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Joist: A slender beam used repetitively at narrow spacings to support a ceiling surface or the decking of a floor or roof.
Knee brace: A temporary diagonal member used to support a wall or column during construction; a permanent diagonal member that joins a column above its midpoint.
Labyrinth joint: An open joint between wall panels that is configured so that there is no straight-line path through the joint.
Lamp: A light bulb or light-emitting tube.
Landing: A broad platform on a stair.
Latent heat: The heat required to change the phase of a material without changing its temperature.
Lateral force: A force that is considered to act in a horizontal direction, usually caused by wind, earthquake, soil pressure, or water pressure.
Lateral support: Structural elements that resist lateral forces in a building frame.
Lath: Any mesh, strip, or sheet material used as a base to which to apply plaster or stucco.
LEED System: Acronym for “Leadership in Energy and Environmental Design,” a proprietary initiative to evaluate the sustainability of designs for buildings.
Leaching field: A level bed of crushed stone just below the surface of the ground into which effluent from a septic tank is distributed through perforated pipes or open chambers in order that the effluent will be absorbed by the soil below.
Light shelf: A light-colored horizontal surface, usually located above head level, that serves to reflect daylight into the part of a room that is more distant from the windows.
Lime: Calcium hydroxide, produced by burning limestone or sea shells and adding water. Used in construction as an ingredient of mortar and plaster.
Lintel: A beam that supports a wall above a window or door opening.
Live load: The weight exerted on a structure by people, goods, furniture, machinery, vehicles, snow, and other things that change over time.
Louvers: Closely spaced, parallel strips of material used to obstruct the passage of light or rain while allowing the pas sage of air.
Low-emissivity (Low-c) coating: A thin layer deposited on glass to make the glass selectively reflective of long-wave infrared radiation.
Lumen: A unit used to measure the rate at which light is produced by a lamp.
Luminance: The brightness of a visual task.