- APP
- Atactic polypropylene. A thermoplastic polymer used to modify asphalt, increasing UV resistance and high-temperature flow resistance. Usually torch-applied.
- Ballast
- Loose stone or pavers laid over a membrane to hold it down instead of adhering or fastening it.
- Base flashing
- The upturned edge of the roof membrane where it meets a wall, curb or penetration. The single most common source of commercial roof leaks.
- Base sheet
- The first ply of a built-up or modified bitumen assembly. Anchors the system to the substrate and gives the plies above it something to bond to.
- Bitumen
- Asphalt or coal tar pitch. In a modern built-up roof, the bitumen is the waterproofing agent and the felt holds it in place.
- Built-up roof (BUR)
- A roof system literally built by adding layers: base sheet, ply felts and cap sheet bonded in bitumen, then surfaced.
- Cap sheet
- A coated felt with mineral granules embedded in the topside, forming the weathering surface of a built-up or modified bitumen roof.
- Coal tar pitch
- A bitumen distilled from coke oven tar. Naturally resists water and vapor transmission; its low melt point makes it self-healing. Must be gravel-surfaced.
- Cold process
- A tar or asphalt based semi-liquid adhesive applied by brush or roller rather than heated in a kettle.
- Coping
- The cap on top of a parapet wall. Usually formed sheet metal; a frequent leak path when its joints open.
- Cover board
- A rigid board installed over insulation to protect it and give the membrane a firm substrate.
- EPDM
- Ethylene propylene diene monomer. A rubber single-ply membrane, typically black. Roughly half the single-ply market.
- Glass-fiber mat
- The reinforcement that replaced organic felt nationwide. It does not absorb water, which reduces blistering and ridging.
- Interply mopping
- Bitumen applied between shingled plies of felt in a built-up roof.
- Low-slope roof
- A roof with a slope of 3:12 or less. Almost all commercial roofs. Not the same thing as flat — a properly built low-slope roof drains.
- Mechanically attached
- A membrane fastened to the deck with screws and plates rather than adhered.
- Modified bitumen (MBS)
- Asphalt modified with SBS or APP polymers, reinforced with fabric and delivered as a factory-made sheet.
- Organic felt
- Cellulose fiber, shredded wood and felted paper reinforcement. Absorbs water; largely superseded by glass-fiber.
- Ply
- One layer of felt in a built-up roof. Each ply is said to represent about five years of service life.
- Plasticizer migration
- The loss of plasticizers from a PVC membrane over time, which makes it brittle. Reason PVC roofs should be monitored.
- Ponding water
- Water remaining on a roof more than 48 hours after rainfall. Accelerates degradation and voids most warranties.
- PVC
- Polyvinyl chloride. A thermoplastic single-ply membrane with strong chemical and grease resistance.
- Purlin
- A horizontal structural member that supports roof decking or, in a structural metal roof, the panels themselves.
- Recover
- Installing a new roof system over an existing one rather than tearing it off. Code generally permits one recover.
- Saturated felt
- Felt impregnated with bitumen. Generally used as a base sheet.
- SBS
- Styrene-butadiene-styrene. A polymer that gives bitumen rubber-like flexibility and improved resistance to aging and weathering.
- Scupper
- An opening through a parapet that lets water off the roof. When it clogs, the roof becomes a pond.
- Single ply
- A factory-fabricated membrane installed in the field as one layer, with heat- or chemically-welded laps.
- Standing seam
- A structural metal roof panel with raised seams and concealed fasteners, keeping attachments out of the water plane.
- Tapered insulation
- Insulation cut to a slope so a dead-level deck drains toward the drains.
- Tear-off
- Complete removal of the existing roof system down to the deck.
- TPO
- Thermoplastic polyolefin. The fastest-growing single-ply product type; reflective, weldable, puncture resistant.
