Roofing Terms J – L
- -J-
- Joist
- any of the small timbers, metal or wood beams arranged parallel to each other and spanning from wall to wall to support a floor, ceiling, or roof of a building.
- Joule
- a unit of energy or work; equals the work done by a force of 1 newton which acts over a distance of 1 meter in the direction of the force.
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- -K-
- k or k-Value
- thermal conductivity; the time rate of heat flow through a unit area of a homogeneous material in a direction perpendicular to isothermal planes induced by a unit temperature gradient. In English (inch-pound) units of measurement, it is the number of BTUS that pass through a 1 inch (25 mm) thickness of a 1 square foot (0.09 m2) sample of material in 1 hour with a temperature difference between the two surfaces of 1° F. It is expressed as Btu•inch/h•ft2•°F.
- Kerf
- (1) a slit or notch made by a saw or cutting torch; (2) the width of cut made by a saw or cutting torch.
- Kesternich test
- simulates acid rain conditions by subjecting test specimens to a sulfur dioxide atmosphere as well as condensing moisture for the purpose of evaluating rust/corrosion characteristics.
- Knee cap
- a metal cover trim that fits over a panel rib after it has been cut and bent.
- Knee joints
- see Knuckle.
- Knuckle
- a metal closure, either shop-or pre-fabricated, installed over the cut seam of a continuous metal roof panel at the transition from a steep-slope roof to a vertical roof or wall.
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- -L-
- 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
- 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. The NRCA Roofing and Waterproofing Manual—Fifth Edition 990 Glossary
- Lap seam
- occurs where overlapping materials are seamed, sealed or otherwise bonded.
- 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.
- 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.
- Liquid-applied
- application of bituminous 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.
- 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.).
- Loose-laid membrane
- a ballasted roofing membrane that is attached to the substrate only at the edges and penetrations through the roof.
- 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).
- 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.
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