Glossary of Roofing Terminology
The term definitions contained herein are written in layman terms. The information contained in each definition is believed to be accurate; however, some definitions may not include a technically precise definition and may not include all information necessary for a full precise definition, as that would defeat the purpose to provide layman terms.
Any rock, (typically pea gravel) used as a surfacing for a built-up roof system. Aggregate is usually set in a hot mopping of asphalt. Aggregate acts as a protective surfacing and ballast.
Cracking of the liquid applied surfacing layer of a built-up roof system. Alligatoring produces cracks similar in appearance to the skin of an alligator. The cracks are often limited to the surface layer, but may extend through the entire built-up roof system.
Raised curbs, which divides areas of the roof. The curbs are built several inches apart to allow for movement of the roof areas without causing damage to the roof system. Typically the curbs are made of wood, are a minimum of eight inches above the surface of the roof, and are flashed per the material manufacturer’s specifications. The gap between the two curbs is covered with a flexible material (consult the material manufacturer for their recommended products and details.)
ASPHALT (SOMETIMES REFERRED TO AS ‘HOT’ OR ‘HOT TAR’)
Asphalt is from the naturally occurring substance crude oil or petroleum. Asphalt is the residue left after the evaporation of other components contained within the raw crude. There are four types (or grades) of asphalt that may be used in roofing applications:
- TYPE I – For roofs that are flat (i.e.: have little or no slope.) Type I asphalt conforms to ASTM specification section D 132.
- TYPE II – For roofs with a slope less than 1/8” per foot, but not dead level flat. Type II asphalt conforms to ASTM specification section D 132.
- TYPE III – For roofs that have a slope of between 1/8” per foot and 2” per foot. Type III asphalt conforms to ASTM specification section D 132.
- TYPE IV – For low slope roofs with a slope greater that 2” per foot. Type III asphalt conforms to ASTM specification section D 132.
Mixture of asphalt and an emulsifying agent. Typically Bentonite clay and water are used as emulsifying agents.
ASPHALT SATURATED FELT
Asphalt saturated felt is roofing felts that have been impregnated with asphalt.
Typically rounded washed river rock or concrete paving stones. The purpose of ballast is to anchor the roof system to the structural roof deck by the weight of the ballast material. Typically, ballast is only used with a single ply roof system application.
BALLASTED ROOF SYSTEM
Typically a single ply roof system, which is loose laid (the field insulation is not attached to the roof substrate, the single ply membrane is attached only at the curbs and walls but not attached to the roof substrate.) The roof system is held in place by the weight of the ballast.
- In an asphalt built-up roof system, base flashing consists of multiple ply sheets set in an adhesive (such as flashing cement.) The base flashing membrane covers the edge of the field membrane.
- In a single ply roof system, base flashing is accomplished by applying a separate sheet of the single ply membrane to the wall or curb. Depending on the height of the wall or curb and depending on the manufacturer’s requirements, the membrane may be adhered to the wall substrate. The base flashing membrane covers the edge of the field membrane.
A heavy ply sheet that is saturated with asphalt. Base sheets are typically mechanically fastened or adhered to the roof substrate as a base layer for an asphalt built-up roof system.
A pocket of air trapped within the roof system. Blisters are often caused by moisture trapped in the roof insulation or between layers of the roof system.
BUCKLE (ALSO KNOWN AS A RIDGE)
Upward movement of the roof system, roof insulation or roof substrate, which causes a portion of the roof to rise above the adjacent roof surface.
Rubber-like material, which is produced by mixing copolymerizing isobutylene with isoprene. Butyl may be manufactured in sheets (such as Butyl tape) or it may be blended with other elastomers to create sealants and adhesives.
A tapered material used at the base of walls and curbs to strengthen the backing of base flashing and reduce the degree from the horizontal field of the roof to a vertical wall or curb. The cant also ensures that the roof slopes away from the wall/curb to prevent ponding water at the base of the wall/curb.
A surfacing sheet used with asphalt built-up roof systems. The cap sheet is a heavy duty sheet that is covered with granules. The cap sheet protects the roof from physical damage and from ultraviolet degradation.
A material (or as a verb it is the physical process of applying the caulking), which is used to seal joint where flexibility is required. Caulking has elastic qualities that allow it to move with the bonded materials.
COLD PROCESS BUILT-UP ROOFING
A semi-flexible roof system of multiply SBS (Styrene Butadiene Styrene) ply sheets (typically a base sheet, one or more SBS ply sheets and a mineral surfaced cap sheet) set in liquid applied roof cement, adhesive or modified asphalt. Cold process roof systems are more flexible than a standard hot asphalt built-up roof system.
A steep sloped shingled roof system. Shingles are impregnated with asphalt and have a mineral surfacing. Composition shingles are available in a variety of colors.
An area of the roof that is raised above the adjacent roof area by tapered insulation. Crickets are used to divert the flow of water to create a positive slope to drain so the ponding water is reduced. Crickets are typically installed on the upward side of skylight curbs, mechanical unit curbs and at walls. CSI (CONSTRUCTION SPECIFICATIONS INSTITUTE)The Construction Specifications Institute is an institute that provides education and certifications for construction designers, specifiers, estimators, project managers and contract administrators. Contractors and construction professionals should be certified by the Construction Specifications Institute.
CURBS AND WALLS
A curb or a wall is any vertical projection from the field of the roof. Curbs are typically low in height and are used to raise skylights and mechanical units above the surface of the roof. Walls may be roof perimeter parapet walls, rooftop penthouse walls or similar tall vertical projection from the field of the roof.
DEFLECTION OF THE ROOF STRUCTURE
Deflection is caused by settlement of the roof structural members. Deflection of the roof structure often results in low areas of the roof where ponding water may occur.
Deterioration of the material caused by a breakdown of the physical properties of the material. Degradation is damage to the roof system and typically reduces the materials’ ability to withstand normal wearing. Degradation is typically caused by atmospheric conditions (such as ultraviolet radiation, excessive heat or cold) or by continuous physical wear (such as foot traffic at roof access locations) or by vegetation growth.
Separation of materials (such as the failure of adhesives to bond materials together or the failure of adhesive to hold granule to granule cap sheet.)
Temperature at which cooling water vapor condenses from a vapor to liquid. Condensation is often found on the warmer surface where a material (such as glass or metal), has warm air (heavy with moisture) on one side and cool air on the other side.
Typically made of PVC or sheet metal, it is a conduit, which drains water from the rooftop to a lower roof or to the ground.
Exit point for water to leave the roof. Each primary drain must have an overflow drain or scupper (to provide a way for water to exit the roof if the primary drain becomes blocked.
A coating (usually applied over an existing roof) of a liquid rubber-like material used to extend the lifespan of an older roof. The coating is only affective if applied to a roof that is in fair or better condition. The characteristics of the elastomer allows it to stretch to twice its original width/length and then return to its original size.
A coating (usually applied over an existing roof) of liquid modified asphalt or other adhesive used to extend the lifespan of an older roof. The coating is only effective if applied to a roof that is in fair or better condition. To add dimensional stability and strength to the coating, fibers may be mixed into the liquid material or the coating may be applied in multiple layers with a mesh laid between coatings. The coating is surfaced with a liquid applied reflective surfacing or by broadcasting granules onto the wet surface of the coating.
ETHYLENE PROPYLENE DIENE MONOMER (EPDM)
A single ply membrane made of Ethylene Propylene Diene Monomer.
Sometimes used as a surfacing for asphalt built-up roof systems. A glaze coating, which is left exposed, will fail prematurely due to direct exposure to ultraviolet radiation and severe weather conditions.
GRANULES (ALSO REFERRED TO AS MINERALS)
Colored aggregate, which is used as a surfacing to built-up roofing cap sheets and shingles. Granules may also be used as a surfacing for emulsion coatings by broadcasting the granules onto wet emulsion coating.
Gravel has been used as a surfacing for built-up roofs. Typically, the gravel is broadcast onto a glaze coating of hot asphalt.
GRAVEL STOP EDGE
A roof perimeter edge, which rises up slightly from the surface of the roof. The gravel stop perimeter edge keeps gravel from falling of the edge of the roof and keeps water from dripping off the edge of the roof. The gravel stop edge is flashed similar to base flashing (see base flashing.) The roof perimeter edge is capped with sheet metal.
Heat welding is a method of welding together seams of uncured thermoset single ply membrane. Heat welding does not melt the sheets, but forms a chemical bond to create a monolithic sheet.
Primary structural roof member made of wood or metal, which supports the roof system substrate. Joists support the purlins and other secondary structural members.
MECHANICALLY FASTENED ROOF SYSTEM
Typically used to describe single ply roof systems that are anchored to the roof substrate by screws.
Bitumen modified by adding polymers such as:
- SBS – Styrene Butadiene Styrene. Ply sheets are impregnated with SBS to create a flexible ply sheet that is very durable. SBS ply sheets are generally set in liquid applied modified cold asphalt.
- APP – Atactic Polypropylene. Ply sheets are impregnated with APP and have a heavy application of APP on the underside of the sheet to create a flexible ply sheet that is very durable. APP sheets are applied by using an open flame to melt the modified asphalt on the underside of the sheet.
Perform a detailed scan (using a capacitance meter, infrared survey or nuclear testing) to detect the presence and location of trapped moisture within a roof system.
Roof perimeter wall, which extends above the surface of the adjacent roof.
Perlitic insulation is formed by heating and expanding siliceous volcanic glass. Perlite comes in flat and tapered board stock.
- In an asphalt built-up roof system, a pipe boot is a lead sheet formed and welded to fit the shape of a roof penetration.
- In a single ply roof system, a pipe boot is a covering, which is pre-manufactured from the single ply material and formed to fit the shape of the roof penetration.
PITCH POCKET (ALSO KNOWN AS A PITCH PAN)
Created to provide waterproofing around irregular roof penetrations. Pitch pockets are created by forming a sheet metal box around the projections and extending the bottom flange of the sheet metal under the adjacent roof system. The sheet metal box is filled with grout (to prevent the pourable sealer from leaking into the building) and pourable sealer.
POLYVINYL CHLORIDE (PVC)
Synthetic thermoplastic polymer using vinyl chloride. PVC single ply roof systems are typically applied by fully adhering the system to the roof substrate, mechanically fastening the roof to the substrate or anchoring the roof to the substrate by ballast. Heat welding of PVC seams creates a monolithic single ply sheet.
The national Roofing Contractors Association defines ponding water as water that remains on the roof 24-hours following the end of a rainfall. Ponding occurs at areas of poor roof slope and at deflection of the structural roof deck.
PROTECTED ROOF MEMBRANE ASSEMBLY
A roof system where the waterproof layer of the roof is installed as the bottom layer of the roof, Styrofoam Brand Insulation (or similar closed cell foam insulation) is loose laid over the roof, then ballast if laid over the insulation.
Secondary structural roof member made of wood or metal, which supports the roof system substrate. Purlins are supported by roof joists or similar primary structural roof members.
RCI (ROOF CONSULTANTS INSTITUTE)
The Roof Consultants Institute provides education and certifications for roofing designers, specifiers, estimators, project managers and contract administrators. Roofing designers and contractors should be certified by the Roof Consultants Institute.
Sheet metal, which is attached to a vertical surface and performs the function of receiver for sheet metal counter flashing.
SHEET METAL COPING
Formed sheet metal covers the top of perimeter parapet walls and similar conditions. The sheet metal helps to protect the roof system flashing membrane and secures the membrane to the top of the wall. Sheet metal coping also provides an aesthetically pleasing wall cap, which is visible from the ground. Sheet metal is typically formed with lap seams or standing seams.
SHEET METAL COUNTER-FLASHING
Formed sheet metal fastened onto the surface of walls and curbs to cover the top edge of the roof system base flashing and wall flashing.
SINGLE PLY ROOF SYSTEM
The term ‘single ply roof system’ is a generic term that typically refers to one of the following:
A) TPO (Thermoplastic Polyolefin) single ply roof system, which is installed in one of the following ways:
- Fully adhered
- Mechanically fastened
B) PVC (Polyvinyl Chloride) single ply roof system, which is installed in one of the following ways:
- Fully adhered
- Mechanically fastened
C) EPDM (ETHYLENE PROPYLENE DIENE MONOMER) single ply roof system, which is installed in one of the following ways:
- Fully adhered
- Mechanically fastened
- DEAD LOADS are those permanent non-moving loads on the roof (examples: the roof structure, permanent rooftop equipment, mechanical units, etc.)
- LIVE LOADS are those loads that are temporary and often mobile (example: rain, standing water, wind and moving equipment.)
Surface of the structural roof deck or parapet walls upon which the insulation or roofing is applied.
TEST CORE CUT
Destructive testing in which the existing roof system is cut by a knife, tube core-cutting tool or similar tool. A cross section of the roof is cut out and examined to evaluate the condition of each layer of the roof system (ply sheet, asphalt, insulation and other components.) This is critical to evaluate the condition and potential longevity of the roof system. Destructive test cuts are infilled and sealed to ensure the test cut is watertight.
THERMAL SHOCK (RAPID EXPANSION/CONTRACTION)
Thermal shock occurs with rapid heating or cooling of the roof system (example: hail, snow or rain after a hot spell [the surface of a roof system can reach over two hundred degrees on a hot day].) The shock of rapid expansion and /or contraction can severely damage the roof system. For an older roof system that may be in fair or worse condition (through time, the oils in the built-up roof asphalt evaporate, which leaves the asphalt brittle) thermal shock can tear the roof apart and create areas of severe roof leakage.
THERMOPLASTIC POLYOLEFIN (TPO)
Synthetic thermoplastic polymer using polyolefin. TPO single ply roof systems are typically applied by fully adhering the system to the roof substrate, mechanically fastening the roof to the substrate or anchoring the roof to the substrate by ballast. Heat welding of TPO seams creates a monolithic single ply sheet.
ULTRAVIOLET RADIATION (COMMONLY: UV RADIATION)
Ultraviolet radiation is on the light spectrum that humans cannot see. Ultraviolet radiation acts to evaporate the oils out of built-up roofing asphalt, which leaves the asphalt dry and brittle. The effect of ultraviolet radiation is reduced by the application of surfacing to the roof system (most common forms of surfacing are mineral surfaced cap sheet and liquid applied reflective coatings.
WASHINGTON CONDOMINIUM ACT
In the state of Washington, the legislature has passed an Act, which regulates the construction and remodel of condominiums. Briefly stated, the Act requires a ‘building envelope design professional’ to create and/or review all building envelope flashing details and to monitor the building envelope waterproofing components.
As it relates to roofing, a certified roof consultant would be retained to create new or re-roofing specifications and design details. The consultant would also monitor the installation of the roof system.
Often unseen and not regarded, wind uplift can be a critical factor. The high winds of hurricanes are obvious occurrences, which may lift the roof system away from the roof substrate. In situations where a roof system is not appropriately ballasted, mechanically fastened or fully adhered to the roof substrate, moderate wind speeds may lift the roof system away from the roof substrate.
The contour of the roof perimeter parapet wall magnifies the effect of wind blowing over the wall to create a significant amount of upward pressure. Due to the risk of wind uplift, a design professional should design the roof system.
- This glossary of roofing terminology is the opinion of the author. NW Professional Roofing Services, Inc. is not responsible for consequential damages associated with the use of the definitions contained herein.
- The description and application of the Washington Condominium Act is the opinion of the author. NW Professional Roofing Services, Inc. does not offer legal advice and is not responsible for any consequential damages associated with the information.