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Laitance
a weak layer of cement and aggregate fines on a concrete surface that is usually caused by an overwet mixture, overworking the mixture, improper or excessive finishing or combination thereof.

Laminate
to join layers of materials together using fusion; the process of joining layers of materials together using adhesion.

Laminated shingles
see Dimensional shingles or Architectural shingles.

Lap cement
an asphalt-based roof cement formulated to adhere overlapping plies or asphalt roll roofing.

Lap seam
occurs where overlapping materials are seamed, sealed, or otherwise bonded.

Lap
that part of a roofing, waterproofing, or flashing component that overlaps or covers any portion of the same or another type of adjacent component.

Latex
a stable dispersion of polymeric substance in an essentially aqueous medium.

Lead
a soft malleable, heavy metal; has low melting point and a high coefficient of thermal expansion.

Leader head
see Conductor head.

Leeward
the opposite direction from which the wind is blowing. The side sheltered from the wind.

Life Cycling Costing
a method of economic analysis that takes into account expected costs over the useful life of an asset.

Lift
the sprayed polyurethane foam that results from a pass. It usually is associated with a certain pass thickness and has a bottom layer, center mass and top skin in its makeup.

Light Reflectance
the percentage of light that is not absorbed by the surface of a material.

Liquid-applied built-up roof
a continuous, semi-flexible roof membrane, consisting of multiple plies of felts, mats or fabrics laminated together with alternate layers of roof cements and surfaced with a liquid-applied coating with or without aggregate surfacing.

Liquid-applied
application of bituminous cements, adhesives or coatings installed at ambient or slightly elevated temperatures.

Live loads
temporary loads that the roof structure must be designed to support, as required by governing building codes. Live loads are generally moving and/or dynamic or environmental, (e.g., people, installation equipment, snow, ice or rain, etc.).

Load Deflection
see Deflection.

Loose-laid membrane
a ballasted roofing membrane that is attached to the substrate only at the edges and penetrations through the roof.

Low temperature flexibility
the ability of a membrane or other material to resist cracking when flexed after it has been cooled to a low temperature.

Low-slope roofs
a category of roofs that generally include weatherproof membrane types of roof systems installed on slopes at or less than 3:12 (14 degrees).