Why is my basement leaking?
Last updated: By Below-Grade Records Desk
A wet basement is four or five completely different problems that look identical from the top of the stairs, and they range from a $200 downspout extension to a $35,000 excavation. Answering three questions honestly narrows it enough to know which trade to call. The causes ranked below are the City of Toronto's own list, quoted.
Three questions
What that points at
Surface water is being delivered to the foundation
Most likely: Surface water is being delivered to the foundationscore 6 of 13
The roof of an average house sheds an enormous volume of water in a storm, and a downspout emptying at the wall puts all of it straight down beside the footing. Ground that slopes back toward the house does the same thing more slowly.
What fixes it: Extend the downspouts and correct the grade. This is the cheapest fix on the list by a wide margin and it is worth doing before anything expensive.
The City of Toronto lists this as: “poor lot grading or drainage; overflowing eavestroughs; leaking or plugged downspouts”
Price it: Downspout and grading fix cost2. The foundation drains have failedscore 4 of 13
Groundwater is sitting against the footing with nowhere to go, so it comes in at the weakest line: the joint where the floor slab meets the wall. Old clay weeping tile crushes, silts up or roots through, and once it has stopped draining the water has to go somewhere.
What fixes it: New drainage, inside or outside, usually with a sump. This is the expensive diagnosis and the one worth a second opinion.
The City of Toronto lists this as: “weeping tile system (foundation drains) failure”
Price it: French drain and weeping tile cost3. The sewer is backing up into the housescore 2 of 13
Water is arriving through a drain rather than through the building. In a storm the municipal sewer surcharges and the nearest low opening in your house is the basement floor drain. If it only happens when the house itself is using a lot of water, the blockage is on your side of the property line instead.
What fixes it: A mainline backwater valve, and a drain camera first if it happens without rain. The City of Toronto pays up to 80 per cent of the valve, capped at $1,600 a device.
The City of Toronto lists this as: “a sewer back-up caused by a blocked, broken/cracked or overwhelmed sewer lines”
Price it: Backwater valve cost and Toronto rebateThe list of causes is the City of Toronto’s, quoted above against each one. The weights that rank them are this desk’s editorial judgement about how strongly a symptom points at a cause. They are not probabilities and they are not drawn from any dataset. What this can honestly do is stop somebody waterproofing a wall when the water is arriving up the floor drain.
Method
How this tool decides
The causes are not WaterproofingIndex's. Every one is taken from the City of Toronto's Basement Flooding page, and the City's own wording is printed under each result so you can check the tool against the source rather than trusting it: sewer back-up from blocked or overwhelmed lines, weeping tile failure, sump pump failure, a crack or leak in the foundation, walls, windows or door, poor lot grading, overflowing eavestroughs and plugged downspouts.
What is ours is the ranking. Each answer adds fixed weights to one or more causes and the highest total leads. Those weights are an editorial judgement about how strongly a symptom points at a cause. They are not probabilities, they are not derived from any dataset, and this page will not pretend otherwise, because a confidence percentage invented for the look of the thing is exactly the kind of number this site refuses to print.
Where two causes come out level, the tool says they are level rather than picking one. That is a genuine outcome: several of these symptoms overlap, and the honest answer to overlapping symptoms is that somebody has to look.
Questions
Four free things to check before you spend anything
- Stand outside in the rain and watch. Where the water goes on the outside is usually more informative than anything visible on the inside, and it takes ten minutes.
- Follow every downspout to where it ends. Anything emptying within a metre of the wall is delivering a roof's worth of water to your footing.
- Lift the sump float by hand. If the pump does not start, you have found it. The float is the part that fails silently.
- Pencil the date beside the wet patch. Whether it grows, and in which weather, answers more than any single inspection can.
Frequently asked questions
Why is my basement leaking only when it rains hard?
- Two very different causes both do this and they cost wildly different amounts to fix. If the water arrives up the floor drain, the municipal sewer is surcharging and the answer is a backwater valve. If it arrives along the joint where the floor meets the wall, groundwater is stacking up against the footing because the weeping tile has stopped draining. The first is a few thousand dollars with most of it subsidised. The second is five figures. Which one you have is decided by where you see the water, not by how much of it there is.
What does water up the floor drain mean?
- That it is coming from the sewer rather than the ground. The City of Toronto lists sewer back-up caused by blocked, broken or overwhelmed sewer lines as a distinct cause of basement flooding, and the floor drain is simply the lowest opening in your house. If it happens in storms, the main is overwhelmed. If it happens with no rain after the house has used a lot of water, the blockage is on your side of the property line and a drain camera is the next call, not a waterproofer.
Is white powder on the basement wall a leak?
- It is evidence that water has been moving through the wall and evaporating on the inside face, leaving the salts behind. It tells you moisture is passing through, which is why it scores toward drainage failure and cracks on this tool. It does not on its own tell you how much water or how fast, so it is worth watching that wall during the next real storm before spending anything.
Could it just be condensation?
- It could, and it is the one answer that costs almost nothing to act on. Condensation shows up as general dampness on cool surfaces in humid weather, with no source and no pattern tied to rain. If nothing you can describe matches the City's list of flooding causes, this tool says so rather than inventing a cause, because drying the air is a hundredth of the price of waterproofing a wall.
What should I check outside before paying anybody?
- Three things, all free. Where the downspouts empty, because a roof sheds an enormous volume in a storm and a downspout at the wall delivers all of it to the footing. Which way the ground slopes for the first two metres out from the house. And whether the window wells hold water or are packed with soil. The City of Toronto lists poor lot grading, overflowing eavestroughs and leaking or plugged downspouts as causes of basement flooding in their own right.
How reliable is a three-question tool?
- Reliable enough to tell you which trade to call, and not reliable enough to buy from. The list of causes is the City of Toronto's, not ours. The ranking is this desk's editorial judgement about how strongly each symptom points at each cause, and the page says so in those words rather than dressing it up as a probability. Its real job is to stop somebody spending fifteen thousand dollars waterproofing a wall when the water was arriving up the floor drain.
Sources
Where these numbers came from
This is a diagnostic tool rather than a pricing one, so it carries a source for its list of causes rather than for prices. That list is the City of Toronto's, read on 26 August 2026, and each cause is printed with the City's own wording.