Roofing Dictionary (A–Z)

Alligatoring
A pattern of cracking in the surface of asphalt or bitumen on a built-up roof (BUR), resembling an alligator’s hide. This is caused by aging and the limited ability of asphalt to expand and contrast with temperature changes.

Asphalt
A dark brown to black cementitious material in which the predominant constituents are bitumen’s. Occurs naturally or is refined from petroleum and commonly used in roofing systems.

Bitumen
A generic term for amorphous, semi-solid mixtures of complex hydrocarbons derived from organic sources. In roofing, this includes asphalt and coal tar.

Blister
A raised, spongy area of a roof membrane caused by trapped air or moisture vapor beneath the surface. Blisters typically indicate ply separation within the roofing system.

Built-Up Roof Membrane (BUR)
A continuous, semi-flexible roofing system consisting of multiple plies of saturated felts, fabrics, or mats, bonded together with alternating layers of bitumen and typically surfaced with gravel or mineral aggregate.

Can’t Strip
A beveled strip (typically wood or fiberboard) installed at roof-to-wall intersections to ease the transition between horizontal and vertical surfaces and reduce membrane stress.

Caulking
A pliable sealant applied at ambient temperatures to fill joints or cracks and remain flexible after curing.

Coal Tar Pitch
A dark brown to black semi-solid hydrocarbon residue produced during coal tar distillation. Used as a waterproofing agent in low-slope or dead-level roofing systems. (ASTM D450)

Coping
A protective cap installed on top of a parapet or wall, typically sloped to shed water and protect the wall from weather exposure.

Counterflashing
Formed metal or elastomeric material installed over base flashing to protect its upper edge and fasteners from water intrusion.

Curb
A raised frame or box built above the roof deck to support rooftop penetrations such as skylights, HVAC units, or roof hatches.

Dead-Level Asphalt
A roofing asphalt with a softening point of approximately 140°F (60°C), meeting ASTM D312 Type I requirements.

Dead-Level Roof
A roof with zero slope—completely horizontal. Dead-level roofs require special drainage and waterproofing considerations.

Deck
The structural surface (steel, wood, or concrete) to which the roofing system and insulation are applied.

Delamination
Separation of layers within a roofing membrane or laminated insulation, often caused by moisture intrusion or poor adhesion.

Downspout (Leader)
A vertical pipe that carries water from gutters to the ground or storm drainage system.

EPDM
Ethylene Propylene Diene Monomer. A synthetic rubber single-ply roofing membrane known for flexibility and long service life.

Expansion Joint
A designed separation in a building structure that allows movement due to thermal expansion, contraction, or seismic activity, preventing membrane splitting.

Factory Mutual (FM)
An insurance-related testing organization that evaluates roofing assemblies for fire resistance and wind uplift performance.

Felt
A fabric manufactured from organic fibers or fiberglass and used as reinforcement in roofing membranes.

Ferrule
A small metal sleeve placed inside a gutter to maintain spacing and prevent deformation when gutter spikes are driven.

Fishmouth
A roofing defect where the edge of a membrane ply opens due to wrinkling, allowing moisture intrusion.

Flashing
Components installed to weatherproof roof edges, penetrations, walls, and transitions where the roofing system is interrupted.

Gravel
Coarse aggregate (typically ½"–⅝") used as surfacing on built-up roofing systems for UV protection and durability.

Gravel Stop
A metal edge detail that prevents loose aggregate from washing off the roof while providing a finished perimeter edge.

Heat Welding
A method of joining thermoplastic roofing membranes by melting and fusing overlapping sheets using hot air.

Infrared Thermography
A roof diagnostic technique using infrared imaging to detect temperature variations indicating trapped moisture.

Joists
Structural members (wood or steel) installed parallel to support floors or roof decks.

Modified Bitumen
An asphalt-based roofing material enhanced with polymers such as SBS or APP for increased flexibility and durability.

MSDS (SDS)
Material Safety Data Sheet (now Safety Data Sheet). Provides information on handling, hazards, and safety procedures for materials.

Parapet Wall
A perimeter wall that extends above the roof surface.

Pipe Boot
A flexible or pre-formed flashing component used to seal around pipe penetrations.

PVC
Polyvinyl Chloride. A thermoplastic single-ply roofing membrane with heat-welded seams and chemical resistance.

Reglet
A groove cut into a wall or curb to receive and secure counterflashing.

Ridging
An upward tenting or buckling of the roofing membrane, often occurring over joints or seams due to improper installation.

Sprayed Polyurethane Foam (SPF)
A roofing system formed by spraying two liquid components that expand into a rigid, fully adhered, insulating, and water-resistant membrane.

Standing Seam
A metal roofing system characterized by raised, interlocking vertical seams that conceal fasteners.

Thermoplastic Olefin (TPO)
A single-ply roofing membrane made from polypropylene and ethylene-propylene polymers with added UV stabilizers and flame retardants.

Thermoplastic
Materials that soften when heated and harden when cooled, allowing seams to be heat-welded.

Thermoset
Materials that cure irreversibly when heated or chemically activated (e.g., EPDM).

UL (Underwriters Laboratories)
An independent safety science organization that tests and certifies roofing assemblies for fire resistance and performance.

Vapor Barrier
A material installed within a roof assembly to limit the migration of water vapor from the building interior into the insulation and roofing system.

Walkway Pads
Durable pads or pavers installed on roof surfaces to protect the membrane from foot traffic and maintenance activity.

Wind Uplift
The upward force exerted on a roofing system by wind pressure, which roofing assemblies must be designed to resist.

XPS (Extruded Polystyrene)
A rigid foam insulation commonly used in roofing assemblies, known for high compressive strength and moisture resistance.

Yield
The coverage rate of a roofing material, typically expressed as square feet per gallon or per unit.

Zinc Coating (Galvanization)
A protective zinc layer applied to steel roofing components to prevent corrosion.