Flat roofs are everywhere in Ontario: on garages, additions, back porches, townhouses and nearly every commercial building. They're called flat, but they all have a slight slope so water can drain. And because water moves slowly across them, the membrane you choose, and how well it's installed, matters even more than on a sloped roof.
Here's how the three most common systems compare.
Modified Bitumen (SBS)
Modified bitumen is an asphalt-based membrane reinforced with polyester or fibreglass, usually installed in two layers: a base sheet and a granulated cap sheet. Layers are bonded with heat (torch-applied), with adhesive (cold-applied) or with a self-adhered backing.
Pros
- Two-ply system gives built-in redundancy
- Very tough against foot traffic and hail
- Familiar to most roofers and easy to repair
Cons
- Torch application needs careful fire safety; near wood or tight spaces, self-adhered or cold-applied options are safer
- Dark surface absorbs heat unless a reflective cap sheet is used
Typical lifespan: around 20 to 30 years with good installation and maintenance.
EPDM (Rubber Roofing)
EPDM is a synthetic rubber membrane, usually black, installed in large sheets that are glued or mechanically fastened, with seams sealed with tape.
Pros
- Stays flexible in extreme cold, which suits Ontario winters
- Large sheets mean fewer seams
- Long track record on homes and commercial buildings
Cons
- Seams are the weak point if not installed and taped properly
- Black EPDM absorbs heat in summer
- Can be punctured by sharp objects or careless foot traffic
Typical lifespan: around 25 to 30 years or more.
TPO (Thermoplastic Polyolefin)
TPO is a single-ply membrane, usually white, with seams that are heat-welded together into one continuous sheet.
Pros
- Heat-welded seams are very strong
- White surface reflects sunlight and keeps buildings cooler
- Popular for commercial roofs and larger residential flat roofs
Cons
- Needs experienced installers with the right welding equipment
- Quality varies between manufacturers
Typical lifespan: around 20 to 30 years.
Which One Is Right for You?
- Small residential flat roofs (garages, porches, additions): modified bitumen is a durable, easy-to-repair choice.
- Larger residential roofs or roofs with few penetrations: EPDM's large sheets and cold-weather flexibility work well.
- Commercial buildings or roofs where cooling matters: TPO's reflective surface and welded seams are hard to beat.
The right answer also depends on drainage, how much foot traffic the roof gets, what's mounted on it and your budget. For more on costs, see our comparison of flat vs sloped roofs in Ontario.
Why Installation Matters Most
Most flat roof failures we see are not the membrane's fault. They come from:
- Ponding water from poor slope or blocked drains
- Weak seams and laps
- Poor flashing at walls, parapets, vents and skylights
- Wet insulation trapped under a membrane that was installed over it
A good flat roof starts with proper slope, sound decking and dry insulation, then the membrane.
Repair or Replace?
Small leaks, a lifted seam or damaged flashing can usually be repaired. Replacement makes more sense when the membrane is cracked or blistered over large areas, the insulation underneath is wet, or the roof is near the end of its life and leaking in several places.
Get a Flat Roof Assessment
We install and repair modified bitumen, EPDM and TPO flat roofs across Oakville, Burlington, Hamilton and the GTA. Learn more about our flat roofing services, book a free estimate or call 905-616-4408.
Need flat roof repair in Hamilton? We service modified bitumen, EPDM and TPO roofs on homes and commercial buildings there too.
Frequently Asked Questions
Is a flat roof a bad idea in Ontario?
No. Flat roofs work well in Ontario when they have proper slope to the drains, good flashing and a membrane suited to freeze-thaw cycles. Most problems come from poor drainage or installation.
How often should a flat roof be inspected?
At least once a year and after major storms. Clearing drains and scuppers each fall also prevents ponding and ice build-up.
Can you install a new flat roof over the old one?
Sometimes, but it's often not the best choice. If the existing insulation is wet or the deck is damaged, covering it traps moisture and shortens the new roof's life.