Commercial Roofing Glossary
57 terms defined in plain language for building owners and facility managers.
A
- APP
- Atactic polypropylene, a type of modified bitumen membrane that uses a plastic-based modifier and is installed by torch-application, creating a heat-welded bond. APP membranes are known for their UV resistance and ability to perform well in high-heat climates.
Related: Modified bitumen, SBS, BUR
- ASTM
- ASTM International (formerly the American Society for Testing and Materials) sets voluntary technical standards for materials, products, and systems used in construction. Roofing specifications frequently reference ASTM standards to ensure membrane thickness, tensile strength, and other properties meet minimum quality benchmarks.
B
- Ballasted
- A roof assembly where the membrane is held in place by the weight of ballast material — typically river-washed stone, pavers, or concrete blocks — rather than adhesive or mechanical fasteners. Ballasted systems reduce membrane exposure to UV and wind but add significant dead load to the structure.
Related: Fully adhered, Mechanically attached, EPDM
- Base flashing
- The portion of flashing that covers the joint where the roof membrane meets a vertical surface such as a parapet wall, curb, or penetration. Base flashing directs water away from these vulnerable transitions and is typically made from the same membrane material or compatible metal.
Related: Counter flashing, Flashing, Parapet
- BUR
- Built-up roofing, one of the oldest commercial flat roof systems, consisting of alternating layers of bitumen (asphalt or coal tar) and reinforcing felts to create a multi-ply waterproof membrane. A typical BUR assembly includes three to five plies and is topped with a flood coat and gravel or a cap sheet.
Related: Modified bitumen, Gravel stop, Membrane
C
- Cant strip
- A triangular piece of material (wood, foam, or perlite) installed at the junction where the roof deck meets a vertical surface to create a gradual transition. Cant strips prevent sharp 90-degree bends in the membrane that could crack, split, or pull away over time.
Related: Base flashing, Parapet
- Coping
- A protective cap, usually metal or precast concrete, installed on top of a parapet wall to shed water away from the wall face and prevent moisture infiltration. Improperly secured or deteriorated coping is one of the most common sources of leaks on flat-roofed buildings.
Related: Parapet, Counter flashing, Edge metal
- Counter flashing
- A piece of metal or membrane installed over and lapping down onto the top edge of base flashing to prevent water from entering behind it. Counter flashing is typically embedded in a reglet (groove) cut into the masonry wall or secured under the coping.
Related: Base flashing, Flashing, Coping
- Cover board
- A rigid board (typically gypsum, high-density polyiso, or wood fiber) installed between the insulation and the roof membrane to provide a smooth substrate, enhance puncture resistance, and add fire performance. Cover boards are increasingly specified on commercial projects to protect insulation and improve long-term membrane performance.
Related: Substrate, Insulation (polyiso), Membrane
D
E
- Edge metal
- Pre-formed metal flashing installed along the perimeter of a flat roof to secure the membrane edge, direct water into gutters, and resist wind uplift. Edge metal must be designed and fastened to meet FM or UL wind-uplift requirements for the building's specific wind zone.
Related: Gravel stop, Fascia, Wind uplift
- EPDM
- Ethylene propylene diene monomer, a synthetic rubber single-ply membrane known for its flexibility, UV stability, and long track record on commercial flat roofs. EPDM is typically black (though white versions exist), comes in large sheets to minimize seams, and is installed fully adhered, mechanically attached, or ballasted.
Related: Single-ply, Membrane, Fully adhered, Ballasted
- Expansion joint
- A structural separation built into a roof system to accommodate thermal movement, building settlement, or transitions between different structural sections. Expansion joints must be properly detailed with flexible curbs and membranes; failed expansion joints are a frequent and costly source of leaks.
Related: Penetration, Flashing
F
- Fascia
- A vertical or near-vertical trim board or metal panel at the edge of a roof that covers the exposed ends of roof rafters or deck edge. On flat commercial buildings, fascia provides a finished appearance and often works in conjunction with edge metal to secure the membrane perimeter.
Related: Edge metal, Coping
- Field of roof
- The central, open expanse of the roof membrane that is not part of the perimeter, flashing, or penetration details. The field of the roof typically experiences different wind pressures than perimeter and corner zones, which affects fastener spacing and membrane attachment requirements.
- Flashing
- Components used to seal and waterproof transitions, penetrations, and edges in a roof system where the membrane meets walls, curbs, pipes, or other surfaces. Flashing failures account for the majority of flat roof leaks and are often the first areas to deteriorate.
Related: Base flashing, Counter flashing, Penetration
- FM Approval
- A certification from FM Global (formerly Factory Mutual) indicating that a roofing product or assembly has been tested and meets rigorous standards for fire, wind uplift, and hail resistance. Many commercial insurance policies require or incentivize FM-approved roof assemblies, and specifying FM-rated systems can lower insurance premiums.
Related: UL rating, Wind uplift
- Fully adhered
- A membrane attachment method where the entire underside of the membrane is bonded to the substrate using adhesive or heat welding. Fully adhered systems offer excellent wind-uplift resistance and a smooth finished appearance but are more sensitive to substrate conditions and moisture.
Related: Mechanically attached, Ballasted, Substrate
G
- Gravel stop
- A flanged metal edge strip installed at the perimeter of a BUR or gravel-surfaced roof to contain loose aggregate and direct water runoff into gutters. Gravel stops serve both a functional and aesthetic purpose by providing a clean, finished roof edge.
Related: Edge metal, BUR, Gutter
- Gutter
- A trough or channel along a roof edge that collects and directs rainwater to downspouts and away from the building foundation. On commercial flat roofs, gutters must be sized to handle the roof's drainage area and are critical to preventing fascia rot, foundation erosion, and ponding.
Related: Drain, Scupper, Gravel stop
H
- HVAC curb
- A raised metal or wood frame installed on the roof deck to elevate HVAC equipment above the membrane surface, allowing proper flashing and drainage around the unit. Poorly flashed HVAC curbs are among the most common leak sources on commercial flat roofs.
Related: Penetration, Flashing, Cant strip
I
- Insulation (polyiso)
- Polyisocyanurate rigid foam board insulation, the most commonly specified insulation in commercial flat roofing due to its high R-value per inch (approximately R-5.7 per inch). Polyiso is lightweight, fire-resistant, and compatible with all major membrane types, though its thermal performance can decrease in very cold temperatures.
Related: R-value, Insulation (EPS), Insulation (XPS), Polyiso
- Insulation (EPS)
- Expanded polystyrene rigid foam insulation, a lightweight and cost-effective option that offers stable R-value across temperature ranges (approximately R-3.8 per inch). EPS is commonly used in tapered insulation systems and protected membrane assemblies where it sits below ballast and above the waterproofing layer.
Related: R-value, Insulation (polyiso), Insulation (XPS), Tapered insulation
- Insulation (XPS)
- Extruded polystyrene rigid foam insulation, recognized by its color (typically blue, pink, or green depending on manufacturer), offering high compressive strength and moisture resistance (approximately R-5.0 per inch). XPS performs well in protected membrane and plaza deck applications where the insulation is exposed to moisture.
Related: R-value, Insulation (polyiso), Insulation (EPS)
M
- Mechanically attached
- A membrane installation method where the roofing membrane is secured to the deck using screws and plates, typically at the seams or in the field. Mechanically attached systems are faster to install and less weather-dependent than fully adhered systems but can be more susceptible to membrane flutter in high-wind areas.
Related: Fully adhered, Ballasted, Wind uplift
- Membrane
- The waterproof layer of a flat roof system that provides the primary barrier against water infiltration. Common commercial membrane types include single-ply (TPO, PVC, EPDM), modified bitumen, and built-up roofing, each with distinct performance characteristics and installation methods.
Related: Single-ply, TPO, PVC, EPDM, Modified bitumen, BUR
- Mil
- A unit of measurement equal to one-thousandth of an inch (0.001 inch), used to specify the thickness of roofing membranes. Common single-ply membrane thicknesses are 45 mil, 60 mil, and 80 mil — thicker membranes generally offer better puncture resistance and longer service life.
Related: Membrane, Single-ply
- Modified bitumen
- A hybrid roofing membrane made by reinforcing traditional asphalt with polymer modifiers (APP or SBS) to improve flexibility, strength, and weather resistance. Modified bitumen is installed in two or more plies using torch application, cold adhesive, or self-adhering technology.
N
- NDL warranty
- A No Dollar Limit warranty is the most comprehensive roof warranty available, where the manufacturer covers the full cost of labor and materials to repair or replace a failed roof system with no depreciation or cap. NDL warranties are typically 20-30 years, require a manufacturer inspection during installation, and are only available through certified contractors.
Related: FM Approval
- Net free area
- The unobstructed open area of a vent or louver through which air can pass, measured after accounting for screens, louvers, and other restrictions. Adequate net free area is critical for proper attic or plenum ventilation, which helps prevent condensation and moisture damage in the roof assembly.
- NRCA
- The National Roofing Contractors Association, the leading trade association for the U.S. roofing industry, providing technical manuals, training, certification programs, and industry standards. NRCA membership and adherence to NRCA guidelines is an important indicator of a contractor's professionalism and commitment to best practices.
Related: SPRI
P
- Parapet
- A low wall extending above the roof edge around the perimeter of a flat-roofed building, providing fall protection and a surface for terminating the roof membrane. Parapets require careful flashing with base flashing, counter flashing, and coping to prevent water infiltration at this complex transition.
Related: Coping, Base flashing, Counter flashing
- Penetration
- Any object that passes through the roof membrane, including pipes, vents, conduits, supports, and drains. Every penetration requires individual flashing detail, and penetrations are statistically the most leak-prone areas on a flat roof.
Related: Flashing, Pitch pocket, HVAC curb
- Pitch pocket
- A flanged metal or plastic sleeve placed around an irregular roof penetration and filled with sealant or pourable sealer to create a watertight seal. Pitch pockets require regular maintenance and re-filling as sealants shrink and crack, making them a less reliable long-term waterproofing solution than properly fabricated flashing boots.
Related: Penetration, Flashing
- Polyiso
- Short for polyisocyanurate, the most widely used rigid insulation board in commercial flat roofing, valued for its high R-value per inch and compatibility with all membrane types. Polyiso boards are available in flat and tapered configurations to create slope for drainage.
Related: Insulation (polyiso), R-value, Tapered insulation
- Ponding
- Water that remains standing on a roof surface for more than 48 hours after the last rainfall, as defined by the NRCA. Chronic ponding accelerates membrane degradation, adds structural load, voids many manufacturer warranties, and is a leading indicator of drainage or structural problems.
Related: Drain, Scupper, Tapered insulation, Slope
- PVC
- Polyvinyl chloride, a single-ply thermoplastic membrane known for its exceptional chemical resistance, fire performance, and hot-air-welded seams that become stronger than the membrane itself. PVC is the preferred choice for roofs exposed to grease, chemicals, or animal fats, such as restaurants and manufacturing facilities.
Related: Single-ply, TPO, Membrane
R
- R-value
- A measure of thermal resistance — the higher the R-value, the greater the insulating effectiveness of the material. Building energy codes in Florida, Alabama, and Mississippi require minimum roof insulation R-values that affect both material selection and total roof system cost.
Related: Insulation (polyiso), Insulation (EPS), Insulation (XPS)
- Recover
- Installing a new roof membrane over an existing roof system without removing the old one, saving the cost and time of tear-off. Building codes generally allow only one recover layer, the existing roof must be dry and structurally sound, and a moisture survey is recommended before proceeding.
S
- Scrim
- A woven or non-woven reinforcement fabric embedded within a roofing membrane to add tensile strength, puncture resistance, and dimensional stability. The type and weight of scrim significantly affects a membrane's overall strength and its ability to resist tearing from foot traffic, hail, or wind-borne debris.
- Scupper
- An opening in a parapet wall or roof edge that allows water to drain from the roof surface through the wall to an exterior downspout or onto the ground. Scuppers should be sized generously and positioned at the lowest points of the roof; undersized or clogged scuppers are a common cause of ponding.
- SBS
- Styrene-butadiene-styrene, a rubber-based modifier used in modified bitumen membranes that gives the membrane excellent flexibility and elongation, especially in cold temperatures. SBS-modified membranes can be installed with torch, cold adhesive, or self-adhering methods.
Related: Modified bitumen, APP
- Single-ply
- A category of roofing membranes manufactured in single sheets (rather than multiple built-up layers) and installed in one layer over insulation. The three main single-ply types — TPO, PVC, and EPDM — account for the majority of new commercial flat roof installations in the United States.
- Slope
- The angle of the roof surface, expressed in inches of rise per 12 inches of horizontal run or as a percentage. The NRCA recommends a minimum slope of 1/4 inch per foot for flat roofs to promote drainage and prevent ponding water.
Related: Ponding, Tapered insulation
- SPF
- Spray polyurethane foam, a roof system where liquid polyurethane is sprayed directly onto the roof substrate, expanding into a seamless, monolithic insulation and waterproofing layer. SPF is protected by an elastomeric coating that must be recoated every 10-15 years to maintain waterproofing integrity.
- SPRI
- Single Ply Roofing Industry, a trade association representing manufacturers, suppliers, and contractors in the single-ply roofing sector. SPRI develops technical standards, wind design calculators, and installation guidelines that help ensure single-ply roof systems are properly designed and installed.
Related: Single-ply, NRCA
- Standing seam
- A metal roof system featuring raised interlocking seams that stand vertically above the flat panel surface, creating a watertight joint without exposed fasteners. Standing seam metal roofs offer exceptional longevity (40-60 years), wind resistance, and recyclability, making them a premium flat roof option for buildings that can accommodate minimum slope requirements.
Related: Membrane, Edge metal
- Substrate
- The surface directly beneath the roof membrane to which it is attached — typically a cover board, insulation, or the structural roof deck itself. Substrate condition is critical to membrane performance; rough, wet, or deteriorated substrates can cause premature membrane failure.
Related: Cover board, Membrane
T
- Tapered insulation
- Rigid insulation boards manufactured with a built-in slope (typically 1/8 to 1/2 inch per foot) to create positive drainage on an otherwise flat roof deck. Tapered insulation systems are designed with a layout plan that directs water toward drains, scuppers, or gutters.
Related: Slope, Ponding, Insulation (polyiso), Drain
- Tear-off
- The complete removal of all existing roofing materials down to the structural deck before installing a new roof system. A full tear-off allows inspection and repair of the deck, eliminates trapped moisture, and resets the recover count, but adds significant labor cost and disposal fees.
- TPO
- Thermoplastic polyolefin, the most widely installed single-ply commercial roofing membrane in the United States, known for its energy-efficient white surface, hot-air-welded seams, and competitive pricing. TPO membranes are available in 45, 60, and 80 mil thicknesses to suit different performance and warranty requirements.
Related: Single-ply, PVC, Membrane, Mil
U
- UL rating
- A fire and wind-uplift classification issued by Underwriters Laboratories for roof assemblies that have been tested and meet specific performance standards. UL ratings (such as Class A fire rating) are often required by building codes and are distinct from but complementary to FM Approvals.
Related: FM Approval, Wind uplift
- Underlayment
- A secondary water-resistant layer installed beneath the primary roofing membrane to provide an additional barrier against moisture infiltration. In flat roof systems, underlayment may refer to a base sheet, vapor retarder, or slip sheet depending on the membrane type and assembly design.
Related: Vapor retarder, Membrane, Substrate
V
- Vapor retarder
- A material installed on the warm side of the insulation to limit moisture vapor from the building interior migrating into the roof assembly, where it could condense and cause insulation degradation or deck corrosion. Vapor retarders are especially important in high-humidity environments like the Gulf Coast and in buildings with high interior moisture loads.
Related: Underlayment, Insulation (polyiso)
W
- Walkway pad
- Protective pads or rolls of reinforced material installed on the roof membrane surface to create designated foot-traffic paths, preventing puncture and abrasion damage from maintenance personnel. Walkway pads are recommended around HVAC units, hatches, and any area with regular foot traffic.
- Wind uplift
- The upward force exerted on a roof system by wind flowing over and around a building, which can tear membranes, detach edge metal, and lift insulation if the assembly is not properly designed and fastened. Wind-uplift resistance requirements are calculated based on building height, location, exposure, and roof zone (field, perimeter, corner).
Related: FM Approval, UL rating, Mechanically attached, Edge metal
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