What a retaining wall is actually doing
A retaining wall holds a mass of soil in place that would otherwise spread or slide. Every cubic metre of saturated soil behind that wall weighs somewhere between 1,700 and 2,000 kilograms, and that load does not stay constant. It rises when the soil is wet, rises further when it freezes, and pushes hardest in the spring when the ground is thawing unevenly. The wall you see above grade is only half the structure. The footing, the drainage layer, and the compacted base below grade are doing most of the work.
Segmental block walls, armour stone walls, and palisade walls all rely on the same principle: mass and batter (the slight backward lean of each course) resist the soil pressure. Take away the drainage path behind the wall and that pressure multiplies. Take away the frost depth on the footing and the whole structure moves every winter. Neither problem shows up immediately, which is why walls that were built without these details often look fine for two or three seasons before the first visible lean appears.
If you are already looking at a wall that has started to move, the pages on leaning wall repair and wall rebuilding cover what that repair involves.
Toronto's frost depth and why footings matter
The ground in Toronto and the surrounding area freezes to a depth that varies by winter severity, soil type, and snow cover, but a footing placed at less than roughly 1.2 metres below finished grade is at real risk of frost movement. That number is not a guarantee in either direction: a wet clay site in a hard winter can freeze deeper, and a well-drained granular site under a thick snow pack may freeze shallower. The point is that a footing sitting at 300 or 400 millimetres, which is common in walls built without proper planning, will move every single year.
Frost heave does not push straight up. It pushes in the direction of least resistance, which for a retaining wall often means outward and forward. Each freeze-thaw cycle ratchets the wall a little further from where it started. After five or six winters the lean becomes visible. After ten it becomes a structural concern.
Segmental block walls that rely on geogrid reinforcement extending back into the retained soil are especially sensitive to this, because the grid anchors need stable soil to do their job. If the footing zone is moving, the grid is moving with it.
Drainage behind the wall: the part nobody sees
Water is the primary enemy of any retaining wall. When rain or snowmelt soaks into the retained soil and has nowhere to go, hydrostatic pressure builds behind the wall face. That pressure is additive: it sits on top of the normal soil load and can exceed it during a heavy rain event. Walls without a drainage path behind them are essentially holding back a reservoir every time it rains.
The standard approach is a layer of clean crushed stone, typically 50 to 100 millimetres of clear stone, placed directly against the back of the wall for its full height. A perforated pipe at the base of that stone layer collects the water and carries it to daylight at the ends of the wall or into a channel drain at grade. The geotextile fabric separating the crushed stone from the native soil keeps fines from migrating into the drainage layer over time and blocking it.
On Toronto properties where the lot slopes toward the house, getting that drainage outfall right matters as much as the wall itself. Water that exits the wall correctly but then pools against the foundation has only moved the problem. Grading and slope work is often part of the same project, and channel drain installation at the base of a wall is a common addition when the outfall options are limited.
- Clean crushed stone backfill, full height behind the wall
- Perforated drainage pipe at the base of the stone layer
- Geotextile fabric between native soil and drainage stone
- A clear outfall to daylight or a channel drain at grade
- No fine-grained fill placed directly against the wall face
Municipal rules: height, setbacks, and permits
Most municipalities set a threshold near a metre, above which a retaining wall moves from landscaping into engineered structure territory. Toronto has its own rules, and your specific property may also be subject to conditions tied to a ravine designation, a heritage overlay, or a lot grading plan from the original subdivision. None of those details are uniform across the city, and none of them can be confirmed without checking with the City of Toronto's building division directly.
What is consistent is the general principle: a low wall holding a modest grade change is usually treated as landscaping. A wall above the threshold, or any wall within a certain distance of a property line, typically requires a permit and in many cases a drawing stamped by a licensed engineer. The setback rules for walls near property lines vary by zoning category and are not something to guess at, because a wall built in the wrong location can require removal regardless of how well it was constructed.
Toronto Interlock Repair recommends confirming permit requirements with the City before any wall over roughly a metre is designed, not after. The municipal building department is the right place to start, not a contractor and not a general guide like this one.
Excavation, locates, and what sits underground
A retaining wall that needs a proper frost footing requires excavation. In Toronto, any dig requires a locate request through Ontario One Call before work starts. This is a legal requirement, not a suggestion. Gas lines, water services, electrical conduits, and telecommunications cables all run through residential lots, and their depth is not always where you expect it to be.
The locate process takes a few business days, and a responsible contractor will not schedule excavation without confirmation in hand. If a contractor proposes to start digging immediately without mentioning locates, that is worth asking about directly.
On properties in Etobicoke, North York, and Scarborough, older lots in particular can have utility runs that were installed before consistent record-keeping. The locate confirmation is the minimum; experienced crews also probe carefully in the first few hundred millimetres before bringing in equipment.
Choosing wall materials for a freeze-thaw climate
Segmental concrete block, armour stone, and natural stone palisade are the three most common choices for residential retaining walls in the Toronto area. Each handles freeze-thaw differently.
- Segmental concrete block: consistent sizing makes drainage detailing straightforward; the hollow cores allow some drainage through the face; cap course units need to be properly adhered or they lift seasonally
- Armour stone: large mass resists movement well; individual stones are heavy enough that frost movement is less of a concern for the stones themselves, though the base still needs depth; a leaning armour stone wall is a significant repair
- Natural stone palisade: vertical stones set in a granular base; works well for low walls under about 600 millimetres; above that height the lack of batter and mass makes it less reliable in freeze-thaw conditions
The armour stone and wall caps pages go into more detail on material-specific considerations. For walls that have already moved or cracked, the repair approach depends heavily on which material was used and how far the movement has progressed.
Efflorescence, the white mineral deposit that appears on concrete block walls, is a separate issue from structural performance. It is cosmetic in most cases and does not indicate the wall is failing. Efflorescence removal is a routine maintenance service.
Cost ranges and what moves the number
Retaining wall costs vary more than almost any other hardscape project because the variables compound: wall height, wall length, material choice, frost footing depth, drainage complexity, access for equipment, and whether a permit and engineering drawing are required all affect the final number. A short low wall on an open lot with easy access is a very different project from a tall wall on a narrow side yard that requires hand excavation.
As a general orientation, segmental block walls tend to cost less per square metre of face area than armour stone, but armour stone often requires less ongoing maintenance. Any wall that requires an engineer's drawing adds that professional fee to the project cost. Any wall that requires a permit adds the permit fee and the time for approval.
The retaining wall repair cost page covers the repair side of the equation. New wall construction follows similar logic but adds the footing and drainage work that a repair on an existing wall may not need.