Exploring Roofing Terminology

Understanding  terminology used in the flat roofing industry, can better equip you when considering a flat roofing system or metal retrofit replacement. Below is an extensive list of roofing terms to help:

Alligatoring - Crackling in roofing products due to shrinkage from sunlight, oxidation or heat/cold exposure.

Adhesive - substance which attaches two surfaces.

Asphalt – Dark substance, petroleum based byproduct of processing crude oil, used in waterproofing.

Ballast - materials such as concrete paving stones, use to weigh down or prevent uplift of loose-laid roofing systems.

Base Sheet - A heavy sheet often used as the first ply in built-up roofing.

Base Flashing - Attached or rests on roof deck and directs water flow or seals against the deck. May also be material applied to wall base extending above a roof, as protection for the junction of the wall, and the roof. 

Bitumen - Bitumens are mixtures of hydrocarbons. In roofing terms this covers both asphalt and coal tar pitch.

Blisters - Air, water vapor or other gases trapped within the plies of the roofing causing bubbles upon the roof surface.

Bond - Adherence between elements of the roof system.

Breather - A roof vent penetrating the roofing membrane to allow escape of moisture and air from insulation.

BUR - An acronym which stands for Build-up Roofing; the process of creating multiple layers of roof materials, sandwiching felts to bitumen.

Buckling - Warping or wrinkling of a roof membrane.

Built-up Roofing - Layers of roofing felt bonded together on site, usually has a protective surface coating of gravel  often in a heavy top coating of bitumen. 

Cant Strip - A support used at the intersection of the roof deck, used in built-up roofing but not required in single ply roofing.

Cap Flashing - Portion of flashing built into a vertical surface to prevent the flow of water behind the base flashing.  The cap flashing overlaps and caps off the top of the base flashing.

Caulking – Adhesive which fills in gaps and joints.

Coal Tar Pitch - Bituminous material distilled from crude tar derived from coal.  Formerly used as the waterproofing material for tar and gravel built-up roofing. Now a “designated substance” and carcinogen--obsolete and undesirable in modern roofing practice.

Cold Process - A roofing procedure that uses cold materials to create waterproofing on-top of the surface.

Concealed Fasteners or Nails - A technique used in roll roofing that conceals or covers all fasteners or nails used to secure roofing materials to decking.

Condensation - Change from water vapor to liquid water, resulting from a drop in temperature of an air vapor mixture..

Cracking - A fissure or fissure pattern appearing on asphalt roofing material due to weathering exposure.

Curb - Wall of concrete, wood or masonry built above the level of the roof, surrounding a roof opening such as for installation of hvac units.

Dead Load - Total weight of installed materials along with the continual weight of a roof--such as snow or water, and used to calculate the strength of the supporting frame.

Deck - Top building surface over which a roof system is applied. 

Downspout -  Pipe conveying water from roof gutters to a drain.

Drip Edge - L-shaped flashing used at eaves to direct runoff water into gutters away from the fascia.

Eave - Horizontal roof overhang that is not directly over the exterior walls or the buildings interior.

EPDM - An acronym which stands for: Ethylene Propylene Diene Monomer. In roofing, a single membrane roof material composed of rubber polymer and carbon black.

Expansion Joint - Controlled joint between two sections of a roof and allows building envelope to expand and contract without causing harm to the roof.

Exposure -  Subjected to the elements of weather.

Fascia - Vertical face trim board used to hide the ends of rafters and sheathing.

Fastening Tab - Part of Duro-Last roofing system where membrane is mechanically attached to the deck.

Felts - Mat of organic or inorganic fibers used with tar and gravel roof systems.

Fire Wall - Built for the purpose of restricting the spread of fire in a building.

Fire-Resistant - Material that is resists ignition.

Flashing - Treatment of the edge of a roof with metal usually galvanized colour coated sheet metal - may also be copper, tin or aluminum.

Flood Coat - a liquid of bitumen poured overtop roofing felts normally topped off with gravel.

Gutter – A valley which conveys water from the roof to downspout.

Header - The beam into which joists are fitted when framing around a roof opening. 

Heat Welding - Hot air process where two pieces of material are bonded or fused together.

Lap - A seam, where one edge of roofing materials overlaps the next.

Live Load - Total weight of all installed equipment and materials along with the various weights that will move across a surface.  A maintenance crew working on the roof would be part of a live load and HVAC equipment vibrations also--any impact load moving on or across a roof.

Membrane - The skin of waterproofing system or roof covering which protects the interior functions of a building.

Mopping - is a term to describe the spreading of hot bitumen with a cotton or other fiber  mop (often the practice which contributes to roofing fires).

Overhang - A portion of roofing extending beyond the deck such as the part of the roof structure which extends beyond the exterior walls of a building.

Parapet - Low wall along the edge of a roof deck, generally an extension of exterior building walls and above firewalls that extend above the roof.

Pitch - Height from the joist to ridge divided by rafter length, translates to rise in inches per horizontal foot or ratio. This creates the angle of a roof.

Pitch Pan - A rectangular or circular metal collar placed around supports that protrude above a roof deck.  The pitch pan is placed on top of the roofing underlayment.

Plastic Cement - A mixture of asphalt, solvent and mineral stabilizer used to adhere or fill  flashings.

Ply - Thickness of roofing material.  Roofs are described according to the number of layers used to build up the membrane.

Ponding - Collection of water in pools on the roof top. 

Primer -  Thin coating used on asphalt, concrete or metal in preparation of applying a built-up roof.

PVC - An acronym standing for: Polyvinyl Chloride; a thermoplastic which has multiple construction uses. In roofing it is a single ply sheet system.

Rolled Roofing - Roofing material, which comes from the manufacturer in rolls. 

Roof Drain – Fitting of an interior drain and leader for draining water from flat roofs.

Roof Insulation - Material used as part of the roofing system to reduce heat loss. 

Roofing System - Waterproof roof membrane, roof insulation, vapor barrier and roof deck as an entity.

Scrim - Component of the Duro-Last membrane.  A weft sandwiched into the PVC and creating a grid-like appearance.  If the Duro-Last membrane is ever punctured this scrim prevents further damage.

Scupper - Outlet in the wall of a building or a parapet wall for drainage of overflow

Stack - A vent pipe which penetrates the roof surface to allow for the escape or discharge of fumes or gases.

Substrate - The base or supporting surface of the roofing membrane.

Surfacing - Is the protective topcoat or cover of a roof. Often this refers to the granular or graveled layer.

Sky Light - Glass or plastic insertion in roof for the admission of light.

Tar - Dark black or brown thick liquid distilled from petroleum, coal or organic materials. In roofing used to bind aggregates like gravel, and seal off or watertight a surface.

Thermoplastic - Type of membrane that can be heat welded and includes a re-inforcement layer of polyester (scrim) in the weft.

Torch down or on - An open flame propane method of adhering roofing materials. In roofing, torch down membrane systems pose fire risk and require special safety handling.

TPO - An acronym for: Thermoplastic Poly Olefin. In roofing, it is a single ply system usually with a laminated surface.

Titanium Dioxide - Used in thermo-plastic roofing membranes to reflect UV rays and to protect the polymers in the roofing material.

Truss - A combination of components like beams, bars and ties, arranged to form a supporting framework for loads over spans of roof construction.

Valley - Horizontal line formed at the bottom of two inclined roof surfaces.

Vapour Barrier or Vapour Retardant - Material that prevents the passage of water vapor through it.

Vent - Outlet for air, a ventilating duct.

Vent Pipe - Vertical pipe providing an escape for foul gasses. 

Vent Stack - A large vertical pipe leading to the outside through the roof.

Walk Pads - A reinforced product, layed out on the roof surface in a pathway configuration. Designed for the safety and protection of the roof membrane from damages due to human activity upon the building.

Waterproofing - The treatment of making a surface impervious to liquid/water.

Water Vapor - Moisture as a gas in air. 

Wrinkle - A slight ridge caused by folding or creasing.  Refers to the common crinkle pattern that forms over the joints or insulation in roofing.