Rabbet - A recess cut into the edge of a board into which another piece fits to form a rabbet joint. v. to cut a rabbet. To join by means of a rabbet joint.
Radiation - Energy transmitted from a heat source to the air around it. So-called "radiators" actually depend more on convection than radiation.
Radon - A naturally-occurring radio active gas common in many parts of the country e.g. very high levels are found in portions of Pennsylvania and New Jersey and very low levels are found in most portions of western Washington. Radon gas exposure is associated with lung cancer. Mitigation measures may involve: crawl space and basement venting and various forms of vapor barriers.
Rafter - The framing member which directly supports the roof sheathing. A rafter usually follows the angle of the roof, and may be a part of a roof truss.
Rated output - The maximum amount of power produced by a wind machine (or other energy system); a compromise between maximum output and safety of operation.
Rated velocity - The wind speed at which a wind machine reaches its rated output.
Relative humidity - The amount of moisture in a volume of air as a percentage of the maximum amount of moisture which can be held in that air at a certain temperature - cold air can't hold as much moisture as warmer air.
Rail - Any relatively lightweight horizontal element, especially those found in fences. Also the horizontal pieces between panels in a panel door.
Refrigerant - A substance that remains a gas at low temperatures and pressure and can be used to transfer heat.
Retaining wall - A structure that holds back a slope and prevents erosion.
Ridge board - The topmost beam at the peak of a roof to which rafters tie.
Rise - The vertical distance from one point to another above it; a measurement you need in planning a stairway or ramp. Also see run.
Riser - The upright piece between two stairsteps. Also see tread.
Roof decking - Subsurface material on which roof shingles or built up roofing is applied. Some roof decking is made with one side finished to serve as both roof deck and finished ceiling.
Roof ridge - The line that runs along the peak of a roof, where sloping parts of the roof meet.
Roof sheathing - The sheets or boards for sheathing over rafters. Also called roof decking, roof underlayment.
Roofing cement - A pliable asphalt- or plastic-based compound used as an adhesive and to seal flashings, minor leaks, etc.
Rough opening - The openings in walls and partitions for doors and windows as formed by the framing members.
Rough sill - The framing member at the bottom of a rough opening for a window. It is attached to the cripple studs below the rough opening.
Roughing-in - The initial stage of a plumbing, electrical, carpentry, or other project, when all
components that won't be seen after the second finishing phase are assembled.
Run - The horizontal distance a ramp or stairway traverses. Also see rise.
R-Value - a measure of insulation. For example, typical new home's walls are usually insulated with 6" of bat insulation with an R value of R-19, and a ceiling insulation of R-28.