Flat Roofing Glossary

The following is a list of terms or phrases commonly used in the roofing industry. Each term is accompanied by a brief definition.

Alligatoring
Shrinkage cracking of the bituminous surface of built-up roofing or the exposed surface of smooth-surface roofing, producing a pattern of deep cracks with the scaly look of an alligators hide.

Asphalt
A highly viscous hydrocarbon produced from the residuum left after the distillation of petroleum used as the waterproofing agent of a built-up roof.

Ballast
An anchoring material (such as rounded river rock, gravel, or pre-cast concrete pavers), which is used to resist wind, uplift forces and hold roof membranes in place.

Bitumen
A generic term for either the asphalt or coal tar pitch used in the roofing industry.

Blister
A spongy, raised portion of roofing membrane, ranging in size from 25mm(1”) in diameter and barely detectable to as much as 4.6m2 (50ft2) in area and 300 mm(12”) high. Blister result from the pressure of trapped air or water vapour.

Built-Up Roof
Is a roof consisting of multiple plies of roof felts laminated together with bitumen. Built-up roof material can consist of bitumen-saturated felt, coated felt, polyester felt or other fabrics. A surfacing is generally applied and can be asphalt, aggregate (gravel or slag), emulsion or a granule-surfaced cap sheet.

Deck
The substrate over supportive framing to which roofing material is applied; also called decking or sheathing.

Drip edge
Flashing made of steel or other non-corrosive material that is placed along the eaves and rake edges at a 90º angle to let water runoff drop clear of fascia and into the gutters.

Eave
A roof edge that extends past the exterior wall line.

Emulsion
An intimate mixture of bitumen and water, with uniform dispersion of the bitumen globules, achieved through a chemical of clay emulsifying agent.

EPDM
Ethylene Propylene Diene Monomer (or Terpolymer which is simply a product consisting of three distinct monomers). EPDM is classified as a Thermoset material which means it is either fully-cured prior to being installed or that it cures during natural weathering after installation.

Fishmouth
An opening formed by an edge wrinkle in a felt where it overlaps another felt in a built-up roofing membrane. It is the result of lifting of laps at edges in bituminous roofing, or unbonded edges in PVC roofing due to inadequate heat welding.

Flashing
Metal or other flexible material used to seal the roof and prevent leaks around any projection or intersection, such as pipes, chimneys, dormers, valleys or adjoining walls.

Flat or low slope roof
A roof with a pitch of less than three feet of rise over a twelve-foot run; this type of roof needs a sealed system installation.

Flood Coat
The top layer of bitumen in an aggregate-surfaced built-up roofing membrane.

Gravel
Coarse granular aggregate, having rounded edges, resulting from the natural erosion of rock.

Ice/Leak barrier
A self-adhering and self-healing membrane applied to the roof deck and designed to protect against water infiltration from ice build-up or wind-driven rain.

Loose Fasteners
Common in mechanically fastened systems, where the fasteners have backed out. This could happen as a result of the insufficient thickness of the fasteners, workmanship, and/or deterioration of the roofing components due to saturation. This deficiency was also observed where major structural settlement has occurred.

Loosely Laid Membrane
Membranes, which are not attached to the substrate except at the perimeter of the roof. They are held in place with appropriate and adequate ballast.

Membrane
The portion of the roofing system that serves as the waterproofing material. Can be composed of one material or several materials laminated together.

Modified Bitumen
Is a bitumen modified by one or more polymers such as Atactic Polypropylene (APP), styrene butadiene styrene (SBS).

Permeability
The rate of flow of a liquid or gas through a porous material.

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.

Plumbing boot
A prefabricated covering, usually of flexible material, used to seal around a penetration; also called a pipe boot.

Ply
A layer of roofing membrane. A four-ply membrane has at least four plies of felt at any vertical cross section cut through the membrane.

Ponding
The accumulation of water at low-lying areas on a roof. It is usually the result of structural settlement and localized roof failure. The practice of installing tapered insulation has been widely employed to correct this deficiency, as it is difficult to add drainage to existing structure.

Primer
A liquid bituminous material applied to a surface to improve adhesion of heavier application of subsequently applied bituminous materials.

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.”

Puncture
Found where blisters have broken or as a result of rising nails/screws or other exterior mistreatment. If punctures were found throughout the roof, assume that a considerable amount of water has penetrated the membrane.

PVC
PVC stands for Polyvinyl Chloride. It is one of the pioneering single-ply roof materials. The first known PVC roofing membrane was produced by a company called Trocal and was installed in Germany in 1966. Within a few short years, Trocal PVC covered millions of square feet all over the world and single-ply roofing was setting the stage for market dominance.

Rafter
The structural member supporting the deck and roof system components, extending from the down slope perimeter to the ridge or hip.

Ridging
Also, referred to as wrinkling and buckling. It occurs in bituminous roofing, and normally found above joints of insulation. They are usually narrow and long. It is the result of interior moisture condensing on the underside of the felts. Ridges and wrinkles on single-ply loosely laid membrane may be found around drain areas, and roof protrusions. This may be attributed to inadequate attachment of the single ply membrane and/or using unflexible flashing, which has a memory of its original shape.

Roof ventilation
A static, wind- or power-operated system for removing hot air and moisture from the air under your roof. Includes ridge vents.

Single Ply Membrane
Roofing membranes that field-applied using a pre-manufactured sheet of single layer membrane material (either homogenous or composite) rather than multiple layers.

Sheet Metal
Copings are sheet metal cover for all wall systems. It is applied to protect the flexible flashings from wind forces. Any movement of wall systems and settlement may create opening in the sheet metal. Counter-flashings are metal installed over the flexible flashing at the adjoining walls. This prevents the water from contacting the top edge of flashing. Any punctures or loosening of sheet metal may allow water penetration into the wall.

Smooth-Surfaced Roof
A built-up roofing membrane with a coating of hot asphalt, asphalt emulsion or asphalt cutback.

Splitting
Long cracks in roof membrane, usually parallel to felts and insulation joints. Splitting of membrane is usually spongy. The cause of this deficiency may be attributed to physical stress, ponding, freeze-thaw, cracking substrate, or bad workmanship.

Substrate
The surface upon which the roofing membrane is placed – structural deck or insulation.

TPO
Thermoplastic Olefin or Polyolefin membranes are single-ply roof membranes constructed from ethylene propylene rubber.

Vapour Retarder
A material designed to restrict the passage of water vapour through a wall or roof assembly.

Water shield
A watertight barrier used to seal water out at the eaves and rakes, in valleys, and around chimneys and skylights.

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