Backup plate—A rigid plate to support an end lap to provide uniform compression.

Background count—The reading of a test device before the material to be tested or identified is introduced.

Backnailing—"blind" (i.e., concealed by overlapping felt) nailing of roofing felts to a substrate in addition to hot-mopping to prevent slippage.

Backscatter—The number of neutrons reflected back as contrasted to passing through a substance.

Bald roof—See Smooth-surfaced roof.

Ballast—Loose aggregate, concrete pavers, or other material designed to prevent wind-uplift or flotation of a loose-laid roof system.

Banbury mixer—A heavy duty batch mixer with two counter rotating rotors. Used mainly in the rubber industry.

Bar joist—(Open web joist) Normally used as beams or horizontal structural members suitable for the support of floors or roof decks, with top and bottom chords of tees, pairs of angles, or round bars and round bar web members.

Barrier protection—Protection from the environment by a physical, inert barrier. If broken, the underlying base metal is unprotected. Contrasts with anodic coatings which, if breached, continue to protect the underlying base metal.

Base time—the date to which all future and past benefits and costs are converted when a present value method is used (usually the beginning of the study period).

Base sheet—A saturated or coated felt placed as the first ply in a multi-ply built-up roofing membrane.

Batten—Raised rib, in a metal roof, or a separate part or formed portion in a metal roofing panel.

Bay—The space between frame center lines or primary supporting members in the lengthwise direction of the building.

Beam—A primary member, usually horizontal, that is subjected to bending loads. There are three types: simple, continuous, and cantilever.

Beam and column—A primary structural system consisting of a series of rafter beams supported by columns. Used as the interior frame system on many pre-engineered building systems.

Bearing plate—A steel plate that is set on the top of a masonry support on which a beam or purlin can rest.

Beaufort scale—A scale in which the force of the wind is indicated by numbers from 0 to 12. No. 7 is "near gale" at 32-38 mph. No. 9 is "strong gale" at 47-54 mph.

Bell curve—The shape of a curve depicting the distribution of results of data in which the data follows a normal or Gaussian distribution.

Bend radii—The inside radii of bent sections.

Bent—A transverse framework to carry lateral and vertical loads (see Rigid frame).

Bill of materials—A list of items or components used for fabrication, shipping, receiving, and accounting purposes.

Bird screen—Wire mesh used to prevent birds from entering the building through ventilators and louvers.

Bitumen—(1) A class of amorphous, black or dark colored, (solid, semisolid, or viscous) cementitious substances natural or manufactured, composed principally of high molecular weight hydrocarbons, soluble in carbon disulfide, and found in asphalts, tars, pitches, and asphaltites; (2) A generic term used to denote any material composed principally of bitumen; (3) In the roofing industry there are two basic bitumens: asphalt and coal-tar pitch. Before application they are either (a) heated to a liquid state, (b) dissolved in a solvent, or (c) emulsified.

Bituminous emulsion—A suspension of minute globules of bituminous material in water or in an aqueous solution.
Bituminous, adj.—Containing or treated with bitumen. Examples: bituminous concrete, bituminous felts and fabrics, bituminous pavement.

Black body—A theoretical object that absorbs all the radiant energy falling on it and emits it in the form of thermal radiation.
Blanket insulation—Fiberglass insulation in roll form, often installed between the metal roof panels and the supporting purlins.

Blind rivet—A small headed pin with expandable shank for joining light gauge metal. Typically used to attach flashing, gutter, etc. Applied from one side, with a stem that pulls against material on the blind side.

Blister—An enclosed pocket of air-water vapor, trapped between membrane plies or between membrane and substrate.
Blister (Polyurethane Foam)—Undesirable rounded delamina-tion of the surface of a polyurethane foam whose boundaries may be either more or less sharply defined.

Block copolymer—An essentially linear copolymer in which there are repeated sequences of polymeric segments of different chemical structure.

Block or board thermal insulation—Rigid or semirigid thermal insulation preformed into rectangular units.

Blocking—(1) Wood built into a roofing system above the deck and below the membrane and flashing to (a) stiffen the deck around an opening, (b) act as a stop for insulation, (c) serve as a nailer for attachment of the membrane or flashing. (2) Wood cross-members installed between rafters or joists to provide support at cross-joints between deck panels. (3) Cohesion or adhesion between similar or dissimilar materials in roll or sheet form that may interfere with the satisfactory and efficient use of the material.

Blocking, wood—Treated wood members designed to help prevent movement of insulation.

Bloom—A visible exudation of efflorescence on the surface of a material.

Blowing agent—A compounding ingredient used to produce gas by chemical or thermal action, or both, in manufacture of hollow or cellular articles.

Blueberry—A small bubble or blister in the flood coating of a gravel-surfaced membrane.

BOCA—Building Officials and Code Administrators, International, Inc. Author of the National Building Code.

Bodied solvent adhesive—An adhesive consisting of a solution of the membrane compound in solvent used in the seaming of membranes.

Bond—The adhesive and cohesive forces holding two roofing components in intimate contact.

Boot—A bellows type covering to exclude dust, din, moisture, etc., forming a flexible closure.

Brace rods—Rods or cables used in roof and walls to transfer loads such as wind loads, and seismic and crane thrusts to the foundation. (Also often used to plumb buildings but not designed to replace erection cable.)

Breaking factor—Tensile at break in force per unit of width; units, SI: Newton per meter, customary: pound per inch.

Breaking stress—Stress (in force per linear or area units) at which sheet, or other tested component, ruptures under tensile force.

British thermal unit (BTU)—That amount of heat required to raise the temperature of one pound of water by 1°F.

Brooming—Embedding a ply by using a broom to smooth it out and ensure contact with the adhesive under the ply.

BTU—British Thermal Unit. The amount of heat required to raise one pound of water one degree Fahrenheit. A wooden kitchen match yields approximately one BTU.

Builder/contractor—A general contractor or subcontractor responsible for providing and erecting metal building systems.

Building code—Published regulations and ordinances established by a recognized agency describing design loads, procedures, and construction details for structures. Usually applying to designated political jurisdiction (city, county, state, etc.). Building codes control design, construction, and quality of materials, use and occupancy, location and maintenance of buildings and structures within the area for which the code was adopted. (See Model codes)

Built up roofing (BUR)—A continuous, semiflexible membrane consisting of plies of saturated felts, coated felts, fabrics or mats assembled in place with alternate layers of bitumen, and surfaced with mineral aggregate, bituminous material, or a granule surfaced sheet.

Built up section—A structural member (usually an "I" section) made from individual members connected together to act as one unit.

Bull—Roofer's term for flashing or plastic cement.

Butyl rubber—A synthetic rubber based on isobutylene and a minor amount of isoprene. It is vulcanizable and features low permeability to gases and water vapor and good resistance to aging, chemicals and weathering.

By-product nuclear material—Secondary radioactive material derived from nuclear refining processes in the manufacture of nuclear fuels. This type of material is used in nuclear moisture meters.