Roofing Terms that Start with the Letter “O”

Roof Online’s Glossary of Roofing Terms

By Roof Online Staff • Updated October 8, 2022

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O.C.: On Center

Off-Ratio Foam: Polyurethane foam which has been improperly mixed during application.

Oil Canning: Slight physical distortions of flat metal panels that appear as ripples in the metal and detract from a desirable flat and uniform appearance of the panels.

Olefin: A relatively stable, unsaturated chemical compound, such as ethylene or propylene, containing at least one carbon-to-carbon double bond. Widely used as a basic material in the petrochemical industry. See TPO.

On Center: Refers to measurements being made from the center of the members (studs, rafters, joists, etc.).

Open Time: The time after an adhesive has been applied to a substrate during which a proper bond can be made.

Open Valley: A roof valley construction technique in which the shingles on each side are cut back, so that the shingles do not cover the valley, and the valley flashing material is exposed. Also see “Woven Valley”.

Open Web Steel Joist:  A lightweight steel truss with a triangulated web and parallel chords, typically used to support floors and roof decks. Also called a “bar joist”.

Orange Peel Surface Texture: Typically used in reference to spray polyurethane foam. A surface texture similar to that of the outside of an orange, acceptable as a surface to which a protective coating may be applied.

Organic: 1. Formed from material that was once part of a living organism. 2. Containing hydrocarbons.

Organic Felt: A substrate, produced in sheets, made from cellulose fibers and used in the production of roll roofing, asphalt shingles, base sheets, and reinforcement plies in built-up roofs.

Organic Shingle: An asphalt shingle manufactured with organic felt.

Oriented Strand Board: Engineered wood panels produced by compressing layers of wood strands and adhesive, with the wood strands of each layer being at right angles to those of the next layer. It is commonly used as an alternative to plywood in structural sheathing applications. Also called OSB.

Original Construction: The parts of a building that date from the time the building was first constructed.

ORNL: Oak Ridge National Laboratory

OSB: Oriented Strand Board

OSHA: (U.S.) Occupational Safety and Health Administration

Overhang: A part of a building that protrudes beyond the exterior walls and protects the area below it. When the overhang is an extension of the roof, it is referred to as the “eaves”.

Overlay: The installation of new material over existing material. See Recover.

Overnight Tie-in: A temporary waterproofing application at the edge of a section of newly-installed roofing to prevent water from infiltrating the area while the installers are away. 

Overspray: The unintended loss or misapplication of spray-applied material during application.

Ozone:  (O3). A highly reactive gaseous molecule present in the atmosphere and having a corrosive effect on virtually all roofing material.

Ozone Resistance: The ability of a material to remain unreactive in the presence of ozone.