Glossary of Roofing Terms

Tab: the exposed portion of strip shingles defined by cutouts. (See Figure 25.)

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

Tensile Strength: the maximum force (longitudinal pulling stress) a material can bear without tearing or breaking apart.

Tensile Test: a test in which a specimen is subjected to increasing longitudinal pulling stress until fracture occurs.

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

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

Test Cut: a sample of the roof, which may contain all components or just the membrane, usually used to diagnose the condition of the existing membrane, evaluate the type and number of plies or number of membranes, or rates of application such as determine the weight of the average interply bitumen mappings.

Thatch Roof: the covering of a roof usually made of straw, reed, or natural foliage (palms) bound together to shed water.

Therm: a unit of heat equivalent to 100,000 BTUs (105.6-106). Commonly used by utilities in quoting prices or costs.

Thermal Barrier: a material applied over polyurethane foam designed to slow the temperature rise of the foam during a fire and delay its involvement in the fire. Thermal barriers for use with SPF must have a time rating of not less than 15 minutes.

Thermal Block: a compression-resistant insulation block installed between the structural steel and the panel to maintain insulation value.

Thermal Conductance (C): a unit of heat flow that is used for specific thicknesses of material or for materials of combination or composite construction, such as laminated insulation. The formula for thermal conductance is:

Thermal Conductivity (k): the heat energy that will be transmitted by conduction through 1 square foot of 1 inch thick homogeneous material in one hour when there is a difference of 1 degree Fahrenheit perpendicularly across the two surfaces of the material. The formula for thermal conductivity is: k = Btu/Square FooVlnch /Hour/Degree Fahrenheit

Thermal Image: a visual representation of temperature distribution over a surface area. The image is displayed on a screen, presenting the response to infrared light waves.

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): an index of a material's resistance to heat flow; it is the reciprocal of thermal conductivity (k) or thermal conductance (C). The formula for thermal resistance is:

Thermal Shock: the stress-producing phenomenon resulting from sudden temperature changes in a roof membrane when, for example, a cold rain shower follows brilliant hot sunshine, which may result in sudden cooling or rapid contraction of the membrane.

Thermal Stress: stress introduced by uniform or non-uniform temperature change in a structure or material that is contained against expansion or contraction.

Thermogram: a visible light record of the display of an infrared camera system via a Polaroid print, 35mm film, video tape, or computer generated image.

Thermography: a technique for producing heat "pictures" from the radiant energy emitted from stationary or moving objects without in any way influencing the temperatures of the objects under view. The electronic generation and display of a visible image of an infrared spectrum.

Thermoplastic: materials that soften when heated and harden 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 material that solidifies or "sets" irreversibly when heated. This property is usually associated with cross-linking of the molecules induced by heat or radiation.

Thinner: a liquid used to reduce the viscosity of coatings or mastic. Thinners evaporate during the curing process. Thinners may be used as solvents for clean-up of equipment.

Thread Count: the number of threads per inch in each direction, with the warp mentioned first, and the fill second, (e.g., a thread count of 20 x 10 means 20 threads per inch [25.4mm] in the warp and 10 threads per inch [25.4mm] in the fill direction).

Through-Wall Flashing: a water-resistant material, which may be metal or membrane, extending through a wall and its cavities, positioned to direct water entering the top of the wall or cavity to the exterior, usually through weep holes.

Tie-Off: (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.

Toggle Bolt: a two-piece assembly consisting of a threaded bolt and an expanding clip that can fit through a drilled hole. The clip can spring outward to provide anchorage from the blind 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.

TPO: Thermoplastic Olefin.

Traffic Bearing: in waterproofing, a membrane formulated to withstand a predetermined amount of pedestrian or vehicular use 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.

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

Two-Part System: a coating of SPF formed by the mixing and (usually) the reaction of two different materials.

U-Value: overall thermal conductance. U-value is equal to the inverse of the sum of the R-value in a system (U = 1/R total). Units are Btu/Ft2.Hr.°F.

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

Ultraviolet (UV): (1) situated beyond the visible spectrum, just beyond the violet end, having wavelengths shorter than wavelengths of visible light and longer than those of X-rays; (2) relating to, producing, or employing ultraviolet radiation.

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

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 at any given temperature exerted by a vapor that is in equilibrium with its liquid or solid form.

Vapor Retarder: material installed to impede or restrict the passage of water vapor through a roof assembly.

Veneer: (1) a single wythe of masonry for facing purposes, 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 other gas from inside a building or a building component to the atmosphere.

Ventilation Short Circuit: (as it relates to a passive ventilation system where the system is designed for air flow between intake and exhaust vents) a ventilation short circuit occurs when air is introduced into the ventilation system from an area higher than the intake vent thereby minimizing or defeating the effectiveness of the intake vent. One example can be a gable vent in a soffit-to-ridge ventilation system. Air intake from the gable vent can short circuit the stack-effect draw of air through the soffit vents, and interrupt the thorough venting of the roof cavity.

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

Vermiculite: an aggregate used in lightweight insulating concrete, formed by the heating and consequent expansion 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. Viscous materials are usually cohesive, and have a sticky consistency.

Void: an open space or break in consistency.

Volatile: descriptive of a substance which passes off easily as a gas or vapor, evaporating quickly.

Volatile Organic Compounds (VOC): organic materials which evaporate at normal temperatures and pressures; organic materials which have vapor pressures greater than 0.1 mm Hg at one atmosphere.

Vulcanization: any of various processes by which natural or synthetic rubber or other polymeric materials may be cured or otherwise treated (i.e., exposed to chemicals, heat, or pressure) to render them non-thermoplastic, and which improve their elastic and physical properties.

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

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 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. (See ASTM Standard E 96.)

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.

Weather Infiltration: the negative condition where rain or snow penetrate the roof. The condition is typically wind-driven.

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 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, as contrasted to movement of water vapor.

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.

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. This force is then transmitted to the roof surface. Uplift may also occur because of the introduction of air pressure underneath the membrane and roof edges, where it can cause the membrane to balloon and pull away from the deck.

Windward: being in or facing the direction toward which the wind is blowing. The side exposed to the prevailing wind.

Wire Tie System: a scheme of attachment for steep-slope roofing units (e.g., tile, slate, and stone) utilizing fasteners (nails and/or screws) in conjunction with wire to make up a concealed fastening system.

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.