What Interior Basement Waterproofing Costs in Toronto 2026

A 900 square foot basement in a semi-detached house on Woodbine in East York needs interior waterproofing. The owner pulls three quotes: one at $8,200, one at $14,500, one at $22,000. All three contractors are licensed, all promise a dry basement, and all include a sump pump. The difference sits in the membrane system, the drain detail, whether structural drawings are part of the price, and what the permit actually covers. In 2026, most interior waterproofing projects across Toronto and the GTA land between $8,000 and $15,000 for a typical semi, with the final number determined by the square footage, the drainage strategy, and whether you are tying into exterior work at the same time.

Interior waterproofing is the approach when exterior excavation is blocked by a shared driveway, a deck over the foundation, mature trees with roots against the wall, or a neighbour's structure sitting six inches from your foundation line. It does not replace exterior drainage, but it catches water that makes it through the foundation wall and routes it to a sump before it reaches your finished floor. The work usually includes a perimeter trench inside the slab, a drainage membrane on the foundation wall, weeping tile or perforated pipe in the trench, a sump pit and pump, and a new concrete floor poured over the system. The membrane can be a dimpled polyethylene sheet, a spray-applied coating, or a cementitious waterproofing compound, each with a different price and a different warranty.

What interior waterproofing includes in a Toronto basement

The standard scope starts with breaking out a strip of concrete slab around the perimeter, typically 300 to 450 millimetres wide, down to the footing. A perforated pipe or weeping tile goes into the trench on a bed of clear stone, sloped toward a sump pit that sits at the low corner of the basement. The foundation wall gets cleaned and a drainage membrane is fastened or applied from the footing up to about 200 millimetres above the expected finished floor height. Water that seeps through the wall runs down the membrane into the trench, then into the sump, where a pump lifts it to a discharge line that exits above grade away from the house. The trench is backfilled with more clear stone, covered with a filter fabric to keep concrete fines out of the pipe, and a new slab is poured to match the original elevation. Most quotes include a battery backup sump pump or an alarm, because a power outage during a storm is exactly when the system has to work.

The membrane choice moves the price. Dimpled polyethylene sheets run $3 to $6 per square foot of wall installed, spray-applied rubberized membranes $8 to $14, and cementitious crystalline coatings $6 to $10. Contractors who specialize in waterproofing lean toward spray or crystalline systems because they seal cracks and penetrations better than sheet goods, but they cost more and take longer to cure before the concrete pour. If you are planning a basement renovation that includes new plumbing or electrical in the slab, coordinate the waterproofing and the rough-in so the concrete only gets poured once.

Per square foot ranges for membrane and drainage in 2026

Interior waterproofing is priced by the linear foot of perimeter and the square footage of wall, not by total basement area. A rectangular basement has less perimeter per square foot than an L-shaped one, so two basements with the same floor area can have different quotes. In 2026, the installed cost for a complete interior perimeter system runs $85 to $150 per linear foot of foundation wall, which includes trenching, pipe, membrane, sump pit, pump, and the new concrete. A basic dimpled membrane with a single sump and no backup pump lands at the low end. A spray-applied membrane with a battery backup sump and a secondary discharge line sits at the top. The per square foot number for wall coverage alone runs $18 to $35 depending on the membrane and the wall prep required, but most quotes bundle everything as a perimeter price.

Concrete removal and replacement add $12 to $22 per square foot of slab, so a project that trenches the full perimeter but only breaks out a 400 millimetre strip costs less than one that removes half the floor to reroute drains or add underfloor insulation. If the existing slab is already cracked and settling, some contractors recommend a full replacement, which brings the total closer to $28,000 to $40,000 for a 900 square foot basement once you include grading and compaction. That number overlaps with exterior excavation pricing in cases where access is open, so the decision comes down to which side of the wall you can reach and whether you want the work done from inside while the house stays occupied.

A worked example on a 900 square foot semi-detached basement

A semi-detached house on Glebeholme in the Danforth has a basement that measures roughly 30 feet by 30 feet, so 900 square feet of floor and 120 linear feet of foundation wall. The owner wants interior waterproofing with a spray-applied membrane, a primary sump with a backup pump, and a new concrete floor over the trench. The contractor prices the perimeter drainage and membrane at $110 per linear foot, so 120 feet times $110 equals $13,200. The sump pit, primary pump, and battery backup add $2,800. Concrete removal and replacement for a 400 millimetre perimeter strip, roughly 160 square feet of slab, runs $18 per square foot, adding $2,880. Subtotal before permits and tax: $18,880.

A building permit for interior waterproofing in Toronto typically costs $800 to $1,600 depending on whether the scope includes structural changes or new plumbing rough-ins that need separate inspection stages. This project is drainage only, so the permit lands at $950. HST at 13 per cent on the construction and permit total adds $2,570. The contract includes a 10 per cent contingency for unforeseen conditions like a cracked footing or a clay plug in the existing drain tile, adding $1,890. Final contract price: $24,290. The owner decides to skip the full slab replacement and the number drops to $18,600 plus permit, tax and contingency, landing at $22,100. That is the range: $18,000 to $24,000 for a typical Toronto semi with interior waterproofing, depending on how much concrete comes out and which membrane goes in.

Permit and inspection fees in Toronto for waterproofing work

Interior basement waterproofing that includes a new sump pump and discharge line usually needs a building permit in Toronto, because you are adding or altering plumbing and drainage. Breaking out concrete and pouring a new slab does not need a permit on its own, but the moment you connect a sump pump to a discharge that exits the building envelope, the Building Code applies and the municipality wants to see the work. The permit fee for this type of alteration typically runs $800 to $1,800, and the application needs a site plan showing the discharge location, a cross-section of the drainage detail, and confirmation that the pump discharges at least 1.8 metres from the foundation and away from the property line. If you are also adding a backwater valve to protect against sewer backup, that requires a separate plumbing permit and an inspection before you cover the valve.

Toronto's Basement Flooding Protection Subsidy Program offers up to roughly $3,400 toward a backwater valve, sump pump, and pipe severance, but you need permits and inspections to qualify for the rebate, and the work must be done by a licensed contractor. The application goes in after the permit is issued and before the work starts. Most waterproofing contractors are familiar with the program and will walk you through it, but the subsidy does not cover the membrane or the concrete, only the pump and the valve. If your project is part of a larger basement renovation that includes framing, electrical, or a bathroom, the waterproofing permit can be bundled into the main building permit, which saves a separate application fee but extends the review time because the examiner is checking the whole scope.

When you need structural drawings for foundation work

Interior waterproofing that trenches along the footing and installs a drainage membrane does not usually require an engineer's stamp, because you are not altering the structure of the foundation. The footing stays in place, the wall stays in place, and the new concrete slab is not load-bearing. The permit examiner wants to see that the trench does not undercut the footing and that the sump pit is not dug so deep that it weakens the base, but those details can be shown on drawings prepared by a qualified designer with a BCIN. If the project includes underpinning to lower the basement floor, or if you are removing part of the foundation wall to add an egress window or a walkout, then you need an engineer's stamp and the cost goes up by $2,500 to $6,000 for the structural drawings and site reviews.

Semi-detached and row houses add another layer. If your waterproofing trench runs along a shared foundation wall, the engineer needs to confirm that the excavation will not destabilize the party wall, and you need to give your neighbour written notice before the work starts. Most interior waterproofing projects trench only on the three exterior walls and leave the party wall alone, which avoids the engineering and the neighbour conversation. If the shared wall is the one that leaks, exterior excavation from your side may not be possible, and you are back to interior drainage with a membrane on your side of the wall and a frank conversation about whether the problem is being solved or just managed.

What moves the price up: French drains, sump pumps, and exterior tie-ins

The base price assumes one sump pump in one corner. If the basement is large or L-shaped, you may need two sumps, which adds $2,200 to $3,800 for the second pit, pump, and discharge line. A battery backup sump pump adds $800 to $1,500 depending on the pump capacity and whether it includes a monitoring panel that alerts you when the battery is low or the pump has cycled more than a set number of times in an hour, which usually means the primary pump has failed. A secondary discharge line that exits through a different wall adds $600 to $1,200 in pipe and labour, but it is worth it if your main discharge freezes or gets blocked by landscaping.

If you are waterproofing the interior and also replacing the exterior weeping tile on one or two walls where access is open, the two systems need to tie together at the footing so water does not pool between them. The tie-in usually happens through a sleeve cast into the foundation or by core-drilling a hole near the footing level, and it adds $800 to $1,800 per wall depending on how much coordination the contractor needs to do between the interior and exterior crews. Some projects include a perimeter French drain in the yard to catch surface water before it reaches the foundation, which runs $45 to $85 per linear foot depending on depth and stone type, and that number stacks on top of the interior waterproofing cost. If your downspouts discharge at grade next to the foundation, extending them to daylight or into a buried drain adds $400 to $900 per downspout, and it may do more to keep the basement dry than any membrane.

HST, contingency, and the line items people forget

Every waterproofing quote should break out HST at 13 per cent, which applies to the full contract including materials, labour, and permits. On an $18,000 project, HST adds $2,340. Contingency is the line that gets cut when an owner is trying to hit a budget, but it is the line that keeps the project from stalling when the contractor opens the floor and finds a cracked footing, a clay plug in the old weeping tile, or a buried oil tank that nobody mentioned. We include 10 per cent contingency on post-war houses and 12 to 15 per cent on pre-war ones, because the older the house, the more likely the foundation has been patched, the floor has been poured in two stages, or the original drain tile is clay that has collapsed. On a $20,000 waterproofing project, 10 per cent contingency is $2,000, and it usually gets spent.

Permit fees, inspections, and the time to coordinate them are often left out of early quotes, especially from contractors who do not pull permits regularly. A building permit for interior waterproofing in Toronto costs roughly $800 to $1,600, and the application takes two to four weeks to review if the drawings are complete. The inspection happens after the trench is dug and the pipe is in place but before the concrete is poured, so the pour date depends on the inspector's schedule, not just the contractor's. If you are applying for the city's basement flooding subsidy, add another two to four weeks for that application to be reviewed and approved before you start work. We build all of that time into the schedule when we price a waterproofing project as part of a full basement renovation, because a delayed inspection on the drainage can push back the framing, the mechanical rough-in, and the final occupancy by two weeks or more.