Glossary of Roofing and Waterproofing Terms

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Laitance: a layer of weak nondurable material containing cement and fines from aggregates, brought by bleeding water to the top of overwet concrete. Laitance may be detected by scraping the concrete surface with a putty knife; if a quantity of loose powdery material is observed or easily removed, excessive laitance may be considered to be present.

Laminate: to bond two or more layers of a material together to make a finished product.

Laminated Shingles: see Dimensional Shingles or Architectural Shingles.

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.

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.

Latex: a colloidal dispersion of a polymer or elastomer in water which coagulates into a film upon evaporation of the water.

Lead: a soft workable metal used for miscellaneous flashings.

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.

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, wind, snow, ice or rain, etc.).

Load Deflection: see Deflection.

Loose-laid Membranes: membranes that are not attached to the substrate except at the perimeter of the roof and at penetrations. Typically, loose-laid membranes are held in place with ballast, such as water-worn stone, gravel, pavers, etc.

Low Temperature Flexibility: the ability of a membrane or other material to remain flexible (resist cracking when flexed), after it has been cooled to a low temperature.


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