Laitance: A layer of fine, powdery cement and aggregate particles on the surface of newly-cured concrete caused by the outward movement of water. The presence of laitance points to the use of too much water in the concrete mix. Laitance is a defect and prevents the proper adhesion of other materials, such as waterproofing, to the concrete.

Laminate: To create a product by bonding together two or more layers, or plies, of a material.

Laminated Shingles: Premium-grade asphalt-based strip shingles having more than one layer of tabs, giving them extra thickness and a textured appearance. Also called three-dimensional or architectural shingles.

Lap: 1. The part of a basic roofing material unit (shingle, felt, membrane sheet, roofing roll, etc.) that covers part of the preceding or adjacent course in an overlapping roofing application. 2. To cover part of one roofing course with another.

Lap Cement: An asphalt-based roofing adhesive used to adhere overlapping plies of asphalt roll roofing.

Lap Joint: A joint where the units of the material(s) overlap and are bonded or fastened together.  

Lap Seam: An area where two roofing material units that overlap are heat-seamed, adhered, or otherwise bonded together.

Latent Heat: Heat transferred during a phase change without a change in the substance’s temperature. Water vapor transfers heat to its surroundings as it condenses and absorbs heat from its surroundings as it vaporizes.

Latex: A stable emulsion of polymer microparticles in a water-based solution. After application, the latex coalesces into a film as the water evaporates. As in latex paint.

Lead:  A soft, heavy, malleable, bluish-white to dull gray metal that in sheet form is used as a roof flashing material. Lead is also a component of the solder typically used in copper roofing seams.

Leader: A leader pipe. Also called a conductor or downspout.

Leader Head: A roof plumbing component used to direct water from a roof scupper or gutter to a downspout. Also known as a conductor head or a collector head.

Leader Pipe: A pipe for carrying water from a gutter, scupper, or other drainage system component from the roof to the ground or a sewer system. Also known as a downspout or conductor.

Leno Fabric: A type of fabric used as scrim reinforcement in laminated roofing membranes. Leno fabric is marked by weft threads that are each wrapped by two warp threads to ensure stability by preventing the threads from moving relative to each other.

Life Cycle Cost Analysis (LCCA): An economic evaluation tool that assists in determining the most cost-effective option among different alternatives in the purchasing, owning, operating, maintaining, and disposing of an asset, when each is equally appropriate on technical grounds. For a roof, in addition to the initial installation cost, LCCA accounts for all owner costs including warranty fees, scheduled maintenance or cleaning costs, reductions or increases in building heating and cooling costs associated with the roof, expected repair costs, and removal cost.  

Lift: A device used for raising or lowering people, equipment, or materials.

Lift: The thickness of a layer of sprayed polyurethane foam after a pass.

Lifting: The swelling or separation from the substrate of a dried layer of coating as the result of the application of a subsequent coat.

Light Reflectance Value (LRV): On a scale of 0% to 100%, a measure of the visible and usable light reflected from a surface when illuminated by a light source. The higher the LRV, the more reflective a surface is, with bright white being around 85%, and dark black being around 5%.

Lightning Protection System: A system of strike termination devices, conductors, grounding electrodes, interconnecting conductors, surge protection devices, and connectors designed to protect structures from fire or mechanical damage due to lightning strikes

Lightning Rod: A metal rod that serves as a strike termination device in a lightning protection system.

Lightweight Concrete: Concrete that is of a significantly lower density than normal concrete due to the addition of an air-entraining agent or lightweight aggregate such as vermiculite, perlite, gypsum, or cellulosic fibers. Its equilibrium density tend to be between 70 and 120 lb/ft3 (1120 and 1920 kg/m3). Used as a poured deck fill on roof decks.

Lightweight Insulating Concrete (LWIC): A roof deck system in which lightweight concrete is used in conjunction with a structural deck and, typically, expanded polystyrene insulation boards to increase the roof assembly R-value and create a contoured, monolithic substrate for the roof system.

Lintel: A horizontal structural member over a window or door which carries the weight of the wall above it.

Live Loads: Also called imposed loads. Temporary loads on a roof such as people, equipment, stored materials, or snow which the structure must be designed to support as required by code.

Load Deflection: See Deflection.

Loads: Or structural loads. Forces that act on structures or their members.

Loose-Laid: Of single-ply roof membranes and insulation boards. Not adhered or mechanically attached to the substrate.

Loose-Laid Roof Membrane: A roofing membrane (such as an EPDM membrane) that is attached to the substrate only at the perimeter and at penetrations, and is typically held in place by stone ballast or pavers.

Louver: A opening with angled, horizontal slats designed to admit light or air but not precipitation or direct sunlight.

Low Balling: Deliberately bidding low on a project, often with the intention of increasing the project’s total billable amount later through cost overruns and change orders.

Low Slope Application: Method of installing asphalt shingles on roofs with slopes that are between 2/12 and 4/12.

Low-Slope Roof: A roof with a maximum slope of less than 3 inches vertical rise for every 12 inches of horizontal run. A roof with a slope of less than 14 degrees.