Retaining walls fail for predictable reasons. Not because of bad luck or unforeseeable conditions, but because specific details were skipped or undersized. The frustrating part is that those details are invisible in the finished wall — you can't look at a retaining wall and tell whether the drainage is right or the footing is deep enough. You find out years later when the wall moves.
Why Retaining Walls Fail
Inadequate drainage is the leading cause of retaining wall failure on the North Shore. Soil that is saturated with water is dramatically heavier than dry soil. Hydrostatic pressure can be enormous. A wall designed to hold back dry soil without drainage provision will eventually fail — sometimes gradually, sometimes suddenly during an extreme rainfall event.
Insufficient footing depth and size. A footing must be deep enough to get below the frost line and wide enough to distribute the load without settlement. A wall with an undersized footing settles unevenly over time, cracking and leaning as sections move differently.
No deadman anchors. For taller walls built with interlocking block or timber, deadman anchors — lengths of block or timber extending perpendicular into the retained soil — are critical to preventing the wall from being pushed forward. Walls without deadmen rely on block weight alone, which is insufficient for any wall of meaningful height.
Incorrect batter. Retaining walls should lean slightly into the retained soil — a backward lean called batter. A wall built perfectly vertical has less inherent resistance to lateral pressure. A wall that bows forward has none.
Poor base preparation. The base course is the foundation of everything above it. If not properly levelled, compacted, and set, the wall above will reflect those problems at every course.
The Materials
Interlocking Concrete Block
The most common residential retaining wall material. Available in a wide range of sizes, profiles, and appearances; installs without mortar. For walls under approximately 1.2 metres (4 feet) of exposed height, interlocking block walls can typically be designed and built by a qualified contractor without a structural engineer's involvement, following manufacturer specifications. For walls above 1.2 metres — or supporting a surcharge such as a driveway or structure — engineering is warranted. The wall's drainage system is critical regardless of height.
Poured Concrete
The strongest and most permanent retaining wall system. Appropriate for taller walls, walls supporting significant loads, and situations where the wall is an integral structural element such as a basement wall or a wall supporting a structure above. Poured concrete retaining walls require engineering — wall thickness, reinforcing steel size and spacing, footing dimensions, and drainage provisions are all design elements that need to be calculated for the specific application.
Timber
Timber retaining walls were common on North Shore properties through the 1980s and 1990s. They have a limited lifespan relative to concrete systems — even pressure-treated timber in ground contact eventually deteriorates, and older timber walls that are 20–30 years old are frequently at or past end of life. For any significant structural application — walls over 1 metre, walls supporting driveways or structures — timber is not the appropriate choice.
When You Need an Engineer
In British Columbia, the general principle: walls over approximately 1.2 metres of exposed height typically require engineering; walls of any height supporting a surcharge require engineering; walls in geotechnically sensitive areas may require engineering regardless of height; walls adjacent to property lines or public right-of-way typically require engineering and a permit regardless of height. Structural engineering for a residential retaining wall typically includes calculating the forces acting on the wall and producing stamped drawings that the building inspector and contractor work from.
The Drainage Detail
Because drainage is the most common cause of retaining wall failure, it deserves specific attention. The standard drainage provision involves three elements:
Drainage aggregate — a layer of open-graded crushed rock (typically 19mm or ¾-inch clear crush) immediately behind the wall face, extending the full height and length of the wall. Filter fabric — a geotextile between the drainage aggregate and the native soil, preventing fine particles from migrating into the aggregate and blocking it over time. Without filter fabric, even a correctly installed drainage system can silt up over years. Drainage outlets — weep holes through the base course or a perforated pipe at the base of the aggregate layer connected to a proper discharge point.
The drainage provisions behind the wall can also be overwhelmed if roof drainage or surface runoff is directed toward the wall. Managing these sources is part of the complete drainage solution.
Construction Sequence
On a properly executed retaining wall project: site assessment and engineering (if required) before design is finalized. Excavation to the depth required for the footing. Base preparation — compacted to a firm, stable surface. First (base) course installation — the most critical course, levelled in both directions, properly positioned for batter. Wall construction and drainage installation simultaneously — aggregate placed as the wall rises, with filter fabric against the native soil and deadman anchors at specified intervals. Compacted backfill placed in lifts. Discharge provisions confirmed to be flowing before the job is complete. Final grading directing surface water to the drainage system.
A Note on Permits
Retaining walls of significant height typically require a building permit in all North Shore municipalities. Walls above approximately 1.2 metres generally require a permit and engineering in DNV, City of North Vancouver, West Vancouver, and Vancouver. Unpermitted retaining walls create the same complications at sale as any other unpermitted work — and a failed retaining wall is a significant liability, particularly if it damages adjacent property.
Frequently Asked Questions
Why do retaining walls fail?
Inadequate drainage is the leading cause on the North Shore — saturated soil is far heavier and creates enormous hydrostatic pressure. Insufficient footing depth is the other common cause. Both are invisible in the finished wall.
When does a retaining wall need an engineer?
In BC, generally walls over about 1.2 metres (4 feet) of exposed height, any wall supporting a surcharge, walls in geotechnically sensitive areas, and walls near a property line or right-of-way typically require engineering — and often a permit.
What's the drainage detail that matters most?
A layer of open-graded clear crushed rock (typically 19mm / ¾-inch) behind the full height and length of the wall, a filter fabric between the drainage rock and retained soil, and a path for collected water to reach an outlet.
Do retaining walls need a permit?
Walls of significant height — generally above about 1.2 metres — typically require a permit and engineering across North Shore municipalities. An unpermitted wall creates the same sale complications as any unpermitted work, and a failed wall is a safety issue.