Flat roofing is waterproofing on a low-slope surface. Rather than shedding water down a steep pitch, it relies on a continuous sealed membrane over a layered assembly: deck, vapour barrier, insulation, cover board and membrane, with slope built in to move water to drains or scuppers.
What makes a roof flat rather than sloped?
Slope, and what that slope means for how the roof keeps water out.
A steep roof works on shedding. Water runs off quickly, and the covering can be made of overlapping discrete pieces, shingles, tiles, panels, because gravity does the sealing. Small gaps do not matter much when water is never sitting still.
A low-slope roof cannot rely on that. The fall is shallow, water moves slowly, and snow and ice sit on the surface for months in an Ontario winter. So the covering has to be continuous and sealed at every joint, edge and penetration. It is waterproofing rather than shedding, and it fails in a completely different way: not through a missing piece, but through an opening in a seal.
This is why the trades are different, the materials are different and the failure patterns are different. It is also why we work only on commercial and industrial low-slope buildings and do not take residential shingle work. The full comparison of systems lives on our flat roof types page; this piece is the primer that sits underneath it.
What is a flat roof actually made of?
From the bottom up, a typical commercial low-slope assembly:
- Structural deck. Steel, concrete or, on older and smaller buildings, wood. The deck carries the load and everything above is attached to it. On steel deck, the flutes act as channels that carry leaked water horizontally, which is why ceiling stains rarely sit under the actual opening.
- Vapour barrier or air and vapour control layer. Sits above the deck and stops warm, humid interior air from carrying moisture up into the cold insulation, where it would condense. Critical in Ontario, and critical in buildings with process moisture.
- Insulation. Rigid boards, mechanically fastened or adhered, in one or more layers with staggered joints. Thickness is set by the thermal requirements applicable to the building under the Ontario Building Code.
- Tapered insulation, where used. Boards cut to a slope, laid to a drawn pattern with sumps at the drains and crickets around obstructions, to move water where it needs to go.
- Cover board. A hard board over the insulation that protects the membrane from hail, foot traffic and fasteners backing out from below, and gives the membrane a stable substrate.
- Membrane. The waterproofing itself, in one of the systems below.
- Surfacing or ballast, where used. Granules, gravel, pavers or stone ballast that protect the membrane from ultraviolet exposure and hold it down.
Every one of those layers can be specified well or badly, and the membrane on top gets blamed for all of them.
Which membrane systems are used on Ontario commercial buildings?
SBS modified bitumen
A two-ply system, base sheet and granulated cap sheet, applied by torch, hot mopping or self-adhesive. The bitumen is modified with styrene-butadiene-styrene to stay flexible in cold weather. It is redundant by design, since there are two plies, and it tolerates foot traffic and rooftop equipment well. It remains the workhorse assembly on GTA commercial buildings.
TPO
A single-ply thermoplastic sheet, heat-welded at the seams so the laps fuse into a continuous sheet. Fast to install across large uninterrupted fields, which is why it is common on warehouses and distribution centres.
PVC
Also thermoplastic and heat-welded. The usual choice where the roof is exposed to grease, animal fats or chemical exhaust, so restaurant, food processing and some industrial applications.
EPDM
A synthetic rubber single ply, seamed with tape and adhesives rather than heat. Flexible at low temperatures and long established. Frequently ballasted on older installations.
Built-up roofing
Multiple bitumen-saturated plies built up on site, surfaced with gravel or a cap sheet. Found on many older Toronto buildings, still repairable, and still performing where it has been maintained.
Low-slope metal
Standing-seam and structural metal systems, covered on our commercial metal roofing page. Different failure mode entirely: metal roofs leak at fasteners and seams rather than through the panel.
How is the roof held down?
Attachment is half the specification and it gets far less attention than the membrane brand.
- Mechanically fastened. Fasteners and plates through the insulation into the deck, with the membrane either fastened in the laps or held by the insulation attachment beneath it. Fast, tolerant of damp substrates, and the usual approach on steel deck.
- Adhered. Insulation and membrane bonded in ribbons or full-spread adhesive. No fastener penetrations through the assembly, better at resisting flutter, and more sensitive to substrate condition and temperature during installation.
- Ballasted. Loose-laid membrane held by stone ballast or pavers. Simple, but it adds significant load, hides the membrane from inspection, and the ballast migrates into drains.
Whichever is used, the attachment pattern is not uniform across the roof. Wind uplift pressures are highest at the corners, next highest along the perimeter, and lowest in the field, so fastener density increases as you move outward. A specification that treats the whole roof as one zone is a specification to ask questions about, and it is a common reason perimeter membrane peels back in a wind event.
Where does a flat roof leak, and why?
In a predictable order, which is useful to know as an owner.
- Flashing at transitions. Parapets, curbs, walls, penetrations and drains. These carry the most movement and the most hand-built work, so they fail first. This is the single largest category.
- Wherever water stands. Ponding does not create defects on its own, but it finds every existing one, adds load and drives freeze-thaw damage into seams. Drainage failures accelerate everything else.
- Seams and laps in the field. As the membrane ages it loses flexibility and the joints open, usually along a lap, at a corner, or over a joint in the deck or insulation below.
- Damage from other trades. Mechanical contractors cutting flashing for conduit, equipment dragged across a membrane, dropped tools. Very common on multi-tenant buildings.
Notice that the membrane material itself is well down the list. Most leaks are detailing and drainage problems, which is why choosing a premium membrane will not save a roof that was detailed badly, and why the same three causes come up on nearly every flat roof repair call we take.
What does the Ontario climate demand of a flat roof?
Four local mechanisms shape how these assemblies are specified here.
- Freeze-thaw cycling. Southern Ontario crosses zero repeatedly through the shoulder seasons. Water in a lap or a hairline split expands as it freezes and widens the opening each cycle.
- Snow and drift loading. The Ontario Building Code sets ground snow load and rain load by municipality in Supplementary Standard SB-1, and drifting against parapets, mechanical screens and higher adjacent walls concentrates that load well above the field value.
- Ice at the perimeter. Meltwater runs to the cold edge, refreezes at the parapet and around drains, and backs up behind the ice until it goes over the base flashing.
- Thermal movement. A dark membrane swings through a wide temperature range in a single summer day and again across the seasons. Cant strips, expansion joints and correctly relieved terminations absorb that movement. Rigid details tear.
Interior conditions matter too. A heated, humidified building in a cold climate pushes vapour upward into the assembly, which is why the vapour control layer is not optional here and why plants venting process moisture through the roof end up with wet insulation if the assembly was never designed for the load.
How do you know what your own building has?
Frequently nobody does, and that is normal. Drawings get lost, buildings change hands, and roofs get recovered without records.
The way to find out is a core cut: a small square through the full assembly, lifted out and patched, showing every layer, the insulation thickness, how many membrane layers already exist and what the deck is. A handful of cores across the roof, plus a walk that records the drains, the perimeter and every penetration, is enough to describe an unknown roof properly.
That description is what makes every subsequent decision possible. You cannot sensibly weigh repair against replacement, decide whether a recover is even permitted, or budget anything without knowing what is up there. A written roof condition report is the document that captures it, and it is the one thing worth having on file before you need it.
If you want a low-slope roof in Toronto, the GTA or southwestern Ontario looked at properly, call 416-456-0777 or book a walk through the contact page. The wider service is set out on our commercial roofing page.