Should I sheet-membrane the soil-side of my Vancouver retaining wall?

Sheet membrane (waterproofing) on the soil side of retaining walls is not typically necessary or recommended for standard residential retaining walls in Metro Vancouver. The focus should be on drainage, not waterproofing, because you want water to drain away from the wall rather than be trapped behind it.

Proper drainage design is far more critical than waterproofing for retaining wall longevity in Metro Vancouver's wet climate. The goal is to prevent hydrostatic water pressure from building up behind the wall during our heavy winter rains (October through March). A perforated drain pipe at the base of the wall, surrounded by clear drain rock and wrapped in filter fabric, with an outlet to daylight or the storm drain, is the essential drainage component. This system allows groundwater and surface water to drain away before pressure builds.

Sheet membrane can actually create problems if not properly detailed. If water gets behind the membrane (through the top, joints, or penetrations), it becomes trapped and creates exactly the hydrostatic pressure you're trying to avoid. Membrane systems also require careful attention to drainage at the base and proper termination at the top of the wall. For most residential segmental block retaining walls (Allan Block, Belgard, Techo-Bloc), the blocks themselves are designed to allow some water migration through the joints, which helps equalize pressure.

When sheet membrane might be considered is for basement retaining walls or walls that retain soil directly against a building foundation where you absolutely cannot allow moisture penetration. In these cases, a full waterproofing system with membrane, protection board, and foundation drainage is appropriate. Some engineers also specify membrane for very tall retaining walls (over 8-10 feet) in areas with high groundwater or where drainage cannot be adequately provided.

For standard residential retaining walls in Metro Vancouver, focus your investment on proper drainage infrastructure rather than waterproofing. This includes adequate clear drain rock behind the wall (typically 12-18 inches wide), proper slope of the drain pipe (minimum 1% grade), and connection to an adequate outlet. The combination of free-draining backfill material and functional drainage prevents water buildup that causes wall failure.

If you're building a wall over 4 feet high (requiring engineering in BC), your geotechnical engineer will specify the appropriate drainage and waterproofing requirements based on your specific soil conditions, groundwater levels, and wall height. Follow their specifications rather than adding waterproofing that wasn't designed into the system.

Need help finding a retaining wall contractor who understands Metro Vancouver drainage requirements? Vancouver Interlock can match you with experienced professionals from the Vancouver Construction Network.