Term Description
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 h•ft2•F/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 resistance ratio (TRR)

(1) The ratio of a material‚Äôs ‚"wet‚" thermal resistance to its air-dry thermal resistance; (2) the ratio of a material‚Äôs thermal resistance determined from conditioned sample measurements to its reference thermal resistance typically determined at equilibrium moisture content at specific temperature and relative humidity.

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 nonuniform temperature change in a structure or material that is contained against expansion or contraction.

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 elastomer:

A material having the general properties of an elastomer and capable of being repeatedly softened by heat and hardened to shape by cooling without significant degradation of the polymer system; typically more thermally and pressure-sensitive and more sensitive to solvents than thermoset elastomers.

Thermoplastic olefin membrane (TPO)

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.


(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.


The sag resistance or ability of a coating to ‚"hang on‚" to a sloped surface.

Through-wall flashing

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

Tie-in (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 roof or waterproofing system.

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 that 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.


Method used in the installation of polymer-modified bitumen membranes characterized by using open flame propane torch equipment.


thermoplastic olefin. Also see "thermoplastic olefin membrane (TPO)."

Traffic bearing

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

Treebark surface texture

In spray polyurethane foam roofing, the surface condition of the foam that shows a coarse texture where valleys form sharp angles. This surface is unacceptable for proper coating and protection.


Tile Roofing Institute; originally founded in 1971 as the National Tile Roofing Manufacturers Association; also was the Roof Tile Institute.


A structure made up of three or more members usually in some triangular arrangement with each member designed to carry a tension or compression force. The entire structure in turn acts as a beam.