Horizontal member in a roof supporting common rafters, such as at the break in a gambrel roof. Also, horizontal structural member perpendicular to main beams in a flat roof.

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Proportional Limit

The upper limit at which stress is proportional to strain.

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Racking

Forcing out of plumb of structural components, usually by wind, seismic stress or thermal expansion or contraction.

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Radiant Heat

Heat transferred by radiation.

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R-Value

The numerical value used to indicate the resistance to the flow of heat.

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R Value

- A measure of insulation. A measure of a materials resistance to the passage of heat. The higher the R value, the more insulating "power" it has. For example, typical new home's walls are usually insulated with 6" of batt insulation with an R value of R-21, and a ceiling insulation of R-49.

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R factor or value

A measure of a materials resistance to the passage of heat. New home walls are usually insulated with 6" of batt insulation with an R value of R-21, and a ceiling insulation of R-49.

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Rabbet-

A rectangular longitudinal groove cut in the corner edge of a board or plank.

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Radiant heating

A method of heating, usually consisting of a forced hot water system with pipes placed in the floor, wall, or ceiling. Also electrically heated panels.

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Quarter round

A small trim molding that has the cross section of a quarter circle.

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Raceway

An enclosed channel designed to carry wires and cables.

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Quartersawing

Sawing lumber so the hard annual rings are nearly perpendicular to the surface.