Axcess Roofing Blog
The Hidden Cost of Ponding Water on Flat Roofs:
What Toronto Property Owners Need to Know
There’s a problem sitting on top of thousands of flat-roofed buildings across Toronto and the GTA right now — and most of the owners and managers of those buildings have no idea it’s there.
It doesn’t drip through your ceiling. It doesn’t set off alarms. It doesn’t generate a maintenance request. It just sits there, quietly, doing damage that compounds with every passing week.
Ponding water is one of the most overlooked and misunderstood threats to flat roofing systems in Canada. Property owners often dismiss it because the roof “isn’t leaking yet.” But by the time the water starts showing up inside the building, the damage to the membrane, insulation, and structure is often already significant — and the repair bill reflects it.
After more than 20 years of inspecting, repairing, and replacing flat roofs across Toronto and the GTA, the team at Axcess Roofing & Waterproofing has seen firsthand what sustained standing water does to a roofing system. This article explains what ponding water actually is, why it’s so common in our climate, what it’s doing to your roof right now, and what you can do about it before it becomes a much more expensive problem.
It doesn’t drip through your ceiling. It doesn’t set off alarms. It doesn’t generate a maintenance request. It just sits there, quietly, doing damage that compounds with every passing week.
Ponding water is one of the most overlooked and misunderstood threats to flat roofing systems in Canada. Property owners often dismiss it because the roof “isn’t leaking yet.” But by the time the water starts showing up inside the building, the damage to the membrane, insulation, and structure is often already significant — and the repair bill reflects it.
After more than 20 years of inspecting, repairing, and replacing flat roofs across Toronto and the GTA, the team at Axcess Roofing & Waterproofing has seen firsthand what sustained standing water does to a roofing system. This article explains what ponding water actually is, why it’s so common in our climate, what it’s doing to your roof right now, and what you can do about it before it becomes a much more expensive problem.
Flat roofs aren’t truly flat — or at least, they’re not designed to be. A properly engineered flat roof has a slight slope, typically a minimum of ¼ inch per foot, designed to direct rainwater and snowmelt toward drains, scuppers, or gutters where it can exit the roof surface.
Ponding water — also called standing water — occurs when water remains on the roof surface for more than 48 hours after a rainfall event. That’s the industry-recognized threshold. If your roof is still holding water two days after the last rain, something isn’t working the way it should.
The cause could be as straightforward as a blocked drain. It could be that the insulation beneath the membrane has compressed over time and created a low-lying depression. It could be that the roof structure has settled. Or it could be a combination of all three. Regardless of the cause, the result is the same — water that has nowhere to go, sitting on a surface that was never designed to hold it indefinitely.
Ponding water — also called standing water — occurs when water remains on the roof surface for more than 48 hours after a rainfall event. That’s the industry-recognized threshold. If your roof is still holding water two days after the last rain, something isn’t working the way it should.
The cause could be as straightforward as a blocked drain. It could be that the insulation beneath the membrane has compressed over time and created a low-lying depression. It could be that the roof structure has settled. Or it could be a combination of all three. Regardless of the cause, the result is the same — water that has nowhere to go, sitting on a surface that was never designed to hold it indefinitely.
Why Ponding Water Is So Common on Toronto Flat Roofs
Toronto isn’t a forgiving climate for roofing systems. Our weather creates conditions that make flat roof drainage problems more likely — and more serious — than in many other parts of the country.
Toronto isn’t a forgiving climate for roofing systems. Our weather creates conditions that make flat roof drainage problems more likely — and more serious — than in many other parts of the country.
Freeze-Thaw Cycles
The Greater Toronto Area experiences dozens of freeze-thaw cycles every winter. When water pools on a roof and temperatures drop, that water freezes. Frozen water expands. As it expands, it pushes against membrane seams, flashing edges, and drain collars, widening existing weaknesses and creating new ones. When it thaws, the water — now with an easier path to travel — pushes deeper into those gaps.
Each cycle does a little more damage. Over a season, that incremental damage adds up significantly.
The Greater Toronto Area experiences dozens of freeze-thaw cycles every winter. When water pools on a roof and temperatures drop, that water freezes. Frozen water expands. As it expands, it pushes against membrane seams, flashing edges, and drain collars, widening existing weaknesses and creating new ones. When it thaws, the water — now with an easier path to travel — pushes deeper into those gaps.
Each cycle does a little more damage. Over a season, that incremental damage adds up significantly.
Heavy Rainfall Events
Southern Ontario has seen an increase in high-intensity rainfall events in recent years. When large volumes of rain fall quickly, roof drains that are partially blocked or undersized can’t keep up with the volume of water, leading to temporary — and sometimes prolonged — ponding across large roof sections.
Snow Load and Snowmelt
When snow accumulates on a flat roof and then melts unevenly — which is typical during Toronto’s transitional spring weather — meltwater can pool against parapet walls, around drains, and in any low areas before those drains are clear enough to handle the volume.
Southern Ontario has seen an increase in high-intensity rainfall events in recent years. When large volumes of rain fall quickly, roof drains that are partially blocked or undersized can’t keep up with the volume of water, leading to temporary — and sometimes prolonged — ponding across large roof sections.
Snow Load and Snowmelt
When snow accumulates on a flat roof and then melts unevenly — which is typical during Toronto’s transitional spring weather — meltwater can pool against parapet walls, around drains, and in any low areas before those drains are clear enough to handle the volume.
Structural Settling and Insulation Compression
Older commercial buildings and flat-roofed homes throughout Toronto are subject to gradual structural settling. Over time, this can alter the slope of a roof that once drained effectively. Meanwhile, the insulation beneath flat roof membranes compresses with age and foot traffic, creating shallow depressions that collect water even when drains are functioning.
Older commercial buildings and flat-roofed homes throughout Toronto are subject to gradual structural settling. Over time, this can alter the slope of a roof that once drained effectively. Meanwhile, the insulation beneath flat roof membranes compresses with age and foot traffic, creating shallow depressions that collect water even when drains are functioning.
Blocked and Neglected Roof Drains
This is the most preventable cause, and unfortunately one of the most common. Debris, sediment, biological growth, and even roofing gravel from aging SBS systems can slowly accumulate in drains and drain strainers. A partially blocked drain doesn’t drain — it ponds.
This is the most preventable cause, and unfortunately one of the most common. Debris, sediment, biological growth, and even roofing gravel from aging SBS systems can slowly accumulate in drains and drain strainers. A partially blocked drain doesn’t drain — it ponds.
