Basement Waterproofing in Toronto: Interior, Exterior, and When to Do It

If your foundation is going to be excavated for underpinning, exterior waterproofing at that moment costs a fraction of what it costs as a standalone job, because the expensive part of exterior waterproofing is the digging and you are already paying for it.

That is the most useful thing anyone can tell a Toronto homeowner about waterproofing, and it is the thing most likely to be left out of a quote. This page covers both methods honestly, what each costs here, and when the cheaper one is the right one.

Exterior versus interior, without the sales pitch

Toronto has companies that only sell exterior systems and companies that only sell interior systems, and each will tell you the other is inadequate. Both methods work. They solve different problems.

Exterior waterproofing excavates down to the footing on the outside, cleans and repairs the wall, applies a membrane, adds drainage board, installs new weeping tile at the footing, and backfills with proper drainage material. It stops water at the outside face of the wall, which means the wall itself stays dry.

Interior waterproofing removes a strip of the basement slab around the perimeter, installs weeping tile inside the footing, connects it to a sump pump, and pours the slab back. It does not stop water reaching the wall. It manages the water that arrives and gets it out before it reaches your floor.

Prices vary widely with depth, access and what has to be dismantled and restored, which is exactly why the same house can get a $12,000 quote and a $30,000 quote for what sounds like the same work.

The case for doing it during underpinning

When a crew underpins a foundation, the outside face of the wall becomes accessible in the course of the work. That access is the single most expensive component of exterior waterproofing, and during an underpinning project it is already bought and paid for.

The marginal cost of adding membrane, drainage board and new weeping tile at that moment is a fraction of the standalone price. Doing it two years later means excavating the entire perimeter again, dismantling the landscaping again, and paying for the access a second time.

There is a second reason, and it is about the concrete rather than the money. Underpinning puts new concrete below the original footing and a new slab on the floor. Getting drainage right around that new work, at the moment it is placed, is straightforwardly better than correcting it afterward.

What is actually causing your wet basement

Before buying either system, it is worth ruling out the cheap causes, because a surprising share of Toronto basement moisture is not a foundation problem at all.

• Grading. Soil that slopes toward the house delivers roof and yard water straight to the wall. Regrading is inexpensive by comparison and sometimes solves it outright.
• Downspouts. A downspout discharging within a foot of the foundation is a hose pointed at your basement. Extending them is a weekend, not a project.
• Window wells. Wells without drainage fill up and then find the window. A well drain or a cover is cheap.
• Old clay drain tile. Toronto's older housing stock frequently has clay weeping tile that has collapsed, silted or been invaded by roots. It cannot be repaired in place, but it can be replaced, and replacing it is often the actual fix.
• Condensation rather than infiltration. A basement that is damp all summer but dry in the spring may have a humidity problem, not a water problem. A dehumidifier is not a foundation repair, but it is also not $20,000.
• Plumbing. Slow leaks from supply lines or drains look exactly like foundation seepage to the naked eye.

A contractor who inspects your grading and downspouts before quoting a system is worth more attention than one who quotes a system from the driveway.

Signs that point to a real foundation issue

• Water appearing at the floor-wall joint after rain, consistently and in the same place
• Efflorescence, the white chalky mineral deposit, on the interior of the wall
• Damp or dark patches on the wall that persist regardless of humidity
• Deteriorating mortar in a block or rubble foundation
• A musty smell that returns after cleaning
• Standing water at any point in the basement
• Horizontal cracking with bowing, which is a structural signal and not just a water one and should be looked at promptly

Sump pumps, honestly

Interior systems depend on a sump pump. That is not a criticism, it is a design characteristic, and it comes with implications you should account for:

• It needs power. In the storms most likely to flood a basement, power failure is also most likely. A battery backup or a water-powered backup is not optional equipment on a system your dry basement depends on.
• It is mechanical. Pumps fail. Plan on inspection annually and replacement roughly every seven to ten years, sooner with heavy cycling.
• It has a discharge. Where the pump discharges matters. Too close to the house and the water simply returns.

Toronto has historically run a subsidy program for basement flooding protection covering measures such as backwater valves and sump pumps. Programs and eligibility change, so confirm the current terms with the City before assuming an amount, but it is worth checking rather than leaving money unclaimed.

Backwater valves

A backwater valve addresses a different failure than either waterproofing method: sewage backing up into the house from an overloaded municipal sewer during heavy rain. Waterproofing does nothing about that, and a backwater valve does nothing about groundwater through the wall.

In parts of Toronto with combined sewers and a history of surcharge during storms, a backwater valve is frequently the higher-value purchase of the two, and it has historically been covered under the City's subsidy program. If your basement floods with dirty water rather than clear water, this is your problem, not your foundation.

Common questions