Term Description

The spray 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.

Lightweight aggregate

See "aggregate, lightweight."

Lightweight or insulating concrete fill (LWIC)

Concrete made with or without aggregate additions to Portland cement, water and air to form a hardened material that, when oven-dried, will have a unit weight of 50 lb/ft3 or less.


Application of bituminous, polymer-modified bitumen or polymeric roof and waterproofing membranes, flashings, cements, adhesives or coatings installed at ambient or slightly elevated temperatures.

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 roof membrane

A continuous roof membrane constructed in place with a liquid resin and reinforcing material. The liquid resin is available as a one- or two-component product and typically is applied in two coats. Depending on resin chemistry, a catalyst or hardener may be added to induce the curing process. The reinforcement typically is polyester or fiberglass mat set into the resin base coat.

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, rain).

Long-term thermal resistance (LTTR)

A method for determining and reporting estimated long-term R-values of unfaced or permeably faced rigid gas-filled closed-cell plastic foam insulations by reducing the specimen thickness to accelerate aging under controlled conditions. This method is reported to account for the diffusion of the blowing agent from the foam insulation‚Äôs cells over time, resulting in a recognized reduction of thermal resistance of some plastic foam insulations as they age. The R-values produced by the LTTR method correspond to the thermal resistances measured for insulation materials stored in controlled laboratory conditions until they have aged five years. LTTR values are reported for polyisocyanurate rigid board insulation in accordance with ASTM C1289, ‚"Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board.‚"

Loose-laid membrane

Ballasted membrane that is attached to the substrate only at the edges and penetrations.


In roofing: (1) production lot—all material produced in one eight-hour shift of the same type (and color when applicable); (2) delivery lot—all material of the same type delivered at one time by one truck or railroad car.

Low-slope roofs

A category of roofs that generally includes weatherproof membrane types of roof systems installed on slopes at or less than 3:12.

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.


A decorative steep-sloped roof on the perimeter of a building.

Marine-grade plywood

Plywood that is made entirely of Douglas fir or western larch with all veneer plies being Grade B or better and is sanded on both faces or includes medium- or high-density overlay with maximum core-gap sizes of 1/8 of an inch; is exterior-rated with fully waterproof structural adhesive glue; and is not treated with decay-resistance chemicals. Where decay is a concern, it should be preservative-treated.


Construction, usually set in mortar, of natural building stone or manufactured units, such as brick, concrete block, adobe, glass block, tile, manufactured stone or gypsum block.


A thick adhesive material used as a cementing agent for holding waterproofing membrane in place. See ‚"asphalt roof cement.‚"


A thin layer of woven, nonwoven or knitted fiber that serves as reinforcement to a material or membrane.

Mechanical damage

In roofing, physical damage to a roof system not caused by normal wear and tear.

Mechanically fastened membrane

Generally used to describe a membrane that has been attached to the substrate at defined intervals.


A flexible or semiflexible roof covering or waterproofing whose primary function is to exclude water.


Tendency of a material to regain its previous configuration.


Any of various opaque, fusible, ductile and typically lustrous substances that are good conductors of electricity and heat.

Metal flashing

See "flashing"; frequently used as through-wall-, step-, cap- or counterflashing.

Metal rain collar

A metal counterflashing used to wrap a penetration and prevent water infiltration through the top of the penetration base flashing.

Metal roof panel

A sheet-metal product having a minimum installed weather exposure of 3 square feet per sheet.