Low-Slope Roof Inspections Terms and Definitions

The Glossary of Terms below will help the inspector use the proper terminology in their commercial property inspection report. Consider uploading it to your reporting software, along with the CCPIA copyright technical renderings – both are available for members to use freely. Take CCPIA’s Inspecting Low-Slope Roofs Course for Commercial Property Inspectors to learn how to inspect low-slope roofs at commercial buildings. Browse the Glossary of Commercial Property Inspections Terms for a complete list of technical and business terminology.

• adhesion: The property of a coating or sealant that allows it to bond to the surface to which it is applied.
• aggregate: Crushed stone, slag or water-worn gravel that comes in a wide range of sizes and is often used to cover the surface of built-up roofs.
• air barrier: A low-slope roof accessory component that prevents or limits air leakage through the building envelope and serves to control the infiltration and exfiltration of air flow.
• alligatoring: An oxidized condition of aged surfacing bitumen, asphalt, or other low-slope roof surface that has lost its volatile oils due to exposure to sun and solar radiation, which is the ultimate result of the surface’s limited tolerance to thermal expansion and contraction. Alligatoring is characterized by a coarse, checked pattern of cracks resembling an alligator hide.
• atactic polypropylene (APP): Used as a modifying compound in the asphalt membrane of modified bitumen (MB) roofs; exhibits a plasticized nature.
• ballast: A material similar to aggregate or pavers (smooth river stone, crushed stone, concrete pavers, lightweight concrete pavers, etc.) that is often used as a weight on loose-laid roofing systems to hold the membrane in place.
• ballast scouring: A term used to describe the shifting of ballast or loosened pavers on the exposed surface of a roof, usually as the result of wind.
• base flashing: The upturned edge of a watertight membrane formed at a roof’s termination point by the extension of the felts vertically over the cant strip and up the wall, and secured with mechanical fasteners.
• batten: A general term used to describe a cap or covering. In a singly-ply membrane installation, it’s a narrow bar used to hold the roof membrane and/or base flashing in place.
• billowing: A term used to describe a membrane that is filled or lifted with air and swells outward.
• bitumen: Any of a variety of mixtures of hydrocarbons occurring naturally or obtained through the distillation of coal or petroleum.
• blister: A term used to describe an enclosed raised spot evident on the surface of a roof.
• blistering: The process and occurrence of a void or unhindered area between the plies or between the substrate, mainly caused by the expansion of trapped air, water vapor, moisture, or other gases.
• blueberry blister: A term used to describe a small raised spot on the roof covering’s surface, which forms when liquid or vapor is trapped in asphalt in a BUR system.
• boot: A flexible material or metal that is used as flashing.
• built-up roofing (BUR): Also called tar-and-gravel roofing, BUR is a roofing system composed of three to five layers of asphalt felt laminated with coal tar, pitch or asphalt, and finished on top with a smooth surface, crushed slag or gravel.
• coal tar pitch: A bituminous material that is a byproduct of the coking of coal and used as the waterproofing material for tar-and-gravel built-up roofing.
• component: A permanently installed or attached fixture, element or part of a system.
• conventional roof system: A low-slope roof system where the insulation is installed under the roof’s membrane.
• core sampling: A vertical cut taken of the roofing system that can be used for analyzing the roof’s layers of composition.
• corrosion: The deterioration of metal by chemical or electrochemical reaction resulting from exposure to weathering, moisture, chemicals, or other agents or media.
• counter-flashing: A pre-formed metal that is secured to a wall, curb or rooftop unit used to cover and protect the upper edge of a base flashing and its associated fasteners.
• coping: A construction unit placed at the top of a parapet wall to serve as a cover for the wall.
• cover board: A relatively thin substrate that’s placed over the primary thermal insulation in a roofing system.
• cracking: A term used to describe nonlinear visible breaks on the surface of a material.
• crater: A pit in the surface of BUR resulting from a burst blueberry blister.
• cricket: An elevated and peaked construction installed on low-slope and structural metal roofs to divert water around roof-penetrating components.
• curb: A square or rectangular-shaped member that is built above the level of the roof that provides a means of support for flashing.
• deflection: The amount of bending movement of any part of a structural member perpendicular to the axis of the member under an applied load; also referred to as sag.
• delamination: A term used to describe the separation of laminated layers, either membrane plies or insulation.
• downspout: Also called a leader, the pipe that carries water down from the gutter or scupper.
• easily visible: Describes systems, items and components that are both conspicuous and in plain sight, absent of the need for intrusive inspection techniques, probing, disassembly, or the use of special equipment.
• edge drainage: A drainage system on a low-slope roof where water flows from the high points to the building’s perimeter, which usually has scuppers, a gutter system, and/or downspouts.
• EPDM (ethylene propylene diene monomer): A single-ply membrane consisting of synthetic rubber, usually in 45 or 60 mils. Application can be ballasted, fully adhered, or mechanically attached.
• expansion joint: A device used to permit a structure to expand or contract without breakage.
• dead-level roof: A roof with no slope; a flat roof.
• dead load: Static load imposed by gravitational forces acting upon the structure and permanently installed building materials, such as a roof, wall, floor, and ceiling-covering materials.
• fishmouth: A term used to describe a defective seam lap opening that is shaped like a half cylinder.
• fixed ladder: A non-self-supporting ladder that is permanently attached to a structure.
• flange: The projecting edge on a component.
• flashing: A pre-shaped or molded low-slope roof accessory that is used to terminate the membrane at the roof’s perimeter and at roof penetrations to prevent rainwater/moisture leakage into the structure.
• flood coat: A layer of bitumen that serves as the surfacing substrate in a BUR system.
• fully adhered: A term used to describe a membrane that is glued to the substrate without penetrating the membrane.
• heat-welded: Also referred to as heat-seaming, it is the process of joining and sealing overlapping thermoplastic membrane edges with heat.
• internal drainage: A drainage system on a low-slope roof where water flows from the high points, usually the edge areas, to a built-in drainage system that leads down through the interior of the building.
• lap seam: Two overlapping membranes that meet and are seamed, sealed or bonded in some way.
• liquid-applied: A continuous roof membrane constructed in place using one or more coats to provide fully adhered, waterproof membranes that conform to all contours.
• live load: A transient, variable load that may be imposed by gravity, by the building’s occupants, or by the environment, such as seismic, hydrostatic (a water source), snow, or wind.
• loose-laid membrane: A membrane that is loosely laid over the substrate and anchored only at the roof’s perimeter and penetrations.
• low-slope roof: A roof having a slope of 1/4:12 up to 3:12.
• mechanically attached: A term used to describe a membrane that is attached to the substrate by means of fasteners (nails and plates); also commonly referred to mechanically fastened.
• membrane: A slightly flexible covering for a low-slope roof, often acting as the weatherproofing component.
• metal edge flashing: Brake metal, sheet metal, or a metal extrusion that is secured at the perimeter of a roof to form a weathertight seal.
• modified bitumen (MB): Also referred to as torch-down roofing or rubberized asphalt, composed of polymer-modified bitumen that saturates, impregnates, or fills and coats reinforcement fabric (polyester and/or fiberglass).
• oil-canning: A term that describes the distortion of thin-gauge metal panels that are fastened in a manner that restricts normal thermal movement.
• parapet wall: A low wall around the perimeter of a roof deck.
• picture-framing: A term used to describe the rectangular or square-patterned ridging that appears over the perimeter of an insulation board.
• pitch: The incline or slope of a roof; also, coal tar pitch.
• pitch pan: A container formed of sheet metal that is installed around supporting connections for roof-mounted equipment and machinery. Filling the container with pitch or plastic roof cement helps seal out rainwater, even under conditions of vibration caused by the machinery’s operation or other factors.
• plumbing jacks: Sleeves that fit around drain-and-waste vent pipes and attached to the low-slope roof’s surface.
• PMR (protected membrane roof): Also known as an IRMA (inverted roof membrane assembly), a low-slope roof having insulation that is installed on top of the membrane.
• ponding: The development of a large puddle or area of standing water on a roof for prolonged periods, typically 48 hours after a rain event when conditions are favorable for drying, due to poor drainage and/or deflection of the roof deck.
• purlins: A horizontal structural member spanning between beams or trusses to support a roof deck. In slope glazing, purlins are the horizontal framing members.
• PVC (polyvinyl chloride): A thermoplastic low-slope roof membrane.
• ridging: Also commonly known as buckling, a term used to describe upward linear tenting of a low-slope roof membrane, usually found over insulation and deck joints.
• rolled/roll roofing: A general term used to describe low-slope roof materials that are packaged into rolls, typically consisting of smooth- or mineral-surfaced felts.
• roof assembly: Consists of all components connected with the roof, starting with the roof deck (but not the roof understructure), and includes the roof system.
• roof deck: Structural component of the roof that serves as the substrate for the installation of the roof system.
• roof joist: One of a series of parallel framing members used to support ceiling loads, and supported, in turn, by larger beams, girders or bearing walls.
• roofing system: Consists of the components installed on top of a roof deck (but not the deck itself) that together protect the building’s interior from weather, including providing resistance to moisture intrusion, and often acts as part of the thermal/pressure envelope. In a membrane roof, this is an air or vapor retarder, insulation, and the membrane designed as the primary weather barrier.
• R-value: The thermal resistance of insulation or a glazing system. The R-value is the reciprocal of the U-value. The higher the R-value, the less heat is transmitted throughout the insulation or glazing material.
• saddle: Two sloping surfaces meeting in a horizontal ridge, used between the backside of roof-penetrating components, in a valley, or between scuppers to divert water; constructed like a pyramid with a diamond-shaped based.
• SBS (styrene butadiene styrene copolymer): Used as a modifying compound in an MB asphalt roof membrane; exhibits a rubberized nature.
• scrim: A woven or mat-type fabric that is used as a membrane sandwiched between other materials to provide reinforcement and stretch resistance.
• scupper: An outlet in the wall of a building or a parapet wall for the drainage of water from a flat roof.
• secondary drain: Also called an emergency overflow, a scupper or drain on a low-slope roof plumbed independently to limit the accumulation of water if the primary drain is clogged.
• service life: The number of years, as determined by the manufacturer, of service that a system or component is deemed to provide before requiring repair due to wear and tear or replacement.
• shear: The lateral force that makes two sheets or components slide parallel in opposite directions.
• shrinkage: A term used to describe stretch in a membrane.
• single-ply membrane: A descriptive term for a roof membrane composed of only one layer of material (EPDM, TPO, PVC).
• slope: The incline or pitch of a roof surface, drainage plane, etc.
• slag: A byproduct of smelting ore, such as iron, lead or copper, and may be used as a surfacing material on bituminous roofing systems.
• slippage: Lateral movement of the roof membrane and layers underneath.
• special equipment: Any tools or devices, other than those normally used by an inspector, that are used to perform a typical and customary, non-invasive, physical examination of the systems, structures and components of a building.
• splitting: The formation of long rupture that goes completely through a membrane. Splits are frequently associated with lack of allowance for expansion stresses. They can also be a result of deck deflection or change in deck direction.
• steep-slope roof: A roof having a slope of 3:12 or greater.
• substrate: A part or substance that lies beneath a roof element or component and supports another element or component of the roof.
• tapered insulation: Pre-cut or pre-molded insulation designed to provide a positive slope for adequate drainage to the roof deck before the membrane is installed.
• thermal insulation: Any material that is highly resistant to heat transmission that, when placed inside the walls, ceilings and floors of a structure, reduces the rate of heat flow.
• thermal cycling: A term used to describe recurring expansion and contraction of building materials from temperature fluctuations.
• thermal movement: The measured amount of dimensional change that a material exhibits as it is warmed and cooled.
• thermoplastic: A material that can be repeatedly softened when heated and hardened when cooled, such as TPO and PVC.
• thermoset: A material that, once cured or vulcanized, cannot be softened to bond with itself or reshaped, such as EPDM.
• torch-applied: A method for installing a membrane on a low-slope roof that uses special equipment, such as an open-flame propane torch; usually used on MB.
• TPO (thermoplastic polyolefins): A thermoplastic low-slope roof membrane.
  UV (ultraviolet) degradation: A reduction in performance caused by exposure to sunlight.
• vapor retarder: A low-slope roof accessory that minimizes vapor diffusion through the roof assembly.
• wind uplift: The upward force exerted by wind traveling across a roof.
• wrinkling: A term used to describe lines and folds that randomly appear in a membrane, usually caused by membrane slippage, and is the result of the slope of the roof such that the asphalt is too viscous or the membrane was not properly attached to the decking.