Roofing Dictionary


Alligatoring is term used to describe the cracking of surfacing bitumen on a built-up roof. These cracks are the result of the limited tolerance of asphalt to thermal expansion or contraction, and produce a pattern that resembles an alligator’s hide.


A dark brown to black material in which the predominating constituents are bitumens, which occur in nature or are obtained in petroleum processing.


The generic term for an amorphous, semi-solid mixture of complex hydrocarbons derived from any organic source. Asphalt and coal tar are the two used in the roofing industry.


A spongy raised portion of a roof membrane, ranging in area from 1 inch in diameter and of barely detectable height upward. Blisters result from the pressure build up of gases entrapped in the membrane system. These gases most commonly are air and/or water vapor. Blisters usually involve delamination of the underlying membrane plies.

Built-up Roof Membrane:

A continuous,semi-flexible roof membrane assembly, consisting of plies of saturated felts,coated felts, fabrics or mats between which alternate layers of bitumen are applied, generally surfaced with mineral aggregate, bituminous materials, or a granule-surfaced roofing sheet. (Abbreviation: BUR.)

Cant Strip:

A beveled strip of wood or wood fiber that fits into the angle formed by the intersection of a horizontal surface and a vertical surface. The 45-degree slope of the exposed surface of the cant strip provides a gradual angular transition from the horizontal surface to the vertical surface.


A composition of vehicle and pigment, used at ambient temperatures for filling joints, that remains plastic for an extended time after application.

Coal Tar Pitch:

A dark brown to black, semi-solid hydrocarbon formed as a residue from the partial evaporation or distillation of coal tar. It is used as the waterproofing agent in dead-level or low-slope built-up roofs. (For specification properties, see ASTM Standard D 450, Types 1and II.)


The covering piece placed on top of a wall that is exposed to the weather. It is usually sloped to shed water.

Counter flashing:

Formed metal or elastomeric sheeting secured on or into a wall, curb, pipe, rooftop unit or other surface to cover and protect the upper edge of a base flashing and its associated fasteners


A raised member used to support roof penetrations, such as skylights, mechanical equipment, hatches, etc. above the level of the roof surface.

Dead Level Asphalt:

A roofing asphalt that has a softening point of 140F (60C) and that conforms to the requirements of ASTM Standard D 312, Type 1.

Dead Level:

The term used to describe an absolutely horizontal roof. Zero slope. (See SLOPE.)


The structural surface to which the roofing or waterproofing system (including insulation) is applied.


Separation of the plies in a roof membrane system or separation of laminated layers of insulation.


A pipe for draining water from roof gutters. A downspout is also called a leader.


Ethylene Propylene Diene Monomer

Expansion Joint:

A structural separation between two building elements designed to minimize the effect of the stresses and movements of a building’s components and to prevent these stresses from splitting or ridging the roof membrane.

Factory Mutual:

An organization that classifies roof assemblies for their fire characteristics and wind-uplift resistance for insurance companies in the United States.


A fabric manufactured from vegetable fibers (organic felts), asbestos fibers (asbestos felts), or glass fibers(glass fiber felts). The manufacturer process involves mechanically interlocking the fibers of the particular felt material in the presence of moisture and heat.


A small metal sleeve placed inside a gutter at the top. A spike is nailed through the gutter into the fastening board. The ferrule acts as a spacer in the gutter to maintain its original shape.


Membrane defect consisting of an opening in the edge lap of a felt in a built up membrance; a consequence of an edge wrinkle.


Components used to weatherproof or seal the roof system edges at the perimeters, penetrations, walls and other places where the roof covering is interrupted or terminated.


Coarse, granular aggregate,containing pieces approximately 5/8 inch to 1/2 inch in size and suitable for use in aggregate on built up roofs.

Gravel Stop:

A flange device, frequently metallic, designed to provide a continuous finished edge for roofing materials and to prevent loose aggregate form washing off of the roof.

Heat Welding:

Method of melting or fusing together the overlapping edges of separate sheets of thermoplastics and polymer modified bitumens.

Infrared Thermography:

A practice of roof analysis where an infrared camera is used to measure the temperature differential of a roof surface to locate areas of underlying moisture.


Any of the small timbers or metal beams ranged parallel from wall to wall in a structure to support a floor or ceiling.

Modified Bitumen:

Composite sheets consisting of a polymer (e.g., atactic polypropylene (APP), or styrenebutadiene styrene (SBS)) often reinforced and sometimes surfaced with various types of mats, films, foils and mineral granules.


Material Safety Data Sheets

Parapet Wall:

Perimeter wall, which extends above the roof.

Pipe Boot:

Covering of flexible material,which may be preformed to a particular shape, used to seal around a pipe penetration.


Polyvinyl Chloride – Usually associated with a thermoplastic single ply roof membrane system.


A groove in a wall or other surface adjoining a roof surface for use in the attachment of counter flashing.


An upward, “tenting” displacement of a roof membrane, frequently occurring over insulation joints, deck joints and base sheet edges. Generally associated with improper application. (See picture framing)

Sprayed Polyurethane Foam (SPF):

a foamed plastic material, formed by spraying two components, PMDI ([A] component) and aresin ([B] component) to form a rigid, fully adhered, water resistant, and insulating membrane.

Standing Seam:

a metal roof system that consists of an overlapping or interlocking seam that occurs at an upturned rib.

Thermoplastic Olefin Membrane (TPO):

A blend of polypropylene and ethylene-propylene polymers. Colorant, flame retardants, UV absorbers, and other proprietary substances, which may be blended with the TPO to achieve the desired physical properties.


Materials that soften when heated and harden when cooled. Thermoset: A material that solidifies or “sets” irreversibly when heated. This property is usually associated with cross-linking of the molecules induced by heat or radiation.


Underwriters Laboratories, Inc.

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