Term Description
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.

Base coat

The first coat of a multicoat system.

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 and extends up the vertical surface. See "flashing."

Base ply

The bottom or first ply in a built-up or polymer- modified bitumen roof or waterproofing system 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 and waterproofing systems.

Basic wind speed

Three-second gust wind speed in miles per hour at 33 feet above ground in Exposure C as defined in the latest edition of the American Society of Civil Engineers standard ASCE 7, "Minimum Design Loads for Buildings and Other Structures."


(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 steep-slope roof system, a strip of wood or metal usually set in or over the structural deck, used to elevate and/or attach a primary roof covering; (4) in a single-ply membrane roof system, a narrow plastic or metal bar that is used to fasten or hold the roof membrane and/or base flashing in place.

Batten seam

A name applied to a common standing-seam metal panel profile that may use a square- or rectangular-profile snap-on cap or may be attached to and formed around a beveled wood or metal batten.


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

Bearing plate

(1) A plate used to distribute fastener load in metal panel roof systems placed over rigid board insulation and through-fastened to the roof deck; (2) a steel plate that is set on the top of a masonry support on which a beam or purlin can rest.


A porous clay formed by the decomposition of volcanic ash that swells five to six times its original volume in the presence of water.

Bermuda seam

A metal panel profile featuring a stepdown profile that runs perpendicular to the slope of the roof; has a shingled appearance and is water-shedding.


(1) A class of amorphous, black or dark-colored, (solid, semi-solid or viscous) cementitious substances, natural or manufactured, principally composed 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, typically asphalt or coal tar.


Containing or treated with bitumen, e.g., bituminous concrete, bituminous felts and fabrics, and bituminous pavement.

Bituminous emulsion

(1) See "asphalt emulsion"; (2) a suspension of minute globules of bituminous material in water or in an aqueous solution; (3) a suspension of minute globules of water or of an aqueous solution in a liquid bituminous material (invert emulsion).

Blanket insulation

Fiberglass or other compressible fibrous insulation, generally available in roll form.

Blast furnace slag

The nonmetallic product, consisting essentially of silicates and alumino-silicates of calcium and other bases, that is developed in a molten condition simultaneously with iron in a blast furnace.


(1) A raised portion of a roofing membrane resulting from local internal pressure, such as 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; (2) the similarly formed surface swelling in coated prepared roofing such as asphalt shingles; (3) separation of a coating from a substrate; may be caused by water absorption and the resultant swelling or subsurface corrosion.


The formation in the film of dome-shaped, liquid or gas-filled projections resulting from local loss of adhesion and lifting of the film from the substrate or previously applied coating.


(1) Sections of wood (which may be preservative-treated) built into a roof or waterproofing 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; (2) wood crossmembers 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.

Blowing agent

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


(1) The adhesive and/or cohesive forces holding two components in positive contact; (2) a surety; typical types are: bid, performance and payment; (3) a guarantee relating to roof system performance.

Bonding agent

A chemical substance applied to a suitable substrate to create a bond between it and a succeeding layer. See "adhesive."


(1) A covering made of flexible material or combination of flexible and rigid material that 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.


Structural elements installed to provide restraint or support or both to other members so the complete assembly forms a stable structure; may consist of knee braces, cables, rods, struts, ties, shores, diaphragms, rigid frames, etc.