The most useful way to think about a basement stairwell is that it has two separate water problems, and each needs its own solution. Mixing them up is the root of most failures.
A stairwell needs a defence against each, and as explained below, the two collection systems must stay separate.
This distinction costs homeowners a lot of money.
They look similar on a wall on installation day. They perform very differently in year three. When a quote says the stairwell will be waterproofed, ask which product, at what thickness and rated for what. A named product on the quote makes the difference.
The failure sequence in an unprotected stairwell is remarkably consistent:
| The cost gap: Remediating a finished legal suite (removing and replacing wet framing, insulation, drywall and flooring) typically runs $8,000 to $25,000, plus lost rent and a displaced tenant. Waterproofing correctly during construction runs roughly $2,000 to $5,000. |
The cheapest defence is the one most often skipped:
Every litre kept out of the stairwell is a litre the drain does not have to handle.
The soil-facing sides of the stairwell walls get a membrane before any backfill goes in. It must run the full height, from the base of the wall to above finished grade, and stay continuous across every joint and penetration. "Before backfill" is the key phrase. Once soil is against the wall, the opportunity is gone, and no interior treatment can recover it.
A dimpled drainage board goes over the membrane before backfill. It protects the membrane from punctures during backfilling and creates a drainage plane so water runs down to the subsoil drain instead of sitting against the wall.
Perforated pipe at footing level, set in clear stone and wrapped in filter fabric, collects what the drainage board delivers and ties into the foundation drainage system. This handles the water from the soil.
A drain at the low point of the stairwell floor handles the water from above. The floor should slope to it at a minimum 2% fall away from the door sill. It needs a cover rated for foot traffic and an accessible cleanout.
This is where otherwise competent builders get tripped up, and the rules were clarified in the 2024 Ontario Building Code. The Code requires an area drain at a stair open to the exterior. Water collected by that drain is storm water and must be directed to a storm drainage system or an approved storm water disposal location. Two consequences contradict advice often heard on job sites:
The area drain cannot discharge into the weeping tile. Subsoil drainage pipes are not a storm drainage system. Tying an area drain into them overloads the foundation drains and clogs them with silt and leaves, degrading drainage for the whole house. This is the most common wrong answer, and it turns a stairwell problem into a foundation problem.
The area drain cannot discharge to the sanitary sewer. Toronto’s sewers by-law prohibits discharging storm water or groundwater to a sanitary or combined sewer, and other GTA municipalities have similar rules. Toronto has also had a foundation drainage policy since 2022 that pushes new development to manage this water on site. Connecting storm flow to sanitary lines also overloads the system during heavy rain and can back sewage up into basements.
| Homeowner rule: Ask your contractor exactly where the stairwell drain discharges, and confirm it with your municipality before the slab is poured. A drain that connects to nothing, or to the wrong system, is not a drain. |
A sump pump only works while it has power. The heaviest rain events in the GTA often arrive with the storms most likely to knock out electricity, so the pump fails at exactly the moment it is needed. On a stairwell sump serving a tenant’s only exit, these are essential, not optional:
Even when water reaches the door, it should not get in. The door is part of the waterproofing system:
Most stairwell water damage enters at this single detail, and it deserves far more attention than it usually gets.
Where the new stairwell wall meets the existing foundation is the most vulnerable point in the entire assembly. It is a cold joint between concrete poured decades apart, subject to differential movement, running from footing to grade right beside the door. It needs deliberate treatment: a waterstop or keyed joint at the pour, appropriate sealant, and the membrane carried continuously across the joint and lapped onto the existing foundation rather than stopping at the edge of the new work. If a contractor cannot explain how they treat this joint, take note.
Interior hydraulic cement or crystalline products. These can reduce water transmission through sound concrete, but they fight hydrostatic pressure from the wrong side and do nothing about water collecting in the stairwell from above.
A stairwell cover or grate. Useful for reducing rain and snow, and worth considering, but it does nothing about groundwater and cannot replace a membrane or drain. Any cover must also maintain required headroom over the stair and must not obstruct the exit.
The entire waterproofing system disappears the moment soil goes back in. Nobody can inspect it afterward: not you, not a future buyer’s home inspector, not an insurer after a claim. Take dated photos of the membrane, joint treatment, drainage board, subsoil pipe and drain connection before backfilling, and keep them with your permit documents. It takes five minutes and is the only proof the work was done.
At Maple Leaf Quality Renos, every below-grade stairwell we build includes a full exterior membrane, drainage board, subsoil drain and an area drain with a confirmed, permitted discharge point, all documented in photographs before backfill.
Contact us for a free site assessment.
📞 +1 (647) 496-3360
📧 contact@mapleleafqualityrenos.ca