Term Description
Two-component system

In spray polyurethane foam roofing, a coating of polyurethane foam formed by the mixing and reaction of two different materials.


See ‚"thermal transmittance (U or U-factor).‚"


See "Underwriters Laboratories Inc. (UL)."

UL label

An identification label or seal affixed to a roofing product or package with the authorization of Underwriters Laboratories Inc. (UL) The presence of the label indicates the product has met certain performance criteria.

Ultraviolet (UV)

Invisible electromagnetic radiation adjacent to the violet end of the visible spectrum with wavelengths from about 400 to 100 nm (nanometers [1 nm = 1√ó10-9 m]).


An asphalt-saturated felt or other sheet material (may be self-adhering) installed between a roof deck and roof covering, usually used in a steep-slope roof construction. Underlayment is primarily used to separate a roof covering from the roof deck, shed water and provide secondary weather protection for the roof area of the building.

Underwriters Laboratories Inc. (UL)

An independent product safety certification organization that tests products and writes standards for safety worldwide. UL tests, rates and classifies roof assemblies for their resistances to fire, impact, leakage, corrosion of metal components and wind uplift.

Unforeseen conditions

(1) Unusual situation not reasonably anticipated based on contract documents; (2) unknown physical condition of an unusual nature that differs materially from those ordinarily encountered.


See "wind uplift."


See "polyurethane."

UV degradation

Damage done by the ultraviolet rays (UV) of the sun.


The internal angle formed by the intersection of two sloping roof planes.

Vapor barrier

See "vapor retarder."

Vapor diffusion

The movement of water vapor through materials as a result of vapor pressure differences.

Vapor migration

The movement of water vapor from a region of high vapor pressure to a region of lower vapor pressure.

Vapor pressure

The pressure exerted by a vapor of a solid or liquid when in equilibrium with the liquid or solid.

Vapor retarder

A material or system that significantly impedes the transmission of water vapor under specified conditions.

Vapor retarder class

A measure of a material or assembly’s ability to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method of ASTM E96 as follows:

Class I: 0.1 perm or less.

Class II: 0.1 < perm ≤ 1.0 perm.

Class III: 1.0 < perm ≤ 10 perm.

Vegetative roof assembly

Substrate and a vegetative roof system.

Vegetative roof system

A roof area of plantings/ landscaping installed above a waterproofed substrate at any building level that is over habitable space. A vegetative roof system consists of a waterproofing system and its associated components—such as protection course, root barrier, drainage layer, thermal insulation and aeration layer—and an overburden of growth medium and plantings.

Vegetative roof system contractor

A professional waterproofing or roof system contractor familiar with the installation of vegetative roof systems.

Vegetative roof waterproofing membrane

A continuous layer used as a treatment of a surface or structure to prevent the passage of water under hydrostatic conditions; used specifically for a vegetative roof system. (See ‚"waterproofing membrane.‚")


(1) A single wythe of masonry for facing purposes that may not be structurally connected; (2) any of the thin layers of wood glued together to form plywood.


An opening designed to convey air, heat, water vapor or other gas from inside a building or a building component to the atmosphere.

Verge-of-popcorn texture

In spray polyurethane foam (SPF) roofing, the verge-of-popcorn surface texture is the roughest texture suitable for receiving the protective coating on an SPF roof. The surface shows a texture where nodules are larger than valleys with the valleys relatively cured. This surface is acceptable for receiving a protective coating only because of the relatively cured valleys. However, the surface is considered undesirable because of the additional amount of coating material required to protect the surface properly.