Commercial Flat Roofing Glossary
Plain definitions of the low-slope roofing terms that appear in specifications, condition reports and quotations.
This glossary defines the terms used in commercial and industrial flat roofing in Ontario, from the layers of the assembly to the details at every junction. Use it to read a roofing specification, condition report or quotation without needing a translator.
How to use this glossary
Roofing specifications are written in a trade language nobody outside the trade is taught, which is a problem when the document commits a capital budget. The terms below appear most often in Ontario commercial low-slope specifications, reports and quotations.
Two points make the rest easier. A flat roof is an assembly, not a sheet: deck, vapour barrier, insulation, cover board, membrane and surfacing, and the difference between two quotes usually sits in the layers you cannot see. And water gets in at junctions, which is why so many terms below describe edges and details.
If a term is missing, call 416-456-0777, or bring the document to a free roof assessment.
A
Adhered. Insulation or membrane bonded to the layer below with adhesive rather than fasteners, avoiding penetrations.
Aggregate surfacing. Gravel or crushed stone over a bituminous roof, shielding the membrane from sunlight and impact.
Air barrier. The layer stopping air movement through the assembly. Humid indoor air leaking into it deposits far more moisture than diffusion does.
B
Ballast. Loose stone or pavers holding a membrane down by weight. Ballast hides the membrane from inspection and adds dead load.
Base flashing. Membrane turned up from the roof field onto a wall, parapet or curb. Its height and termination decide whether water gets behind the system.
Base sheet. The first ply of a multi-ply bituminous roof, fastened or adhered to the substrate, with further plies applied over it.
Bitumen. Asphalt used as the waterproofing agent in built-up and modified bitumen roofing, applied hot, cold, torched or self-adhering.
Blister. A raised bubble caused by trapped air or moisture expanding when heated. It thins the membrane and eventually splits.
Built-up roofing (BUR). Multiple bitumen-saturated felts, usually surfaced with aggregate. Still found on older Toronto buildings. See flat roof types.
C
Cant strip. An angled strip at the deck-to-wall junction, softening the ninety degree turn so the membrane does not crack.
Cap sheet. The top ply of a modified bitumen system, surfaced with mineral granules against sunlight and foot traffic.
Coping. The cap over a parapet wall, in metal, stone or precast, stopping water entering from above. Failed coping joints leak in ways no field repair fixes.
Counterflashing. Metal lapping over the base flashing and set into the wall, shedding water clear of the flashing termination.
Cover board. A rigid board over the insulation and under the membrane, spreading loads and resisting hail and foot traffic.
Cricket. A tapered structure behind a curb, diverting water around it toward a drain instead of ponding on the upslope side.
Curb. The raised frame supporting a rooftop unit, skylight or hatch so waterproofing can be turned up. See curb work.
D
Deck. The structural surface the assembly is built on: steel, concrete, wood plank or plywood. It governs fastener selection.
Drip edge. Metal edge trim directing runoff away from the fascia and wall below.
Dry-in. A temporary watertight repair stopping active water entry until the assembly is dry. Standard first response on an emergency call.
E
EPDM. Ethylene propylene diene monomer, a rubber single-ply membrane joined with tape or adhesive, flexible in cold weather.
Expansion joint. A designed break letting the structure either side move independently. Roofing over one instead of detailing it tears membranes.
F
Fascia. The vertical face at the roof edge, covering the deck edge and the assembly behind it. See flashing and siding.
Fishmouth. An open lap edge in a bituminous membrane where the ply has not bonded. A direct route for water into the assembly.
Flashing. Any component sealing the junction between the roof field and a wall, curb, drain or edge. Most flat roofs leak here.
Fully adhered. A membrane bonded across its entire underside, giving high uplift resistance and limiting lateral water travel if a breach occurs.
G
Granule loss. Wearing away of mineral surfacing from a cap sheet, exposing the bitumen. Widespread loss across the field signals the end of service life.
Gravel stop. Metal edge flashing at the perimeter of an aggregate-surfaced roof, with an upstand holding the gravel on.
Green roof. A vegetated assembly over the waterproofing, with root barrier, drainage layer and growing medium. Saturated load needs an engineer.
H
Hot work. Any operation involving open flame or high heat, principally torch and kettle work, controlled by procedure and a fire watch.
I
Ice damming. Meltwater running to a cold perimeter, refreezing and backing up behind the ice until it finds a way behind the base flashing.
Insulation. The thermal layer, typically polyisocyanurate, mineral wool or expanded polystyrene board. Once wet it does not dry in place.
L
Lap. The overlap where two membrane sheets meet and are bonded, welded or sealed. Width and bond quality decide whether the joint holds.
Low-slope. A roof shallow enough that water does not shed quickly, so the assembly must be watertight. What most people call a flat roof.
M
Mechanically fastened. Screws and plates through the insulation into the deck, with density increased at perimeters and corners where uplift is highest.
Membrane. The continuous waterproofing layer, whether single-ply sheet, modified bitumen ply or built-up assembly.
Modified bitumen. Asphalt modified with polymer for flexibility, supplied in rolls and applied by torch, hot asphalt, cold adhesive or self-adhesion.
Moisture survey. An infrared or capacitance scan mapping where moisture has spread, so a scope can be drawn rather than guessed. Part of a full roof inspection.
N
Nailer. Wood blocking at perimeters and openings, giving edge metal and terminations something to fasten into.
O
Overflow scupper. A secondary opening through a parapet above the drainage level, letting water escape if drains block. A safeguard against structural overload.
P
Parapet. The wall extending above the roof at the perimeter. On older Toronto buildings parapets are tall, masonry and moving.
Pitch pocket. A metal collar around an irregular penetration, filled with sealant. A maintenance item, since the filler shrinks and cracks.
Ponding. Water standing on the roof beyond about forty-eight hours after rain. It accelerates degradation and feeds freeze-thaw damage.
Protected membrane assembly. An inverted build-up with insulation above the waterproofing, usually under pavers. Common on terraces and amenity decks.
PVC. Polyvinyl chloride single-ply membrane, heat-welded and resistant to grease, animal fats and chemical exhaust.
R
Recover. A new roof installed over the existing membrane rather than tearing it off. Only where the deck is sound and the insulation dry.
Reglet. A groove formed into a wall to secure the top edge of counterflashing, sealed so water cannot run behind it.
Ridging. Long raised lines following insulation board joints, caused by movement or moisture below.
Roof drain. The internal outlet carrying water into the building drainage system, set in a sump. Blocked drains cause more damage than storms do.
Roof zone. The division of a roof into field, perimeter and corner areas for wind design. Uplift is highest at corners.
S
SBS. Styrene butadiene styrene, the polymer modifying bitumen for low-temperature flexibility. The workhorse two-ply assembly on GTA commercial roofs.
Scupper. An opening through a parapet letting water off the roof, as primary drainage or as overflow.
Seam. The joint between membrane sheets: heat-welded on TPO and PVC, taped or adhered on EPDM, torched on bituminous systems.
Single-ply. A system waterproofed by one layer of sheet membrane such as TPO, PVC or EPDM, as distinct from multi-ply bituminous assemblies.
Splitting. A straight-line tear through the membrane, caused by thermal or structural movement rather than impact.
Standing seam. A metal system with panel edges turned up and joined above the water line, keeping fasteners out of the drainage path. See metal roofing.
Substrate. Whatever the next layer is applied to: deck, insulation, cover board or an existing membrane.
Sump. A recess formed around a roof drain so water is drawn into it rather than sitting beside it at deck level.
T
Tapered insulation. Insulation made in sloped boards, laid to a drainage plan to drive water toward drains on a deck with little built-in slope.
Tear-off. Complete removal of the existing assembly down to the deck, removing all wet material.
Termination bar. A metal bar along the top edge of a base flashing, holding it tight to the wall and providing a sealed edge for counterflashing.
Test cut. A small opening cut through every layer to the deck, showing what is present and whether the insulation is wet.
Thermal bridging. A path where heat bypasses the insulation, such as a fastener, causing local cold spots and condensation risk.
Torch-applied. Modified bitumen melted onto the substrate with a propane torch. Subject to hot work controls and a fire watch.
TPO. Thermoplastic polyolefin single-ply membrane, heat-welded at the seams and used over large uninterrupted warehouse fields.
V
Vapour barrier. A layer below the insulation limiting moisture movement from a humid interior into the cold part of the assembly.
Vent. A roof-mounted device relieving pressure or moisture from within the assembly. Vents do not dry saturated insulation.
W
Walkway pad. Protective surfacing along service routes, so maintenance traffic wears the pad instead of the membrane.
Wet insulation. Insulation that has taken water through a breach. It loses thermal value, adds weight and cannot be dried in place.
Wind uplift. The suction force wind exerts on the assembly, greatest at corners and perimeters. Attachment is designed against it.
Work platform. A designated safe working surface at height with edge protection, used near an unprotected edge or a fragile surface.
Where do these terms show up in practice?
In three documents. A condition report uses the diagnostic words: blistering, ridging, splitting, granule loss, ponding, wet insulation. A quotation uses the assembly words: base sheet, cap sheet, cover board, tapered insulation, mechanically fastened or adhered. A close-out record uses the detail words: base flashing, counterflashing, coping, cant strip, sump.
Read those three sets and you can compare two proposals properly. What matters is rarely the last line; it is whether both specified the same assembly, the same attachment against wind uplift and the same flashing scope. See flat roof types.
Frequently asked questions
What is the difference between a membrane and a roof system?
The membrane is the waterproofing sheet or plies. The system is everything working together: deck, vapour barrier, insulation, cover board, membrane, surfacing, flashings and the attachment method holding it down. Quotes that name only the membrane are describing a fraction of what you are buying.
Is a low-slope roof the same thing as a flat roof?
In everyday use yes, and low-slope is the more accurate term. No roof is truly flat: even a roof that looks level is built or tapered to move water toward drains or scuppers. Low-slope means the fall is shallow enough that the assembly must be watertight rather than merely water-shedding.
Why do specifications keep referring to corners and perimeters separately?
Because wind uplift is not uniform. Suction is highest at roof corners, high along perimeters and lowest in the field, so attachment density is increased in those zones. A proposal that applies one fastener pattern across the whole roof has skipped a design step.
What does it mean if a report says the insulation is wet?
That water has entered through a breach and saturated the thermal layer. Wet insulation loses thermal value, adds weight, can corrode a steel deck and does not dry in place. It has to be cut out and replaced, and how far it has spread is what usually decides between repair and replacement.
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