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Back-nailing (also referred to as "Blind-nailing")
the practice of blind nailing the back portion of a roofing ply, steep roofing unit, or other components in a manner so that the fasteners are covered by the next sequential ply, or course, and are not exposed to the weather in the finished roof system.

Back-Surfacing
fine mineral matter applied to the back side of asphalt shingles and roll roofing to keep them from sticking together while packaged.

Ballast
a material, such as aggregate or precast concrete pavers, which employs its mass and the force of gravity to hold (or assist in holding) single-ply roof membranes in place.

Bar joist
(see Steel joist).

Barrel vault
a building profile featuring a rounded profile to the roof on the short axis, but with no angle change on a cut along the long axis.

Barrier board
noncombustible board stock material of low thermal conductivity placed between two elements of a roof assembly.

Base flashing (membrane base flashing)
plies or strips of roof membrane material used to close-off and/or seal a roof at the horizontal-to-vertical intersections, such as at a roof-to-wall juncture. Membrane base flashing covers the edge of the field membrane. (see Flashing.)

Base ply
the bottom or first ply in a built-up roof membrane when additional plies are to be subsequently installed.

Base sheet
an impregnated, saturated, or coated felt placed as the first ply in some low-slope roof systems.

Batten seam
a metal panel profile attached to and formed around a beveled wood or metal batten.

Batten
(1) cap or cover; (2) in a metal roof, a metal closure set over, or covering the joint between, adjacent metal panels; (3) in a wood, a strip of wood usually set in or over the structural deck, used to elevate and/or attach a primary roof covering such as tile; (4) in a single ply membrane roof system, a narrow plastic, wood, or metal bar which is used to fasten or hold the roof membrane and/or base flashing in place.

Beaufort Scale
a scale in which the force of the wind is indicated on a scale of 0 to 12

Bentonite
a porous clay formed by the decomposition of volcanic ash that swells 5 to 6 times its original volume in the presence of water.

Bermuda seam
a metal panel profile featuring a step-down profile that runs perpendicular to the slope of the roof.

Bi-Level Drain
see Dual-Level Drain.

Bird bath
random, inconsequential amounts of residual water on a roof membrane.

Bird Screen
wire mesh used to prevent birds from entering the building through ventilators, louvers, or other openings. (See Insect Screen.)

Bitumen-Stop
see Envelope and Bleed Sheet.

Bitumen
(1) a class of amorphous, black or dark colored, (solid, semi-solid, or viscous) cementitious substances, natural or manufactured, composed principally of high molecular weight hydrocarbons, soluble in carbon disulfide, and found in petroleum asphalts, coal tars, pitches and asphaltenes; (2) a generic term used to denote any material composed principally of bitumen, typically asphalt or coal tar.

Bituminous emulsion
a suspension of minute particles of bituminous material in water.

Blackberry (also referred to as "Blueberry" or "Tar-boil")
a small bubble or blister in the flood coat of an aggregate-surfaced built-up roof membrane.

Blanket (batt) insulation
glass fiber or other compressible fibrous insulation, generally available in roll form.

Bleed-sheet
a sheet material used to prevent the migration of bitumen.

Bleeder strip
(see Rake-Starter).

Blind-nailing
the use of nails that are not exposed to the weather in the finished roofing system.

Blister
an enclosed pocket of air, which may be mixed with water or solvent vapor, trapped between impermeable layers of felt or membrane, or between the membrane and substrate.

Blocking
sections of wood (which may be preservative treated) built into a roof assembly, usually attached above the deck and below the membrane or flashing, used to stiffen the deck around an opening, act as a stop for insulation, support a curb, or serve as a nailer for attachment of the membrane and/or flashing.

Blowing agent
an expanding agent used to produce a gas by chemical or thermal action, or both, in manufacture of hollow or cellular materials.

BOCA
Building Officials and Code Administrators, International, Inc. (author of "The BOCA National Building Code").

BOMA
Building Owners & Managers Association, International

Bond, Chemical
adhesion between surfaces, usually of similar materials, resulting from a chemical reaction or cross-linking of polymer chains.

Bond, Mechanical
adhesion between surfaces resulting from interfacial forces or a physical interlocking.

Bond
the adhesive and/or cohesive forces holding two components in positive contact.

Bonding Agent
a chemical substance applied to a suitable substrate to create bond between it and a succeeding layer.

Boot
(1) a covering made of flexible material, which may be preformed to a particular shape, used to exclude dust, dirt, moisture, etc., from around a penetration; (2) a flexible material used to form a closure, sometimes installed at inside and outside corners.

Brake
hand- or power-activated machinery used to bend metal.

Bridging
(1) when membrane or base flashing is unsupported at a juncture; (2) bridging in steep-slope roofing occurs when reroofing over standard-sized asphalt shingles with metric-sized asphalt shingles.

British thermal unit (BTU)
the heat energy required to raise the temperature of 1 pound of water one degree Fahrenheit (joule). For the metric equivalent, see Joule.

Broadcast
uniformly cast or distribute grandular or aggregate surfacing material.

Brooming
to improve the embedding of a ply or membrane by using a broom or squeegee to smooth it out and ensure contact with the adhesive under the ply or membrane.

Buckle
an upward, elongated displacement of a roof membrane frequently occurring over insulation or deck joints. A buckle may be an indication of movement within the roof assembly.

Building code
the minimum construction requirements established generally by national organizations of experts and adopted completely or in altered form by local governing authorities.

Built-up roof (BUR)
a continuous, semi-flexible roof membrane, consisting of multiple plies of saturated felts, coated felts, fabrics or mats assembled in place with alternate layers of bitumen, and surfaced with mineral aggregate, bituminous materials, a liquid-applied coating or a granule-surfaced cap sheet.

Bun stock
large solid box-like structure formed during the production of polystyrene insulation; individual board stock pieces are then cut from the bun.

Bundle
an individual package of shakes or shingles.

Bush Hammer
a hammer, originally a hand tool but now usually power driven, having a serrated face containing many pyramid-shaped points; used to provide a roughened surface on concrete.

Butt joint
a joint formed by adjacent, separate sections of material, such as where two neighboring pieces of insulation abut.

Button punch
a process of indenting two or more thicknesses of metal that are pressed against each other to prevent slippage between the metal.

Butyl coating
an elastomeric coating system derived from polymerized isobutylene. Butyl coatings are characterized by low water vapor permeability.

Butyl rubber
a synthetic elastomer based on isobutylene and a minor amount of isoprene. It can be vulcanizable and features low permeability to gases and water vapor.

Butyl tape
a sealant tape sometimes used between metal roof panel seams and/or end laps; also used to seal other types of sheet metal joints, and in various sealant applications.

Butyl
rubber-like material produced by polymerizing isobutylene.