Term Description

National Fire Protection Association; an international nonprofit organization. Its mission is to reduce the worldwide burden of fire and other hazards on the quality of life by providing and advocating consensus codes and standards, research, training and education. NFPA publishes the NFPA 70, "National Electrical Code."


National Institute of Standards and Technology.

Nitrile alloy

An elastomeric material of synthetic non-vulcanizing polymers.

No-cut valley

A method of valley construction in which a row of shingles is first applied up the valley with the exposure edge facing its center, from eave to peak, on either side of the valley. Field shingles are then placed over the row running up the valley with its exposure edge used as an alignment guide for bottom corners of the field shingles. This method sometimes is referred to as ‚"California valley.‚"


National Oceanic and Atmospheric Administration.

Noble metal

A metal that readily receives electrons from an anodic metal (see "galvanic series") but does not readily give up its own electrons; characterized by a marked resistance to chemical reaction, particularly to oxidation and to solution by inorganic acids.

Nondestructive testing (NDT)

A method to evaluate the disposition, strength or composition of materials or systems without damaging the object under test. Typically used to evaluate moisture content in roofing assemblies, the three common test methods are electrical capacitance, infrared thermography and nuclear back-scatter.

Nonferrous metal

Metal that is not an iron alloy and is nonmagnetic (e.g., aluminum, copper).

Nonwoven fabric

A textile structure produced by bonding or interlocking of fibers, or both, accomplished by mechanical, chemical, thermal, or solvent means and combinations thereof.


National Roofing Contractors Association.


Generic name for a family of polyamide polymers used as a scrim in some fabric-reinforced sheeting.

Off-ratio foam

Spray polyurethane foam (SPF) that has excess isocyanate or resin. Off-ratio SPF will not exhibit the full physical properties of correctly prepared SPF.


An unsaturated hydrocarbon containing at least one double bond: ethylene, propylene, butadiene or isoprene.

Open time

The period of time after an adhesive has been applied and allowed to dry during which an effective bond can be achieved by joining the two surfaces.

Open valley

A method of valley construction in which the steep-slope roof-covering materials on both sides are trimmed along each side of the valley, exposing the metal valley flashing.

Orange-peel surface texture

In spray polyurethane foam roofing, a condition of the foam in which the surface shows a fine texture and is compared to the exterior skin of an orange. This surface is considered acceptable for receiving a protective coating.


Being or composed of hydrocarbons or their derivatives; matter of plant or animal origin.

Organic felt

An asphalt roofing base material manufactured from cellulose fibers. See "felt."

Oriented strand board (OSB)

A mat-formed panel product with oriented layers resulting in directional properties. OSB is comprised primarily of wood strands bonded with exterior adhesive formulations under heat and pressure. Design capacities are referenced to the primary and secondary structural axes, which typically correspond to the manufacturing machine and cross-machine directions, respectively. The primary direction is often referred to as the strength direction.


Oak Ridge National Laboratory.


Occupational Safety and Health Administration; a government agency in the U.S. Department of Labor. Its mission is to maintain a safe and healthy work environment.


Any material installed on top of a waterproofing assembly.

Overburden dividers

Overburden dividers include gravel boards, edge pieces, rounded edge sections and vegetative roof system edge definers. They are used to separate overburden types, such as growth medium and pavers or growth medium and rock; they are installed over the protection course, drainage layer and filter layer and can be polyethylene (vertical piece with support stands) or stainless steel (L-shaped). Overburden dividers are not mechanically fastened but are loose-laid and ballasted and typically have drainage slots. They are installed over the protection course, drainage layer and filter layer and should be secured to avoid displacement.

Overflow drainage

Emergency or overflow drainage devices, such as drains or scuppers, which provide for drainage relief from a roof system or waterproofing system if primary drains allow water to build up for any reason.


See "roof overhang."