Adhesion:  the state in which two surfaces are held together by interfacial forces which                                   

                   may consist of modular forces or interlocking action or both.

 

Aggregate:  rock, stone, crushed stone, crushed slag, water worn gravel, or marble

                     chips used for surfacing and/or ballasting a roof system.

 

Aging:  the effect on materials that are exposed to an environment for an interval

              period of time.

 

Alligatoring:  the cracking of the surfacing bitumen on a built-up roof, producing a

                        pattern of cracks similar to an alligator’s hide; the cracks may or may not

                        extend through the surfacing bitumen.

 

Asbestos:  a group of natural, fibrous, impure silicate materials used to reinforce some

                   roofing products.

 

Asphalt, Air Blown:  produced by blowing air through molten asphalt held at an

                                    elevated temperature , raise the asphalt’s softening point and

                                     modified other properties.

 

Asphalt Emulsion:  a mixture of asphalt particles and an emulsifying agent such as

                                  bentonite clay and water.  These components are combined by using

                                  a chemical or a clay emulsifying agent and mixing or blending

                                  machinery.

 

Asphalt Felt:  an asphalt-saturated and/or an asphalt-coated felt.  (See Felt)

 

Asphalt Roof Cement:  a trowelable mixture of solvent-based bitumen, mineral

                                        stabilizers, other fibers and/or fillers.  Classified by ASTM

                                        Standard  2822-91 Asphalt Roof Cement, and D 4586-92

                                        Asphalt Roof Cement, Asbestos-Free, Types I and II.

 

ASTM:  American Society for Testing and Materials

 

Atactic Polypropylene:  a group of high molecular weight polymers formed by the

                                         polymerization of propylene.

 

Ballast:  an anchoring material, such as aggregate or precast concrete pavers, which

               employs the force of gravity to hold (or assist in holding) single-ply

               membranes in place.

 

 

Base Flashing:  (membrane base flashing): piles or stripes of roof membrane material

                           used to close-off and/or seal a roof at the roof-to-vertical intersections,

                           such as at a roof-to-wall juncture.  Membrane base flashing covers the

                           edge of the field membrane (Also see Flashing)

 

Base Ply:  the lowermost ply of roofing in a roof membrane or roof system.

 

Base Sheet:  an impregnated, saturated, or coated felt placed as the first ply in some

                      Mult-ply built-up and modified bitumen roof membranes.

 

Batten:  (1) cap or cover; (2) in a metal roof;: a metal closure set over or covering the

               joint between, adjacent metal panels; (3) wood: a strip of wood usually set in or

               over the structural deck, used to elevate and/or attach a primary roof covering

               such as a tile; (4) in a membrane roof system: a narrow plastic, wood or metal

               bar which is used to fasten or hold the roof membrane and/or base flashing

               in place.

 

Bitumen:  (1) a class of amorphous, black or dark colored, (solid, semi-solid or viscous)

                  cementitious substances, natural or manufactured, composed principally of

                  high molecular weight hydrocarbons, soluble in carbon disulfide, and found

                  in petroleum asphalts, coal-tars and pitches, wood tars and asphalts; (2) a

                  generic term used to denote any material composed principally of bitumen,

                  typically asphalt or coal tar.

 

Blister:  an enclosed pocket of air, which may be mixed with water or solvent vapor,

               trapped between impermeable layers of felt or membrane or between the

               membrane and substrate

 

Bridging:  (1) when the membrane is unsupported at a juncture;  (2) bridging in steep-

                  slope roofing is a method of re-roofing over standard-sized asphalt shingles

                  with metric-sized asphalt shingles.

 

Buckle:  an upward, elongated tenting displacement of a roof membrane frequently

               occurring over insulation or deck joints.  A buckle may be an indication of

               movement within the roof assembly.

 

Built-Up Roof Membrane (BUR):  a continuous, semi-flexible multi-ply roof

                                             membrane, consisting of piles or layers of saturated felts,

                                             coated felts, fabrics or mats between which alternate layers

                                             of bitumen are applied.  Generally, built-up roof membranes

                                             are surfaced with mineral aggregate and bitumen, a liquid-

                                             applied coating or granular-surfacing cap sheet.

 

 

 

 

 

 

Cant:  a beveling of foam at a right angle joint for strength and water run off.

 

Cant Strip:  a beveled or triangular-shaped strip of wood, wood fiber, perlite or

                     other material designed to serve as a gradual transitional plane  between

                     the horizontal surface of a roof deck or rigid insulation and a vertical

                     surface.

 

Cap Flashing:  usually composed of meta, used to cover or shield the upper edges of

                          the membrane base flashing, wall flashing or primary flashing.

                          (see Flashing and Coating)

 

Cap Sheet:  a granule-surface coated sheet used as the top ply of some built-up or

                     modified bitumen roof membranes and/or flashing.

 

Caulking:  (1) the physical process of sealing a joint or juncture; (2) sealing and

                   making weather-tight the joints, seams or voids between adjacent units by

                   filling with a sealant.

 

Chalking:  the degradation or migration of an ingredient, in paints, coatings or other

                   materials.

 

Coated Base Sheet:  a felt that has previously been saturated (impregnated) with

                                   asphalt and later coated with harder, more viscous asphalt,

                                   which greatly increases its impermeability to moisture.

 

Coated Felt (sheet):  (1) an asphalt-saturated felt that has also been coated on both

                                   sides with harder, more viscous “coating” asphalt; (2) a glass fiber

                                   that has simultaneously impregnated and coated with asphalt on

                                   both sides.

 

Cold Process Built-Up Roof:  a continuous, semi-flexible roof membrane, consisting

                                                  of a ply or plies of felts, mats or other reinforcement

                                                  fabrics that are laminated together with alternate layers

                                                  of liquid-applied(usually asphalt-solvent based) roof

                                                  cement or adhesives installed at ambient or a slightly

                                                  elevated temperature.

 

Condensation:  the conversion of water vapor or other gas to liquid state as the

                           Temperature drops or atmospheric pressure rises. (Also see Dew Point)

 

 

 

 

 

 

 

Coping:  the covering piece on top of a wall which is exposed to the weather, usually

                made of metal, masonry or stone.  It is preferably sloped to shed water back

                onto the roof.

 

Counterflashing:  formed metal sheeting secured on or into a wall, curb, pipe, roof-

                               top unit or other surface, to cover and protect the upper edge of the

                               membrane base flashing or underlying metal flashing and associated

                                fasteners from exposure to the weather.

 

Course:  (1) the term used for each row of shingles of roofing material that forms the

                roofing, waterproofing or flashing system: (2) one layer of a series of  material

                applied to a surface (e.g. a five-course wall flashing is composed of three

                applications of roof cement with one ply of felt or fabric sandwiched between

                each layer of roof cement)

 

Coverage:  the surface area covered by a specific quantity of a particular material.

 

Cricket:  an elevated roof substrate or structure, constructed to divert water around a

                chimney, curb, away from a wall, expansion joint or other projection/pene-

                tration. (See Saddle)

 

Curb:  (1) a raised member used to support roof penetrations, such as skylights,

             mechanical equipment, hatches, ect., above the level of the roof surface; (2)

             a raised roof perimeter relatively low in height.

 

Deck:  a structural component of the roof of a building.  The deck must be capable of

             safely supporting the design dead and live loads, including the weight of the

             roof systems, and the additional live loads required by the governing building

             codes.  Decks are either non-combustible(e.g., corrugated metal, concrete or

             gypsum) or combustible(e.g., wood plank or plywood), and provide the substance

             to which the roofing or waterproofing system is applied.

 

Deflection (Bowing, Sagging):  the download displacement of a structural member

                                                     or system under load.

 

Degradation:  a deleterious change in the chemical structure, physical properties or

                        appearance of a material due to natural or artifical exposure(e.g.,

                        exposure to radiation, moisture, heat, freezing, wind, ozone, oxygen, ect.)

 

Delamination:  a separation of the laminated layers of a component or system.

 

Drain:  an outlet or other device used to collect and direct the flow of runoff water

             from a roof area. (See NRCA Construction Detail “W-1”.)

             

Drip Edge:   a metal flashing or other overhanging component, with an outward pro-

                      jecting lower edge, intended to control the direction of dripping water and

                      help protect underlying building components.  A drip edge also can be used

                      to break the continuity of contact between the roof perimeter and wall com-

                      ponents to help prevent capillary action. 

 

Eave:   a projecting edge of a roof that extends beyond the supporting wall.

 

Efflorescence:  the formulation of crystalline deposits, generally whitish in color, on the

                          surface of stone, brick, concrete or other masonry surface when moisture

                          moves through and evaporates on the masonry.  May also be caused by

                          free alkalies leached from mortar, grout or adjacent concrete.

 

Elastomeric:  the elastic, rubber-like properties of a material that will stretch when

                        pulled and will return relatively quickly to its original shape when

                        released.

 

End Lap:  the distance of overlap where one ply, panel or piece extends beyond the

                  end of the immediately adjacent underlying ply, panel or piece.

 

EPDM:  Ethylene Propylene Diene Monomer (See also Ethylene Propylene Diene

                Terpolymer)

 

Exhaust Ventilation:  air that is typically vented or exhausted from the roof cavity,

                                     typically through vents installed on the upslope portion of the

                                     roof.  For example, with most steep-slope roof assemblies

                                     exhaust vents are typically located at or near the ridge.

 

Expansion Joint:  a structural separation between two building elements that allows

                              free movement between the elements without damage to the roofing

                              waterproofing system.

 

Fabric:  a woven cloth or material of organic or inorganic filaments, threads or yarns

               used for reinforcement in certain membranes and flashings

 

Fascia:   a vertical of steeply sloped roof or tim located at the perimeter of a building.

               Typically it is a border for the low-slope roof system the waterproofs the

                interior portions of the building.

 

Fasteners:  any of a wide variety of mechanical securement devices and assemblies,

                    including nails, screws, cleats, clips, and bolts, which may be used to

                    to secure various components of a roof assembly.

 

 

 

 

 

 

Felt:  a flexible sheet manufactured by the interlocking of fibers through a combina-

          tion of mechanical work, moisture and heat.  Roofing felts may be manufactured

          principally from wood pulp and vegetable fibers ( organic felts), asbestos fibers

          (asbestos felt), glass fibers(fiberglass or plysheet) or polyester fibers.

 

Field of the Roof:  the central or main portion of a roof, excluding the perimeter and

                                flashing.

 

Field Seam:  a splice or seam made in the field (not factory) where overlapping sheets

                      are joined together using an adhesive, splicing tape or heat- or

                      solvent-welding.

 

Fishmouth:  (also referred to as an edge wrinkle): (1) a half-cylindrical or half-conical

                      shaped opening or void in a lapped edge or seam, usually caused by

                      wrinkling or shifting of plysheets during installation; (2) in shingles, a half-

                      conical opening formed at a edge.

 

Flange:  the projecting edge of a ridge or semi-rigid component, such as a metal edge

              flashing flange, skylight flange, flashing boot, structural member, ect.

 

Flashing:  components used to weatherproof or seal the roof system edges at perime-

                  ters, penetrations, walls, expansion joints, valley drains, and other places

                  where the roof covering is interrupted or terminated.  For example, membrane

                  base flashing covers the edge of the field membrane, and cap flashings or

                  counterflashings shield the upper edges of the base flashing.

 

Gable:  a triangular portion of the endwall of a building directly under the slope roof

             and above the eave line.

 

Gable-shaped Roof:  a single-ridge roof that terminates at gable end(s).

 

Galvanized Steel:  steel coated with zinc for corrosion resistence.

 

Gauge:  a measurement of metal thickness.

 

Glass Felt:  a sheet composed of a bonded glass fibers, suitable for impregnation and

                    coating in the  manufacture of bituminous roofing and waterproofing

                    materials and shingles.

 

Granule:  (also referred to as a mineral or ceramic granule): opaque, natural, or

                  synthetically  colored aggregate commonly used to surface cap sheets,

                  shingles, and other granule-surfaced roof coverings.

 

 

Heat Welding:  (see heat seaming):  method of melting and fusing together the

                           overlapping edges of separate sections of thermoplastic or

                           elastomeric roofing membranes by the application of heat(in the

                           form of hot-air or open flame) and pressure.

 

Hip:  the inclined external angle formed by the intersection of two sloping roof planes.

 

Humidity:  the amount of moisture contained in the atmosphere.  Generally expressed

                   as percent relative humidity (the ratio of the amount of moisture

                   [water vapor] actually present in the air, compared to the maximum

                   amount that the air could contain at the same temperature.)

 

HVAC:  heating, ventilating, and air conditioning equipment.

 

Inorganic:  any chemical or compound that is derived from minerals, does not

                    contain carbon, and is not classified as organic; being or composed

                    of materials other than hydrocarbons and their derivatives; not of plant

                    or animal origin.

 

Insulation:  any of a variety of materials designed to reduce the flow of heat, either

                     from or into a building. (See also Thermal Insulation.)

 

Intake Ventilation:  the fresh air that is drawn into a passive ventilation system through

                                 Vents typically installed in the soffit or eave of a roof.

 

Lap:  that a part of a roofing, waterproofing or flashing component that overlaps or

         covers any portion of the same or another type of adjacent component.

 

Lap Seam:  occurs where overlapping materials are seamed, sealed or otherwise bonded.

 

Lead:  a soft workable metal used for miscellaneous flashings.

 

Life Cycling Costing:  a method of economic analysis that takes into account

                                      expected costs over the useful life of an asset.

 

Mansard:  a decorative steep sloped roof on the perimeter of the building.

 

Mansard Roof:  a steeper roof that terminates into a flat roof at its high point.

 

Mat:  a thin layer of woven, non-woven or knitted fiber that serves as reinforcement

           to the material or membrane.

 

 

 

 

 

Material Safety Data Sheets:  a written description of the chemicals within a product,

                                                   and pertinent other data including such things as safe

                                                   handling and emergency procedures.  In accordance with

                                                   OSHA regulations, it is the manufacturer’s responsibility

                                                    to produce an MSDS and the employer’s responsibility

                                                    to communicate its contents to employees.

 

Mechanically-Fastened Membranes:  generally used to describe membranes that have

                                                                been attached at defined intervals to the substrate.

                                                                Mechanical fastening maybe performed with

                                                                various fasteners and/or mechanical devices, such

                                                                as plates or battens.

 

MIL:  a unit of measure, one mil is equal to 0.001 inches or 25.400 microns, often used,

            to indicate the thickness of a roofing membrane.

 

Mineral-Surfaced Roofing:  roofing materials whose surface or top layer consists of

                                                mineral granules.

 

Mineral-Surfaced Sheet:  a roofing sheet that is coating on one or both sides with

                                            asphalt and surfaced with mineral granules.

 

Modified Bitumen:  (1) a bitumen modified through the inclusion of one or more

                                   polymers (e.g. atactic polypropylene, styrene butadiene, ect.)

                                  (2) composite sheets consisting of a polymer modified bitumen

                                  often reinforced and sometimes surfaced with various types of mats,

                                   films, foils and mineral granules.

 

Moisture Scan:  the use of a mechanical device (capitance, infrared or nuclear) to detect

                            the presence of moisture within a roof assembly. (See Non-Destructive

                            Testing.)

 

Mopping:  the application of hot bitumen, with a roofer’s hand mop or mechanical

                   applicator, to the substrate or to the felts of a bituminous membrane.

 

           Types of Mopping:

 

(A) Solid:  a continuous mopping of a surface.

(B) Spot:  a mopping pattern in which hot bitumen is applied in roughly circular areas,

           leaving a grid of unmapped, perpendicular bands on the roof.

(C) Sprinkle:  a random mopping pattern in which heated bitumen beads are strewn onto the

                 substrate with a brush or mop.

(D) Strip:  a mopping pattern in which hot bitumen is applied in parallel bands.

 

 

Mud Cracking:  surface cracking of a material whereby the degraded material appears 

                            similar to dried, cracked earthen mud.

 

Net Free Vent Area:  the area, measured in square inches, open to “unrestricted” air

                                     flow and commonly used as a “yardstick” to measure relative

                                     vent performance.

 

NRCA:  National Roofing Contractors Association

 

Organic:  being or composed of hydrocarbons or their derivatives originating from plant

                 or animal matter.

 

Penetration:  (1) any object passing through the roof; (2) the consistency(hardness) of a

                        bituminous material expressed as the distance, in tenths of a millimeter

                        (0.1mm), that a standard needle penetrates vertically into a sample of

                        material under specified conditions of loading, time, and temperature.

 

Permeance:  the rate of water vapor transmission per unit area at a steady state through

                      a material, membrane or assembly, expressed in Grain/Square Foot Hour

                      Inch Mercury (grain/ft_h in Hg [ng/Pa a m_]).

 

Phased Application:  the installation of separation roof system or waterproofing system

                                    component(s) during two or more separate time intervals.

                                    Application of surfacings at different time intervals are typically

                                    not considered phased application. (See Surfacing.)

 

Picture Framing:  a square or rectangular pattern of buckles or ridges in a roof covering

                               generally coinciding with insulation or deck joints; generally, a

                               function of movement of the substrate.

 

Plastic Cement:  a roofing industry generic term used to describe Type I asphalt roof

                            cement that is a trowelable mixture of solvent-based bitumen, mineral

                            stabilizers, other fibers and/or fillers.  Generally, intended for use on

                            relatively low slopes-not vertical surfaces. (Also see Asphalt Roof

                            Cement and Flashing Cement.)

 

Ply:  a layer of felt, plysheet or reinforcement in a roof membrane or roof system.

 

Polyester:  a thermoplastic polymeric resin that is used to make a variety of materials

                   and products.  Polyester fibers are widely used as the reinforcing medium

                   in certain membranes.

 

 

 

 

 

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 which 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 splice. 

 

PVC:  Polyvinyl Chloride

 

Rake:  the slope edge of a roof at or adjacent to the first or last rafter.

 

RCI:  Roof Consultants Institute

 

Re-Cover:  the addition of a new roof membrane or steep slope roof covering over a

                    major portion of an existing roof assembly.  This process does not involve

                    removal of the existing roofing.

 

Reroofing:  the process of re-covering or tearing-off and replacing an existing roof

                    system.

 

Reglet:  a sheet metal receiver for the attachment of counterflashing.  (A reglet

              may be insert into a raggle, embedded behind cladding or be surface mounted.)

 

Reflective Humidity:  the ratio of the weight of moisture in a given volume of air-

                                     vapor mixture to the saturated (maximum) weight of water vapor

                                     at the same temperature, expressed as a percentage.  For example,

                                     if the weight of the moist air is 1 pound and if the air could hold 2

                                     pounds of water vapor at the temperature, relative humidity(RH)

                                     is 50 percent.

 

Replacement:  the practice of removing an existing roof system down to the roof deck

                         and replacing it with a new roofing system.

 

Ridge:  highest point on the roof, represented by a horizontal line where two roof areas

             Intersect, running the length of the area.

 

Ridge Vent:  one of many products located at the ridge that allows the escape of warm

                      and/or moist air from the attic area or rafter cavity.  Most ridge vents are

                      either premanufactured metal or flexible shingle over.

 

Roof Curb:  raised frame used to mount mechanical units(such as air conditioning or

                     exhaust fans), skylights, ect.

 

Roof Jack:  a metal bracket used to support toe-boards on steep-slope roofs.

 

 

 

Roof Overhang:  a roof extension beyond the exterior wall of a building.

 

Roof Slope:  the angle a roof surface makes with the horizontal, expressed as a

                      ration of the units of vertical rise to the units of horizontal length

                     (sometimes referred to as run).  For English units of measurements,

                      when dimensions are in inches,slope may be expressed as a ratio of

                      rise of run, such as 4:12 or as a percent.)

 

Roof System:  a system of interacting roof components, generally consisting of

                         membrane or primary roof covering and insulation (not including

                         the roof deck) designed to weatherproof and, sometimes to improve

                         the building’s thermal resistance.

 

Saddle:  a relatively small raised substrate or structure constructed to channel or

               direct surface water to drains or off the roof.  A saddle may be located between

               drains or in a valley, and is often constructed like a small hip roof or like

               a pyramid with a diamond shaped based. (See Cricket)

 

SBS:  (See Styrene Butadience Styrene.)

 

Sealant:  a single – or multi-component polymeric or bituminous-based material used

                to weatherproof many types of construction joints where moderate movement

                is expected.  The material comes in various grades: pourable, self-leveling,

                non-sag, gungrade, and cured or uncured tapes.

 

Seam:  a joint formed by mating two separate sections of materials.  Seams may be made

             or sealed in a variety of ways, including adhesive bonding, hot-air welding,

             solvent welding, using adhesive tape, sealant, ect.

 

Selvage Edge:  an edge designed for certain sheet good materials, e.g., mineral-sur-

                          faced sheets.  With mineral surfaced sheets, the surfacing is omitted over

                          a portion of longitudinal edge of the sheet (e.g., mineral surface cap

                          sheet) in order to obtain better adhesion of the overlapping sheet.

 

Side Lap:  the continuous longitudinal overlap of neighboring like materials.

 

Single-Ply Membranes:  roofing membranes that are field applied using just one layer

                                         of membrane material (either homogeneous or composite)

                                         rather than multiple layers.

 

Single-Ply Roofing:  a roofing system in which the principal roof covering is a single

                                   layer flexible membrane, often of thermoset, thermoplastic or

                                   polymer modified bituminous.

 

 

 

Skylight:  a roof accessory, set over an opening in the roof, designed to admit light.

                 Normally transparent, and mounted on a raised framed curb.

 

Slip Sheet:  sheet material, such a reinforced kraft paper, rosin-sized paper, polyester

                    scrim or polyethylene sheeting, placed between two components of a roof

                    assembly (such as between membrane and insulation or deck) to ensure that

                    no adhesion occurs between them, and to prevent possible damage from

                    chemical incompatibility, wearing or abrasion to the membrane.

 

Smooth Surfaced Roof:   a roof membrane without mineral granule or aggregate

                                           surfacing.

 

Soil Stack:  a sanitation pipe that penetrates the roof; used to vent plumbing fixtures.

 

Solvent:  liquid used to dissolve or disperse film-forming constituents, and which

                evaporates during drying and does not become a part of the dried film.

 

Split:  a rupture (generally linear) or tear in a material or membrane resulting from

           tensile forces.

 

Square:  100 square feet (9.29 m ) of roof area.

 

Standing Seam:  a metal roof system that consists of an overlapping or interlocking

                             seam that occurs at an upturned rib.  The standing seam may be made

                             by turning up the edges of two adjacent metal panels and overlapping

                             them, then folding or interlocking them in a variety of ways.

 

Substrate:  the surface upon which the roofing or waterproofing membrane is applied

                   (e.g., in roofing, structural deck or insulation).

 

Sump:  an intentional depression around a roof drain or scupper that serves to promote

             drainage. 

 

Surfacing:   the top layers of a roof covering, specified or designed to protect the under-

                     lying, roofing from direct exposure to the weather.

 

Test Cut:  a sample of the roof, which may contain all components or just the membrane,

                  usually used to diagnose the condition of the existing membrane, evaluate the

                  type and number of piles or number of membranes or rates of application such

                  as determine the weight of the average interply bitumen moppings.

 

Thermal Shock:  the stress-producing phenomenon resulting from sudden temperature

                             changes in a roof membrane when, for example, a cold rain shower

                             follows brilliant hot sunshine, which may result in sudden cooling or

                             rapid contraction of the membrane.

 

Thermoplastic:  materials that soften when heated and harden when cooled.  This

                            process can be repeated provided that the material is not heated above

                            the point at which decomposition occurs.

 

Thermoset:  a material that solidifies or “sets” irreversibly when heated.  This property

                      is usually associated with crosslinking of the molecules induced by heat or

                      radiation.

 

UL:  Underwriters Laboratories, Inc.

 

UN Label:  an identification label or seal affixed to a roofing product or package with

                    the authorization of Underwriters Laboratories, Inc.  The presence of the

                    label indicates that the product has met certain performance criteria.

 

Underlayment:  an asphalt-saturated felt or other sheet material (may be self-adhering)

                            installed between the roof deck and the roof system, usually used in a

                            steep-slope roof construction.  Underlayment is primarily used to

                            separate the roof covering from the roof deck, to shed water, and to

                            provide secondary weather protection for the roof area of the building.

 

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

 

Vent:  an opening designed to convey air, heat, water vapor or other gas from inside

           a building or building component to the atmosphere.

 

Void:  an open space or break in consistency.

 

Waterproof:  the quality of a membrane, membrane material or other component to

                       prevent water entry.

 

Wind Uplift:  the force caused by the deflection of wind at roof edges, roof peaks or

                        obstructions, causing a drop in air pressure immediately above the roof

                        surface.  This force is then transmitted to the roof surface.  Uplift may

                        occur because of the introduction of air pressure underneath the membrane

                        and roof edges, where it can cause the membrane to balloon and pull away

                        from the deck.