Dry-Stone Wall Construction: Techniques for Cold Climates

Dry-Stone Walling Without Mortar

Dry-stone walls are built without mortar. Their structural integrity depends entirely on the weight, friction, and interlocking placement of individual stones. This construction method predates cement by thousands of years, and in garden contexts, it offers specific advantages over mortared alternatives: natural drainage through the wall face, flexibility to absorb frost heaving without cracking, and the ability to support plant growth in the joints.

In Canada's climate, where deep frost penetrates the ground each winter and freeze-thaw cycles can exert significant lateral pressure on structures, dry-stone walls perform better than mortared walls in many situations. A wall without mortar can shift slightly under frost pressure and resettle without catastrophic failure. A mortared wall, once cracked, requires more significant repair.

Structural Principles

Three structural features define a properly built dry-stone wall: batter, through-stones, and hearting.

Batter

Batter is the deliberate backward lean of the wall faces. A properly battered wall is wider at the base than at the top, with each face leaning slightly inward. The standard batter ratio for garden walls is approximately 1:6 — for every 6 cm of vertical height, the wall face recedes 1 cm horizontally. This means a 60 cm tall wall face leans back about 10 cm from base to top.

Batter serves two purposes. It lowers the centre of gravity, making the wall inherently more stable. It also ensures that if the wall moves under frost pressure, gravity tends to push the movement back toward the intended lean rather than outward toward failure.

Through-Stones

Through-stones, sometimes called bond stones or tie stones, are individual stones that span the full width of the wall. In a 40–50 cm wide wall, through-stones lock the two outer faces together and prevent the wall from spreading. They should be placed at regular intervals — every 1–1.5 m horizontally and at least once per metre of wall height.

Through-stones are difficult to split to order, so the wall design should accommodate whatever through-stone material is available locally. Long flat stones — slabs of limestone or granite — work well. In the absence of suitable through-stones, double-coursing with overlapping joints achieves some of the same locking effect.

Hearting

Hearting is the infill material packed into the centre of the wall between the two outer faces. It consists of small angular stones, rubble, or broken pieces from the facing work. The purpose is to prevent the outer faces from tilting inward and to add mass. Hearting should be packed tightly, with no large voids, and should slope slightly toward the centre to direct water away from the faces.

Foundation and Frost Considerations

The foundation is the most important factor in the long-term performance of a dry-stone wall in Canada. Frost heaving occurs when water in the soil freezes and expands, pushing upward. A wall foundation that sits above the frost line will move with soil heaving each winter, potentially misaligning or destabilizing the wall over time.

The practical recommendation is to excavate to below the expected frost depth in the specific location. In southern Ontario and British Columbia, 30 cm may be sufficient. In the Prairie provinces, where frost can penetrate to 1.5 m or more in severe winters, a deeper footing or a thick layer of compacted granular sub-base provides some protection against heave.

The foundation trench should be filled with coarse angular gravel or crushed stone to provide drainage. Free-draining foundations prevent water from accumulating beneath the wall and freezing, which is the primary driver of frost damage.

Stone Selection

Stone for dry-stone walling should be angular rather than rounded. Rounded stones lack the flat faces needed for stable horizontal bedding, and they roll under load. Fieldstone collected from agricultural land is often well-weathered and can work, but it typically requires more care in sorting and placement. Quarried stone, split to rough regular shapes, is easier to work with.

Stone with natural cleavage — limestone and sandstone in particular — splits into workable slabs that bed well horizontally. Granite is more difficult to work because it lacks natural cleavage planes, but its hardness and frost resistance make it a preferred structural material in regions of the Canadian Shield.

Avoid stones that absorb significant moisture, as repeated freeze-thaw cycles will fracture them over time. Highly porous sandstones from certain regions are problematic in this regard. A simple test: stone that crumbles easily when wet should be avoided.

Plants in Dry-Stone Walls

One of the distinguishing features of a dry-stone wall in a garden context is the opportunity to establish plants in the joints. The wall interior provides excellent drainage, some moisture retention from the hearting material, and shelter from frost damage.

Thymus serpyllum (creeping thyme) establishes readily in wall joints exposed to sun. It tolerates drought, cold, and traffic on the wall top, and its low growth prevents it from destabilizing the structure. Cymbalaria muralis (ivy-leaved toadflax) self-seeds aggressively in shaded wall joints and can be encouraged in walls that face north or east. For shaded walls, ferns including Asplenium trichomanes (maidenhair spleenwort) colonize narrow crevices in limestone.

Plants should be introduced during construction — small seedlings placed in the joints as the wall is built — rather than planted afterward. Post-construction planting disturbs the stone placement and rarely establishes as successfully.

Retaining Walls vs. Freestanding Walls

Retaining walls hold back a mass of soil on one side and have different structural requirements from freestanding walls. The back face of a retaining wall is typically unbattered or slightly battered, with the front face battering as normal. Drainage is critical: without weep holes or a rubble drainage layer behind the wall, hydrostatic pressure from saturated soil can overturn a retaining wall even without frost.

In Canadian conditions, a gravel drainage layer at least 20–30 cm wide should run the full height of the wall behind it, with free drainage at the base. In garden retaining walls up to about 60 cm height, this is straightforward. Taller retaining walls — 1 m and above — may require engineering review depending on local regulations.

References and Further Reading

• Dry Stone Walling Association (DSWA)
• Royal Horticultural Society — Dry-Stone Walls
• National Research Council Canada — National Building Code