Types of Flat Roof Systems, Compared
How SBS, TPO, EPDM, PVC, built-up, metal and restoration coatings behave on a low-slope building in southern Ontario.
The main flat roof systems are SBS modified bitumen, TPO, PVC, EPDM, built-up roofing, low-slope metal, and coating or restoration systems. They differ in composition, how the seams are joined, how they behave in Ontario cold, and how they fail. The right choice follows the deck, the rooftop equipment and the interior humidity load.
What is actually in a flat roof assembly?
People compare membranes. Roofs succeed or fail on the whole assembly, and the membrane is the thinnest part of it. From the deck up, a modern low-slope roof usually reads:
- Structural deck. Steel, concrete or wood. It carries the load and determines how the roof above it is attached.
- Vapour barrier. Sited on the warm side to stop interior humidity migrating into the insulation and condensing. Selection depends on the interior humidity load, which is why plants with wet processes need it thought through carefully.
- Insulation. Two or more staggered layers with offset joints, installed to the Ontario Building Code requirements that apply to the project.
- Tapered insulation and crickets. Built to drive water positively to the drains. The most under-specified and most valuable element on a re-roof.
- Cover board. Protects the membrane from foot traffic, hail and fastener back-out and gives it a stable substrate.
- Membrane. The waterproofing layer, which is what the rest of this page compares.
- Surfacing or ballast. Granules, gravel, pavers or stone ballast where the system uses it.
- Details. Cant strips, base flashings, cap flashings, drains, scuppers and penetration flashings, covered on roof flashing and siding.
A premium membrane over wet insulation on a corroded deck is still a failed roof. That is why our roof condition report examines every layer rather than the surface.
How the assembly is held down matters as much as what it is made of. Mechanically fastened systems screw through the insulation into the deck and are quick and tolerant of a slightly uneven substrate. Adhered systems bond each layer with adhesive, give a smoother finished surface and avoid fastener back-out, but demand a clean, dry, sound substrate. Ballasted systems hold the membrane with stone or pavers and are largely limited by load and by the difficulty of tracing a leak under the ballast. The choice is driven by wind uplift demand, deck type and condition, and how much disruption the building can absorb during installation.
Flat roof systems compared
| System | Composition | Seaming method | Cold-weather behaviour in Ontario | Typical failure mode | Suited to |
|---|---|---|---|---|---|
| SBS modified bitumen | Two plies of polymer-modified asphalt with a granulated cap sheet, on a reinforcing mat | Torch-applied, hot mopped or self-adhered laps | Rubber-modified asphalt stays flexible well below freezing; two plies give redundancy through freeze-thaw cycling | Granule loss and surface weathering across the field; splits at cant strips and low base flashings | Roofs with heavy foot traffic and dense rooftop equipment; multi-tenant and institutional buildings |
| TPO | Single-ply thermoplastic polyolefin sheet, reinforced with a scrim | Hot-air welded seams | Installs quickly in cool conditions; membrane stiffens in deep cold and adhered systems have low-temperature limits | Cold or incomplete welds at seams, particularly at laps and corners; punctures from traffic | Large open warehouse and distribution fields with few penetrations |
| PVC | Single-ply polyvinyl chloride sheet with plasticisers, scrim reinforced | Hot-air welded seams, weldable for the life of the sheet | Handles cold well; remains weldable, which makes future repairs and tie-ins straightforward | Plasticiser migration and shrinkage in older sheets, pulling on base flashings | Roofs exposed to grease, animal fats, oils or chemical exhaust: restaurants, food plants, process buildings |
| EPDM | Single-ply synthetic rubber sheet, black or white, in large sheet widths | Seam tape or liquid adhesive, not welded | Stays flexible at low temperature; adhesives and tapes have minimum application temperatures | Seam and tape adhesion failure over time; shrinkage pulling at perimeter details | Simple large roofs with long uninterrupted runs and limited penetrations |
| Built-up roofing (BUR) | Multiple bitumen-saturated felt plies with gravel or a cap-sheet surfacing | Hot asphalt between plies, fully adhered | Heavy multi-ply build is robust; the aged asphalt becomes brittle and cracks with thermal cycling | Alligatoring and crazing of the surface, blisters between plies, loss of gravel surfacing | Existing older commercial buildings where the assembly is being repaired rather than replaced |
| Low-slope and standing-seam metal | Formed steel or aluminium panels over purlins or a deck | Mechanically seamed or snap-locked ribs, or exposed fasteners | Sheds snow on a pitch; large thermal movement demands expansion provision; ice dams at eaves and gutters | Fastener washer failure and elongated holes; seam capillary back-up; corrosion at laps and cut edges | Pitched commercial elevations and older industrial structures; see our metal roofing page |
| Coating and restoration systems | Liquid-applied acrylic, silicone or polyurethane over an existing sound membrane, often with reinforcing fabric at details | Monolithic, seamless once cured | Application requires temperature and dew point windows, so the season is limited in Ontario | Coating applied over a wet or deteriorated assembly, which traps moisture and hides the problem | Sound, dry roofs where the goal is extending service life rather than replacing |
Relative longevity is a fair question and an unreliable number. In practice, a well-detailed two-ply SBS or a welded single ply on a dry assembly with working drainage will outlast a better membrane on a ponding roof with tired flashings. Drainage, detailing and maintenance move service life more than the choice of sheet does.
SBS modified bitumen, TPO, PVC and EPDM in practice
SBS modified bitumen
The workhorse of GTA commercial roofing. Two plies mean a puncture in one does not put water in the building, which matters on roofs where service technicians, cleaners and other trades are on the surface constantly. The granulated cap takes abuse. It is installed by torch, by hot mopping or self-adhered; on buildings where open flame is a concern we specify self-adhered or mechanically fastened base sheets instead.
TPO
Thermoplastic, so seams are hot-air welded into a continuous sheet. On a big open warehouse field, a crew covers ground fast, and the weld is as strong as the sheet when it is done properly. The risk is concentrated in that word properly: cold welds and unprobed seams are the most common defect, which is why we probe seams as work proceeds.
PVC
Also welded, and the one to specify where anything greasy or chemical lands on the roof. Kitchen exhaust plumes will degrade some membranes and their seams within a few seasons; PVC is formulated for it. It stays weldable, which makes tie-ins and future repairs clean.
EPDM
Rubber, supplied in wide sheets so a simple roof has very few seams to begin with. Flexible in cold weather. Its seams are taped or adhered rather than welded, so seam preparation and cleanliness at installation matter a great deal, and seams are the first thing we check on an aged EPDM roof.
Whichever is chosen, repair strategy differs by chemistry, which we cover on flat roof repairs. Asphalt-based and thermoplastic materials do not bond to each other, so repairs must be made in compatible material.
How do you choose a system for a specific building?
Work through these in order and the field usually narrows to one or two.
- What is under the roof? A production line, a server room or retail stock changes the tolerance for risk and the value of a redundant two-ply assembly.
- How much rooftop equipment is there? Many curbs and penetrations favour a system that details easily and takes traffic. Few penetrations favour a fast single ply.
- What lands on the roof? Grease and chemical exhaust point to PVC. Clean air leaves the field open.
- What is the deck, and what is its condition? Attachment method, fastener holding and whether a recover is even possible all follow from this.
- What is the interior humidity load? This sets the vapour barrier, and getting it wrong soaks the insulation from the inside.
- How will the roof be maintained? A roof that will get scheduled attention under a maintenance program can be specified differently from one that will be ignored until it leaks.
- What are the site constraints? Open flame restrictions, occupancy, access for cranes and hoists, and the season the work will run in.
Ontario context sits over all of it: freeze-thaw cycling working every unsealed lap, ground snow load Ss and rain load Sr set by location in the Ontario Building Code Supplementary Standard SB-1, drifting against parapets and screens, and ice damming at cold perimeters.
One practical note on procurement: specify the assembly, not the brand. A specification that names the deck attachment, the vapour barrier, the insulation type and thickness, the tapered layout, the cover board, the membrane and the flashing details lets you compare quotes that are actually comparable. A specification that names only a membrane invites four different roofs at four different prices, and the lowest priced of them is usually the one missing the drainage work.
Where do you go from here?
If you are deciding between repairing and replacing, start with a commercial roof inspection, because the condition of the insulation and deck decides that question, not the membrane on top. If replacement is already settled, the process and the code-driven upgrades are set out on commercial roof installation and replacement and, for low-slope specifics, flat roof replacement.
Warehouses, plants and distribution centres have their own constraints, covered on industrial roofing. Panel systems are on commercial metal roofing. To see how the main variables move a budget range, use the flat roof cost calculator, then call 416-456-0777 for a free roof assessment. Our full service range is on commercial roofing.
If you are still weighing two systems, bring the shortlist to the assessment visit. The roof itself, its drainage and its equipment usually settle the argument faster than a specification sheet does.
Frequently asked questions
Which flat roof system lasts the longest?
No system wins that on its own. Service life is driven by drainage, detailing, foot traffic and maintenance far more than by the membrane. A two-ply SBS or a properly welded single ply on a dry assembly with working drains will outlast any membrane on a roof that ponds. Fix the drainage first and the membrane choice matters less.
Is a white roof membrane better than a black one?
They are different tools. White thermoplastic membranes reflect more sunlight and are common on large single-ply roofs; black membranes shed snow and ice faster and hide dirt. The right choice depends on the building, the mechanical equipment on the roof and how the assembly is insulated, which is a design conversation rather than a colour preference.
Can we put a new membrane over the existing one?
Only if the deck is sound, the existing insulation is dry, and the number of existing roof layers permits another. A recover saves disposal and time on a live building. Over wet insulation it traps the moisture permanently and hides the deck from inspection, which is why a moisture survey comes before the decision.
Are roof coatings a real alternative to replacement?
On a sound, dry, structurally intact roof, a coating or restoration system can extend service life and buy time in a capital plan. On a wet assembly it is money spent to hide a problem, because the moisture stays under it and keeps working on the insulation and deck. The moisture survey decides which case you are in.
What is ballast, and should we keep it?
Ballast is stone or pavers laid loose over a membrane to hold it down and shield it from sunlight. It works, but it adds load, makes leak tracing slow because the stone must be moved, and can be scoured in high wind at exposed corners. Most ballasted roofs we replace are rebuilt as adhered or mechanically fastened assemblies.
Does the flat roof system affect our insurance or code compliance?
It can. Fire classification, wind uplift resistance for the roof zone, and the insulation levels required by the Ontario Building Code all form part of the specification, and some insurers have views on membrane type and on hot work. We set out the specification in writing so it can be reviewed by your insurer or consultant before the work is ordered.
Ready for your free roof assessment?
Tell us the address, the roof size and what you are seeing. We will book a time to get up there and put it in writing.
Call 416-456-0777 for anything urgent. Our emergency line is answered around the clock.
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