NRC—Nuclear Regulatory Commission. An agency of the Department of Health, Education, and Welfare, with jurisdiction over licensing requirements for nuclear sources.

Nail-type concrete anchor—A hammer-applied fastener with spiral or annular rings which provide pullout strength.

Nailer—Wood member bolted or otherwise anchored to a nonnailable deck or wall to provide nailing anchorage of membrane or flashing.

Nailing—(1) Exposed nailing of roofing wherein nail heads are bare to the weather; (2) Concealed nailing of roofing wherein nail heads are concealed from the weather. (See also Blind nailing).

National Coil Coaters Association—Association composed of North American coil coalers charged with the promotion of the use of coated coils.

Needle punched—A mechanical entanglement of dry laid (usually cross-lapped, carded staple fiber) webs where barbed needles achieve, in multiple punches, mechanical bonding.

Neoprene—A synthetic rubber (polychloroprene) used in liquid or sheet applied elastomeric roofing membranes or flashing.

Neoprene (Polychloroprene)—Generic name for a synthetic rubber used primarily on chloroprene, i.e., chloro-butadiene. Vulcanized generally with metal oxide. Resistant to ozone and aging and to some oils.

Neutral sealants—Acidic acid-free and amine-free sealants.

Neutron—Fundamental particle of matter having a mass of 1.009 but no electric charge. It is a constituent of the nucleus of all elements except hydrogen.

Neutron absorption—The process by which a neutron is "captured" by an atom of the target material, thereby transferring its nucleus to the next higher isotope of the target.

Neutron sources—Neutrons may be produced by reactors, accelerators, or certain radioactive isotopes. In most portable gauging applications neutrons are produced by isotopes. They are the reaction between alpha particles and beryllium, with radium 226 or americium 241 being the source of alpha particles.

Newton—SI unit of measure for force (N).

Nineteen-inch selvage—A prepared roofing sheet with a 432 mm (17 in.) granule surfaced exposure and a 483 mm (19 in.) selvage.

Ninety-pound—A prepared roll roofing with a granule surfaced exposure that has a mass of approximately 4400 g/m2 (90 lb./108 ft2).

Nitrile rubber—A family of copolymers of butadiene and acrylo-nitrile that can be vulcanized into tough oil resistant compounds. Blends with PVC are used where ozone and weathering are important requirements in addition to its inherent oil and fuel resistance.

Noble metal—A metal with marked resistance to chemical reaction, particularly to oxidation and to solution by inorganic acids.

Nominal discount rate—the rate of interest reflecting the time value of money stemming both from inflation and the real earning power of money over time.

Nondestructive testing (NDT)—Methods which can evaluate the strength or composition of materials without damaging the object under test.

Nonwoven fabric—A structure produced by bonding or interlocking of fibers (or both) by mechanical, thermal or solvent means (or combinations thereof).

Norm—The meter CPM reading of hydrogen found in a roof system in its driest area (lowest reading). (See Background count)

Normal distribution—Also known as Gaussian distribution. The distribution of results of random events which follow specific probability rules, e.g., flipping of coins.

Nylon—Generic name for a family of polyamide polymers characterized by the presence of the amide group—CONH. Used as a scrim in fabric reinforced sheeting.