Cost to add drainage behind a retaining wall?

Adding drainage behind a retaining wall in New Brunswick typically costs $15-$35 per linear foot for basic gravel drainage, or $25-$50 per linear foot for a complete French drain system with perforated pipe. The total cost depends on wall height, access, and soil conditions.

Basic Drainage Components and Costs:
The most common approach uses a gravel drainage layer — typically 12-18 inches of clear stone (3/4" crushed gravel) placed directly behind the wall. This costs approximately $15-$25 per linear foot for materials and installation. For a 50-foot retaining wall, expect $750-$1,250 for basic gravel drainage.

A French drain system with perforated pipe adds $10-$25 per linear foot to the base cost. This includes 4-inch perforated drainage pipe, filter fabric, and proper grading to an outlet. The pipe should be placed at the base of the gravel layer and sloped at minimum 1% grade toward a suitable discharge point.

New Brunswick Soil Considerations:
NB's clay-heavy soils in areas like Fredericton and the Saint John River valley make proper drainage absolutely critical. Clay retains water and expands when frozen, creating enormous pressure against retaining walls. In coastal areas with sandy soils, drainage is easier but still necessary to prevent undermining. The frost depth of 4-5 feet in New Brunswick means hydrostatic pressure from frozen groundwater can destroy improperly drained walls.

Installation Factors Affecting Cost:
Access significantly impacts pricing — if equipment can't reach the wall area, hand-digging increases labor costs by 50-100%. Excavation depth matters too; taller walls need deeper drainage systems. Most contractors recommend the drainage layer extend at least 2/3 the height of the wall. For walls over 4 feet, engineering may be required, adding $500-$1,500 to the project.

When to Hire a Professional:
Retaining wall drainage is definitely professional work. Improper installation leads to wall failure, which costs $50-$100 per square foot to rebuild. The contractor needs to ensure proper slope, adequate outlet location, and compliance with municipal drainage bylaws. Many NB municipalities have specific requirements about where drainage water can be discharged.

Additional Considerations:
Factor in filter fabric ($2-$3 per linear foot) to prevent soil migration into the gravel. The drainage system needs a proper outlet — either to a storm drain, natural drainage course, or dry well. Creating an outlet can add $200-$800 depending on distance and municipal requirements.

For existing walls showing signs of water damage (efflorescence, bulging, or cracking), drainage retrofitting is more expensive due to limited access and potential structural repairs needed first.

Need help finding a qualified contractor for retaining wall drainage? New Brunswick Landscaping can match you with experienced professionals who understand NB's challenging soil and climate conditions.