HVAC Curb Flashing Leaks on GTA Flat Roofs: How We Fix Them

If you own a building with a flat roof in the Greater Toronto Area—whether it’s a commercial unit in Mississauga or a modern home in Etobicoke—the HVAC curb is the most common place for a leak to start. During these hot Toronto summer months, your air conditioning unit is working overtime. While you’re feeling the cool air inside, the heat, vibration, and UV exposure on the roof are putting immense stress on the flashing that seals that heavy unit to your structure.

At Rip It Roof It, we often see homeowners and property managers mistake a curb leak for a plumbing issue or a general roofing failure. In reality, HVAC curb flashing is a specific intersection of mechanical work and roofing. If they aren’t talking to each other, you’re the one who ends up with a bucket under the ceiling tiles.

What is an HVAC Curb and Why Does it Leak?

An HVAC curb is essentially a raised metal frame that sits on the roof deck. It acts as a pedestal for the heavy rooftop unit (RTU). The flashing is the material—typically 2-ply modified bitumen, TPO, or EPDM—that bridges the gap between the flat roof surface and the side of that metal frame.

In the middle of a GTA summer, the temperature on a dark flat roof can easily exceed 70°C. This causes the metal curb to expand. Meanwhile, the AC unit is vibrating as the compressor kicks in. This constant physical movement, combined with intense UV radiation that dries out caulking, creates small fissures at the corners of the flashing. When a summer thunderstorm hits, water finds those cracks instantly.

The "Summer Rain" Effect

In Toronto, we get those sudden, heavy downpours in July and August. Because flat roofs are designed to shed water slowly toward drains, a heavy rain can result in "ponding" or temporary standing water. If your HVAC curb flashing is even slightly compromised, that water doesn't need a hole—it just needs a capillary path to get pulled under the membrane.

"Most contractors just slap a layer of silver mastic or caulking over a curb leak. That’s a three-month fix. To actually stop a curb leak, you have to strip the flashing back to the metal and re-integrate it with the roof membrane. Anything else is just stalling."

The Anatomy of a Professional HVAC Curb Flashing Repair

When you contact us, we don't just send a guy with a ladder and a bucket of tar. We target a 47-minute response time for inquiries because we know that a leak during a summer storm can shut down a business or ruin an interior renovation. Our process focuses on the "Why" before the "How."

A proper repair involves a 23-point inspection of the surrounding area. We aren't just looking at the leak; we’re looking at the roof deck, the insulation, the counter-flashing on the unit, and the drainage path. Here is how we actually solve the problem:

• Debris Removal: Summer storms often blow leaves and dirt against the curb, which traps moisture. We clear the area to see the actual state of the membrane.
• Mechanical Check: We ensure the RTU isn't vibrating excessively, which can tear new flashing.
• Stripping Back: We remove the old, cracked sealant and failed membrane layers down to the original curb flange.
• Targeted Re-flashing: Depending on your roof type (likely SBS Modified Bitumen in older GTA buildings or TPO in newer ones), we heat-weld or chemically bond new reinforced flashing.
• Termination Bars: We don't just rely on glue. We use mechanical termination bars to ensure the flashing stays tight against the curb during winter freeze-thaw cycles.

Repair vs. Replacement: The Decision Framework

One of the most common questions we get at Rip It Roof It is: "Can I just patch this, or is the whole roof toast?" In 2026, with material costs stabilizing but labor still at a premium, making the right choice is critical for your budget.

2026 GTA Pricing: What to Expect

Transparency is rare in roofing, but it shouldn't be. For a professional HVAC curb flashing repair in the GTA, prices typically range from $800 to $2,500 per unit. Why such a wide range? It depends on the size of the curb, the type of membrane (TPO is often more expensive to repair than modified bitumen due to the specialized welding equipment needed), and whether we need to coordinate with an HVAC technician to lift the unit.

If you are looking at a full flat roof replacement for a standard 1,200 sq ft commercial footprint or a residential garage/extension, you should budget between $9,500 and $16,000. This includes the removal of old layers, new ISO board insulation, and a high-quality 2-ply system. We provide these ranges so you can spot "too good to be true" quotes that usually involve skipping the very flashing details we're discussing here.

"A cheap quote usually saves money by 'overlaying'—putting new stuff over the old, rotten stuff. In the GTA heat, that trapped moisture will boil, create blisters, and ruin your new roof in two summers."

Why Homeowners in Cities Like Markham and Vaughan Trust Us

We’ve built our reputation on being direct. We are trusted by 126+ GTA homeowners with a 5.0★ Google rating because we treat your roof like a technical challenge, not a sales opportunity. Whether you're dealing with a leak in a century-old building in downtown Toronto or a newer commercial plaza in Vaughan, the physics of water remain the same.

Avoiding the Wrong Contractor

In the summer, many "fly-by-night" crews pop up. They see a leak and offer to "tar it" for $400 cash. Avoid this. If they don't ask about the type of membrane you have or check the moisture levels in your insulation, they aren't fixing the problem—they're hiding it. Ask for their WSIB clearance and their specific experience with flat roof penetrations.

Conclusion

HVAC curb leaks are a nuisance, but they aren't a mystery. They are the result of the extreme Toronto climate meeting a high-stress point on your roof. By understanding the "why" behind the leak, you can make an informed decision that protects your property without overspending on unnecessary replacements.

If you suspect your HVAC curb is failing, or if you’ve seen water spots appearing near your vents after a summer storm, let’s get a real look at it. We offer a 23-point inspection that covers everything from the deck to the drainage. No pressure, just a straight answer on what’s happening up there.

You can reach Rip It Roof It at (647) 558-7310 to speak with a specialist, or book an inspection online. We’ll provide a clear, transparent breakdown of your options so you can get back to enjoying your summer.