Roof Dictionary

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Pan flashing: a sheet metal flashing that covers an equipment platform and is designed to counter flash the base flashing surrounding the platform.

Parapet wall: the part of a perimeter wall that extends above the roof.

Partially attached: a roofing assembly in which the membrane has been “spot affixed” to a substrate, usually with an adhesive or a mechanical device.

Pass: a layer of material, usually applied by the spray method, that is allowed to reach cure before another layer (“pass”) is applied.

Penetration: any construction (e.g. pipes, conduits, HVAC supports) passing through the roof.

Percent elongation: the maximum amount that a material can be lengthened or stretched before breaking; expressed as a percentage of the original length of material tested.

Perm: see permeance.

Permeability: (1) the capacity of a porous material to conduct or transmit fluids; (2) the time rate of vapor transmission through unit area of flat material of unit thickness induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions. The English (inch-pound) unit of measurement for permeability is gr/hr-ft2-(in Hg/in.), which is commonly referred to as “perm-inch” units.

Permeance: (1) the rate of water vapor transmission per unit area at a steady state through a material, membrane, or assembly; (2) the time rate of water vapor transmission through unit area of flat material or construction induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions. The English (inch-pound) unit of measure for permeance is gr/h-ft2-in. Hg, which commonly referred to as “perm” units.

Phasing: installing roof system components in separate time intervals. For instance, installing a base sheet, and then two plies of roofing one day, and coming back and installing the remaining two plies one or more days later. It is generally not considered Phasing if the surfacing is applied at a later date.

Phased application: the installation of a roofing or waterproofing system during two or more separate time intervals or different days. Application of surfacing at different time intervals are typically not considered phased application. See Surfacing. A roofing system not installed in a continuous operation.

Picture framing: rectangular patterns seen in a roof that are created by buckles or ridges in the roof system or sumps in the substrate.

Pinhole: a small hole in a coating, foil, membrane, or other roofing material.

Pipe boot: a prefabricated flashing piece used to flash around circular pipe penetrations. Also known as Roof Jack.

Pitch: term used to describe Roof Slope and also short for Coal Tar Pitch.

Pitch pocket (a.k.a pitch pan): a flanged piece of flashing material placed around irregularly shaped roof penetrations and filled with grout and a pourable sealer to seal around the penetration in order to seal it from against moisture entry. Pitch pockets are a good source of leaks and should be avoided if possible.

Plastic film: a flexible sheet made by the extrusion of thermoplastic resins.

Plasticizers: material incorporated into rubber and plastic in order to increase their flexibility and workability.

Ply: a layer of felt or other reinforcement material in a roof system.

Polyester: any of numerous synthetic polymers chiefly by reaction of dibasic acids with dihydric alcohols and used primarily as light, strong, weather-resistant resins. In roofing, polyester is used to reinforce fabrics.

Polymer: any of numerous natural and synthetic compounds of usually high molecular weight consisting of up to millions of repeated linked units, each a relatively light and simple molecule.

Polymerization: the combining of monomers to produce polymers.

Polypropylene: any of various thermoplastic resins that are polymers of propylene. They are hard and tough, and are used to make molded articles and fibers.

Polyvinyl chloride (PVC): a thermoplastic polymer that can be compounded into flexible and rigid forms through the use of plasticizers, stabilizers, fillers, and other modifiers; rigid forms are used in pipes; flexible forms are used in the manufacture of sheeting and room membrane materials.

Ponding: the excessive accumulation of water at low-lying areas on a roof that remains 48 hours after the end of rainfall under conditions conducive to drying.

Positive drainage: the drainage condition in which consideration has been made during design for all loading deflections of the deck and additional roof slope has been provided to ensure drainage of the roof area within 48 hours following rainfall during conditions conducive to drying.

Primer: (1) a thin, liquid-applied solvent-based bitumen that may be applied to a surface to improve the adhesion of subsequent applications of bitumen; (2) a material that is sometimes used in the process of seaming single-ply membranes to prepare the surfaces and increase the strength (in shear and peel) of the field space; (3) a thin liquid applied material that may be applied to the surface of SPVF to improve the adhesion of subsequent application of SPVF protective coatings.

Protective mat: a sacrificial material used to shield one roof system component from another.

Puncture resistance: the ability of a material to withstand the action of a penetrating or puncturing object.

Purlin: horizontal secondary structural member that transfers loads from the primary structural framing.

PVC: polyvinyl chloride.