“Flat” roofs have a designed slope
The word “flat” is a convenience. Low-slope roofs are built with a gentle fall — created by the structure, by tapered insulation, or both — that directs water to drains, scuppers or gutters. Crickets (small sloped areas) push water around obstructions such as rooftop units and toward drains.
When the slope and drains work together, water leaves the roof shortly after rain or melt stops. When they don't, it collects in low spots.
What counts as ponding?
Small, shallow puddles that disappear soon after the weather clears are common on low-slope roofs. Ponding usually refers to water that stays on the roof for an extended period after precipitation ends — commonly described in the industry as roughly 48 hours. Recurring ponding in the same place is the real warning sign.
Common causes
- Blocked drains or scuppers — leaves, debris, roofing gravel or ice.
- Drains positioned higher than the surrounding roof surface, so water collects around them.
- Insufficient original slope — some older roofs were built nearly dead level.
- Deflection — structure that has sagged over time, often between supports.
- Compressed or wet insulation, which creates dips.
- Obstructions with no cricket to divert water around them.
- Previous roofing layers or repairs that created ridges and dams.
Why ponding matters
- It keeps water in constant contact with seams and laps, increasing the chance of a leak if any detail is imperfect.
- It adds weight. Water is heavy, and ponding can deepen as the structure deflects under it.
- It accelerates wear of some membranes and surface coatings and encourages dirt build-up and vegetation.
- It can affect membrane manufacturer warranties, some of which address ponding specifically.
- In winter it can freeze into ice that blocks drains and dams more water.
Ottawa snow and freeze-thaw
Ottawa's snow and frequent freeze-thaw cycles put drainage under particular strain. Meltwater runs toward drains during sunny days and refreezes overnight, sometimes sealing drains with ice while more melt arrives. Drains in shaded areas, near parapets or under drifts are especially prone to this.
Heavy snow accumulation is also a structural consideration. If you are concerned about snow load, consult a qualified professional — roof snow removal is dangerous and can damage the membrane if done without care, and it should not be done by untrained people.
How drainage problems are corrected
Drainage changes can interact with structure, insulation and plumbing. Some work, especially structural or drainage alterations, may require a permit — see the City of Ottawa's guidance on whether you need a building permit.
- Clearing and maintaining drains, strainers and scuppers — often the simplest and most effective step.
- Resetting or lowering drains, or adding new drains or scuppers where needed.
- Installing crickets to divert water around rooftop units, curbs and other obstructions.
- Adding tapered insulation during re-roofing to create or improve slope.
- Addressing structural deflection, which may require an engineer's input.
Frequently asked questions
Is any standing water on a flat roof a problem?
Not necessarily. Small puddles that dry within a day or two are common. Water that remains for days, or that returns to the same spot after every rain, should be assessed.
Can ponding be fixed without replacing the roof?
Often, yes — by clearing or resetting drains, adding scuppers or installing crickets. Where the whole roof lacks slope, tapered insulation during replacement is usually the lasting fix.
What does a flat roof drain need to work properly?
A clear strainer, a sound clamping ring and membrane connection, a roof surface that slopes toward it, and an open pipe below. Many drainage problems come from one of those being missing or blocked.
Should I sweep or pump water off my roof?
No. Going onto the roof carries fall risk and tools can damage the membrane. Have drains checked by a professional instead.