Glossary – R

R-Value: See Thermal Resistance.

Radiometer: Electronic device that converts infrared energy emitted by an object’s surface into a temperature value.

Radiometric Image: A thermal image saved in electronic format enabling post processing of the image to vary parameters associated with the stored subject image.

Recover: 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.

Recreational Vessel: Any vessel being manufactured or operated primarily for pleasure. The vessel may be leased, rented, or chartered to another for the latter’s pleasure. It does not include a vessel engaged in the carriage of passengers-for-hire as defined in 46 CFR Chapter I, Subchapter C, or in other subchapters of that title.

Reflected Temperature: The apparent temperature value reported by a radiometer that corresponds to the infrared energy incident upon, and reflected from the surface of an object.

Relative Humidity (RH): (1) 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. (2) The ratio of the amount of water vapor actually present in the air to the greatest amount of water vapor possible at the same temperature.

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


Resistance: Opposition to current flow measured in ohms.

Resistor: An electrical component that opposes the flow of current. In circuit diagrams, resistors are designated with R.

Rheostat: A variable resistor used to control the flow of current in a circuit. A rheostat has two leads connected to it.

Roof Area Expansion Joint: See Expansion Joint.


Roof Core Cut: (1) A sample from a low-slope roof system taken for the purpose of obtaining primarily qualitative information about the roof construction. Typically, core cut analysis can verify or reveal the type of membrane; the approximate number of plies; the type, thickness and condition of the insulation (if any); and the type of deck used as a substance for the roof system. (2) For SPF based roof systems, core cuts are used to obtain both quantitative and qualitative information, such as the thickness of the foam, the thickness and adhesion of the coating, thickness of individual passes and adhesion between passes and the adhesion of the foam to is substrate. (3) A small sample encompassing at least 2 inches in diameter of the roof surface area taken by cutting through the roof membrane and insulation, and removing the sample to determine its composition, condition, and moisture content.

Roof Curb: Raised frame used to mount mechanical units (such as air conditioning or exhaust fans), skylights, etc. on a roof.
Roof Recover: The process of installing an additional roof covering over a prepared existing roof covering without removing the existing roof covering.
Roof Repair: Reconstruction or renewal of any part of an existing roof for the purposes of its maintenance.

Roof Replacement: The process of removing the existing roof covering to the deck , repairing any damaged substrate and installing a new roof covering.


Roof Section: A portion of a roof that is separated from adjacent portions by walls, level changes, or expansion joints.

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 example, slope may be expressed as a ratio of rise to run, such as 4:12.

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.

Rotating Equipment: Stationary machinery or electro-mechanical devices that has rotating components.

Rotor: The moving part of a motor or a generator.