Building Stone Retaining Walls in Residential Gardens

Stone retaining walls are among the most common structural elements in Canadian residential landscape design. On sloped lots — particularly common in cities built on glacially shaped terrain such as Hamilton, Vancouver, and Victoria — retaining walls create usable flat areas, manage soil erosion, and define distinct garden zones. Their construction involves more than placing stone: drainage, structural batter, base depth, and material selection each contribute to long-term wall stability in cold-climate conditions.

Dry-Stack vs. Mortared Construction

Dry-Stack Walls

A dry-stack wall is built without mortar. Stones are laid in interlocking courses, with each stone selected for a stable bearing surface against its neighbours. The absence of mortar makes dry-stack walls inherently flexible — they can shift slightly with seasonal frost heave and return to position as temperatures moderate, without cracking. This flexibility is a significant advantage in Canadian climates where freeze-thaw cycles are frequent and ground movement is expected.

Dry-stack walls are generally appropriate for walls up to approximately 900mm (3 feet) in height. Above that height, the structural demands increase and most local building codes require engineered design review. In Ontario, retaining walls exceeding 1.0 metre in height typically require a permit and engineer sign-off under the Building Code Act. Requirements vary by municipality — check with the local building department before construction.

Mortared Walls

Mortared stone walls use Type S masonry mortar (a cement-lime blend rated for freeze-thaw exterior conditions) to bond stones in place. The rigid assembly performs well when built on a concrete footing that extends below the local frost depth, preventing movement that would crack the mortar joints. Without a proper footing depth, mortared walls fail at the mortar joints within a few seasons.

In Ontario, frost depth ranges from approximately 1.0 to 1.8 metres. In most municipalities, the local building department publishes the design frost depth for the jurisdiction. The footing must also extend below any organic material, as organic soils compress and cause differential settlement.

Drainage

Inadequate drainage is the most common cause of retaining wall failure. Water accumulating behind a wall increases hydrostatic pressure — the force of water pushing horizontally against the wall face. In freeze-thaw climates, saturated soil behind the wall also creates frost heave pressure. Both forces push the wall outward and downward over time.

Standard drainage practice involves placing a minimum 300mm layer of clean crushed stone (typically 19mm clear stone) directly behind the wall face, running the full height of retained fill. A perforated drain pipe at the base of this drainage aggregate collects water and diverts it away from the wall base. Drainage outlets should be positioned to discharge to daylight or a storm sewer — never to a foundation.

Batter and Wall Geometry

Batter refers to the backward lean of the wall face into the retained slope. A battered wall face shifts its centre of gravity toward the retained soil, improving stability. For dry-stack walls, a batter of approximately 25–50mm per 300mm of wall height is a commonly referenced range. A wall with 50mm of batter per 300mm of height is noticeably tilted backward — this is intentional and structurally important, not a construction error.

The base of a retaining wall should be wider than its top. A base width of at least half the wall height is a general guideline for dry-stack construction using dense stone.

Stone Selection for Retaining Walls

The best stones for retaining walls have at least one flat face (for the wall face) and sufficient mass to resist the soil pressure. Fieldstone — glacially deposited granite, gneiss, and quartzite — has been used for Ontario farm retaining walls for generations and performs exceptionally well in cold climates due to its low porosity and high density. Cut limestone blocks provide more uniform courses and are common in urban residential settings in southern Ontario. Granite boulders and rubble are used for informal walls in garden settings across British Columbia.

Avoid using sandstone or any porous stone as the primary structural material for mortared walls in Canadian freeze-thaw zones. The mortar bond may remain intact while the stone face spalls away after repeated freeze-thaw cycles.

Terracing Multiple Walls

On steeply sloped lots, a series of lower retaining walls separated by planting terraces is often more stable and visually appropriate than a single tall wall. Each terrace wall retains only a portion of the total grade change, reducing the structural load on any individual wall. A horizontal setback between stacked walls — typically at least equal to the height of the lower wall — prevents the upper wall from overloading the retained soil behind the lower wall.

Permitting and Professional Review

Retaining walls in Canadian municipalities are subject to building permit requirements above threshold heights. As of 2026, most Ontario municipalities require permits for walls over 600mm to 1.0 metre — thresholds vary. British Columbia follows similar patterns under the BC Building Code and local bylaws. Walls built without required permits may need to be removed or retrofitted at the property owner's expense during real estate transactions or insurance claims.

Where walls retain soil adjacent to structures, driveways, or property lines, a structural engineer's review is advisable regardless of permit requirements. The Canadian Society of Landscape Architects directory includes professionals who specialize in grading and retaining wall design.

Last updated: May 25, 2026