# Commercial Roofing Glossary

32 terms, defined plainly by Lloyd Roofing Company, Inc..

**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.
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**Lloyd Roofing Company, Inc.** · 2415 Montague Avenue Ext., Greenwood, SC 29649 · 1-800-421-3830 · Mon–Thu 7:00am – 5:00pm · Fri 7:00am – 12:00pm
Commercial and industrial roofing only. Serving South Carolina, North Carolina and Georgia since 1965.
SC contractor license CLG10175 · USDOT 483661