene and butadiene monomers. These polymers are used as the modifying compound in SBS polymer modified asphalt roofing membranes to impart rubber-like qualities to the asphalt.

Substrate: the surface upon which the roofing or waterproofing membrane is applied (e.g., in roofing, the structural deck or insulation).

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

Sump pan: a metal pan used to create a depression around a drain or scupper to enhance drainage.

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.

Surface erosion: the wearing away of a surface due to abrasion, dissolution or weathering.

Surface texture: the resulting surface from the final pass of SPF. The following terms are used to describe the different

SPF surface textures: smooth orange peel, coarse orange peel, verge of popcorn, popcorn, treebark, and oversprayed. The NRCA Roofing and Waterproofing Manual—Fifth Edition 1004 Glossary

Surfacing: the top layer or layers of a roof covering, specified or designed to protect the underlying roofing from direct exposure to the weather.

Surfactant: contraction for “surface active agent;” a material that improves the emulsifying, dispersing, spreading, wetting or other surface-modifying properties of liquids.


Tab: the exposed portion of strip shingles defined by cutouts.

Tack-free time: in SPF-based roofing, a curing phase of polyurethane foam to when the material is no longer sticky. When the polyurethane foam is tack free, it can be sprayed over with another pass, referred to as a “lift”.

With some care the polyurethane foam can be walked on soon after it is tack free.

Talc: whitish powder applied at the factory to the surface of some roofing materials (e.g., vulcanized EPDM membranes), used as a release agent to prevent adhesion of the membrane to itself.

Tapered edge strip: a tapered insulation strip used to (1) elevate and slope the roof at the perimeter and at curbs, and (2) provide a gradual transition from one layer of insulation to another.

Taping: (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.

Tar: 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.

Tar boils: bubbles of moisture vapor encased in a thin film of bitumen, also known as “blackberries.”
Tarred felt: see Coal tar felt.

Tear-off and reroof: the removal of all roof system components down to the structural deck, followed by installation of a completely new roof system.

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. Expressed in psi (pounds force) per inch width or kN/m (kilonewton per meter width).

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.

Tension leveling: the process of pulling metal coil stock between two spools under a certain pressure to help reduce side camber and potential oil canning in the coil stock caused by manufacturing and cutting processes.

Termination: the treatment or method of anchoring and/or sealing the free edges of the membrane in a roofing or waterproofing system.

Terne: an alloy of lead and tin, used to coat sheets of carbon steel or stainless steel for use as metal roofing sheet.

Terra cotta: low-fired clay, either glazed or unglazed.

Test cut: a sample of the roof system or assembly which exposes the roof deck and is used to diagnose the condition of the membrane, evaluate the type and number of plies or number of membranes, or rates of application (e.g., the weight of the average interply bitumen moppings).

Thermal block: a compression-resistant insulation block installed between structural steel roof panels and their supporting members to help maintain insulation R-values and reduce condensation.

Thermal bridge: the penetration of a material of high thermal conductivity (e.g., a metal insulation or roof membrane fastener) through a material of low thermal conductivity (e.g., thermal insulation); the result is a lowered thermal resistance for the assembly. The NRCA Roofing and Waterproofing Manual—Fifth Edition Glossary 1005

Thermal cycling:
sequence of values caused by a repetitive temperature differential due to changes in radiant energy.

Thermal conductance (C): the time rate of heat flow through a unit area of a body induced by a unit temperature difference between bodies. In English (inch-pound) units of measurement, the number of BTUs that pass through a specified thickness of a one square foot (0.09 m2) sample of material in one hour with a temperature difference between the two surfaces of 1° F. 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 heat flow through a unit area of a homogeneous material in a direction
perpendicular to isothermal planes induced by a unit temperature gradient is called thermal conductivity (k or kvalue). In English (inch-pound) units of measurement, it is the number of BTUs that pass through a 1 inch (25 mm)
thickness of a 1 square foot (0.09 m2) sample of material in one hour with a temperature difference between the two surfaces of 1°F. 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 (25 mm) 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 expansion: the increase in the dimension or volume of a body due to 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): under steady conditions, thermal resistance is the mean temperature difference between two defined surfaces of material or construction that induces unit heat flow 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 shock: the stress-producing phenomenon resulting from sudden temperature changes in a roof membrane when, for example, a cold rain shower follows brilliant sunshine.

Thermal stress: stress introduced by uniform or non-uniform temperature change in a structure or material that is contained against expansion or contraction. The NRCA Roofing and Waterproofing Manual—Fifth Edition
1006 Glossary

Thermal transmittance (U or U-factor): thermal transmittance (U or U-factor) is the time rate of heat flow per unit area under steady conditions from the fluid (e.g., air) on the warm side of a barrier to the fluid (e.g., air) on the cold side, per unit temperature difference between the fluids. In English (inch•pound) units 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 reciprocal of the overall thermal resistance (RT) of a system (i.e., U = 1/RT).

Thermography, Infrared: see Infrared thermography.

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

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. The membrane may or may not be reinforced.

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

Thinner: (1) a volatile liquid added to an adhesive or coating material to modify the consistency or other properties; (2) a liquid used to clean equipment or other surfaces.

Thixotropic: the property of a material that enables it to stiffen in a relatively short time on standing, but upon agitation or manipulation to change to a very soft consistency or to a fluid of high viscosity, the process being completely reversible.

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.

Tie-in: 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 roofing or waterproofing system.

T-joint: the condition created by the overlapping intersection of three or four sheets in the membrane.

Toggle bolt: a bolt having a nut with pivoted, flanged wings that close against a spring when it is pushed through a hole, and open after emerging from the hole; used to fasten objects to a hollow wall or to a wall which is accessible only from one side.

Tongue and groove planks: one of the oldest types of dimensional structural wood used as roof decking. The sides are cut with convex and concave grooves so adjacent planks may join in alignment with each other to form a uniform roof deck.

Torch-applied: method used in the installation of polymer modified bitumen membranes characterized by using open flame propane torch equipment.

TPO: thermoplastic olefin.

Traffic bearing: in waterproofing, a membrane formulated to withstand a predetermined amount of pedestrian or vehicular traffic with separate protection and a wear course.

Transverse seam: the joint between the top of one metal roof panel and the bottom of the next panel, which runs perpendicular to the roof slope.

Treebark surface texture: in SPF roofing, the surface condition of the foam which shows a coarse texture where valleys form sharp angles. This surface is unacceptable for proper coating and protection.
The NRCA Roofing and Waterproofing Manual—Fifth Edition Glossary 1007

Tuckpointing: the process of removing deteriorated mortar from an existing masonry joint and troweling new mortar or other filler into the joint.


U-Value: see Thermal transmittance.

UBC: Uniform Building Code.

UL: Underwriters Laboratories, Inc.

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 that the product has met certain performance criteria.

Ultraviolet (UV): invisible light radiation, adjacent to the violet end of the visible spectrum, with wavelengths from about 200 to 400 nm (nanometres).

Underlayment: an asphalt-saturated felt or other sheet material (may be self-adhering) installed between the roof deck and roof covering, usually used in a steep-slope roof construction. Underlayment is primarily used to separate the roof covering from the roof deck, shed water and provide secondary weather protection for the roof area of the building.

Underwriters Laboratories, Inc. (UL): an organization that tests, rates and classifies roof assemblies for their resistance to fire, impact, leakage, corrosion of metal components and wind uplift.

Uplift: see Wind uplift.


Valley: the internal angle formed by the intersection of two sloping roof planes.

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: a 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.

Vent: an opening designed to convey air, heat, water vapor or gas from inside a building or a building component to the atmosphere.

Ventilator: an accessory that is designed to allow for the passage of air.

Verge of popcorn texture: in SPF roofing, the verge of popcorn surface texture is the roughest texture suitable for receiving the protective coating on a sprayed polyurethane foam roof. The surface shows a texture where nodules are larger than valleys, with the valleys relatively cured. This surface is acceptable for receiving a protective coating only because of the relatively cured valleys. However, the surface is considered undesirable because of the additional amount of coating material required to protect the surface properly.

Vermiculite: an aggregate used in lightweight insulating concrete, formed by heating and expanding of a micaceous material.

Viscosity: the resistance of a material to flow under stress. For bitumen, measured in centipoise. (see Viscous.)

Viscous: resistant to flow under stress.

Void: an open space or break in consistency.

Volatile: a relative term expressing the tendency to form vapor.
The NRCA Roofing and Waterproofing Manual—Fifth Edition 1008 Glossary

Volatile organic compounds (VOC): means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participate in atmospheric
photochemical reactions.

Vulcanization: an irreversible process during which a rubber compound, through a change in its chemical structure (for example, cross-linking), becomes less plastic and more resistant to swelling by organic liquids and elastic properties are conferred, improved, or extended over a greater range of temperature.


Warm roof assembly: a roof assembly configured with each component placed immediately on top of the preceding component; each component is in contact with the adjacent component. No space is provided for ventilation of the roof assembly. Also known as a “compact” roof assembly.

Wash coat: a primer, typically provided on the back side of painted metal products to help protect the underlying metal from wear and corrosion.

Water cure: a method of curing a material, such as concrete, by applying a fine mist of water over the surface to control the rate of moisture evaporation from the material.

Water cutoff: see Cutoff.

Water-shedding: the ability of individual, overlapping components to resist the passage of water without hydrostatic pressure.

Water stop: a diaphragm used across a joint as a sealant, usually to prevent the passage of water.

Water table: the level within the ground, below which the soil is saturated with water.

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.

Waterproof: the quality of a membrane, membrane material, or other component to prevent water entry.

Waterproofing: treatment of a surface or structure to prevent the passage of water under hydrostatic pressure.

Wear course: the top layer of surfacing that carries pedestrian or vehicular traffic. Sometimes referred to as wearing surface.

Wearing surface: see Wear course.

Weatherproof: the ability of a membrane or roof covering to prevent the passage of water with a limited amount of hydrostatic pressure.

Weep holes: small openings whose purpose is to permit drainage of water that accumulates inside a building component (e.g., a brick wall, skylight frame, etc.).

Weld: to join pieces of metal together by heat fusion.

Wet: a condition where free water is present in a substance.

Wet bulb temperature: the temperature of air as registered by a thermometer whose bulb is covered by a water wetted wick.

Wet film thickness: the thickness, expressed in mils, of a coating or mastic as applied but not cured. For comparison, see Dry film thickness.

Wicking: the process of moisture movement by capillary action.

Wind clip: a steep-slope roofing attachment device that fits over the butt end of tile, slate and stone to help secure individual roofing units from wind uplift.

Wind load: force exerted by the wind on a structure or part of a structure.
The NRCA Roofing and Waterproofing Manual—Fifth Edition Glossary 1009

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.

Wire tie system: a system of attachment for steep-slope roofing units (e.g., tile, slate and stone) using fasteners (nails and/or screws) in conjunction with wire to provide a concealed fastening system.

Work slab: see Mud slab.

Woven valley: a method of valley construction in which shingles or roofing from both sides of the valley extend across the valley and are woven together by overlapping alternate courses as they are applied.

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


Yield: in SPF-based roofing, the volume of foam per unit weight, normally expressed as board feet per pound or
board feet per 1000 pounds.


Z section: a member formed in the shape of a “Z” from coiled steel stock.

Zinc: a hard bluish white metal, brittle at normal temperatures, very malleable and ductile when heated; not subject to corrosion; used for galvanizing sheet steel and iron, in various metal alloys, and as an oxide for white paint pigment.