Pass: The term used to describe the application of one layer of Spray Polyurethane Foam (SPF). The speed of a pass will determine foam thickness.
Pass Line: The distinct line formed between two passes of SPF. This line is the top skin of the bottom pass of the SPF.
Penetration: Any object that pierces the surface of 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.
Perlite: A natural volcanic glass having distinctive concentric cracks and a relatively high water content.. Perlite in a fluffy heat expanded form is used in lightweight insulating concrete, fire-resistant rigid insulation board (R = 2.78 per inch) and potting soil.
Perm: A unit of water vapor transmission, defined as one grain of water vapor per square foot per hour per inch of mercury (Hg) pressure difference (1 inch of mercury = 0.491 psi).
Permeability: The rate of flow of a liquid or gas through a porous material.
pH: A measure of the acidity or alkalinity of a solution, numerically equal to 7 for neutral solutions, increasing with increasing alkalinity and decreasing with increasing acidity. The pH scale commonly in use ranges from 0 to 14.
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.
Picture Framing: Rectangular patterns seen in a roof that are created by buckles or ridges in the roof system or sumps in the substrate.
PIMA: Polyisocyanurate Insulation Manufacturers Association
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 a Roof Jack. Click here for an example.
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. For an example on how to properly fill a pitch pocket, click here.
Pittsburgh Lock Seam: A method of interlocking two separate pieces of metal.
Plastic Cement: A term used to describe Type I asphalt roof cement. Plastic cement should not be used on vertical surfaces. See also Asphalt Roof Cement and Flashing Cement.
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.
Plenum: A space or enclosure in which air or other gas is at a pressure greater than that of the outside atmosphere.
Ply: A layer of felt or other reinforcement material in a roof system.
PMR: See Protected Membrane Roof.
Polyester: Any of numerous synthetic polymers produced 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.
Polymer Modified Bitumen: See Modified Bitumen.
Polymeric Methylene Diphenyl Diisocyanate (PMDI): The "A" component in foam roofing; when mixed with "B", it forms polyurethane. PMDI is an organic chemical compound having two reactive Isocyanate (-N=C=O) groups.
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 roof membrane materials.
Ponding: The accumulation of water at low-lying areas on a roof.
Pop Rivet: A small metal pin having a head on one end, inserted through aligned holes in pieces of light gauge metal to be joined and then the head is expanded to join the metal.
Popcorn Surface Texture: A coarse surface texture of SPF considered unacceptable for coatings.
Positive Drainage: The drainage condition of a roof where all water is gone from the roof surface within forty-eight hours of precipitation during normal drying conditions.
Pot Life: The period of time during which a material with multiple ingredients can be applied or administered after being mixed together.
Pourable Sealer: A type of sealant that is initially in liquid form commonly used in conjunction with pitch pans to form a water-tight barrier around penetrations that are difficult to flash.
Press Brake: A mechanical device used to form sheet metal into desired shapes and profiles.
Primer: A material that is applied to a surface in order to increase that surfaces ability to adhere to or work in conjunction with a subsequently applied material.
Proportioner: A pumping unit comprised of two (2) positive displacement pumps that is designed to dispense two (2) components at a precise ratio. Used in SPF and plural component coating applications.
Protected Membrane Roof (PMR): A roof assembly in which the insulation and ballast are placed on top of the membrane component. Commonly referred to as an "inverted roof assembly."
Psychrometer: An instrument that uses the difference in readings between two thermometers, one having a wet bulb and the other having a dry bulb, to measure the moisture content or relative humidity of air.
Puncture Resistance: The ability of a material to withstand being pierced by a sharp object.
Purlin: Horizontal secondary structural member used to transfer loads from the primary structural members.
PVC: Polyvinyl Chloride.