Retaining Wall Waterproofing

1. Waterproofing vs Drainage: Critical Distinction

Drainage removes water from behind the wall so it doesn't build up. Waterproofing prevents water from passing through the wall face. For most retaining walls, drainage is essential; waterproofing is optional and sometimes counterproductive.

A waterproofed wall with no drainage system will still experience hydrostatic pressure build-up — the waterproofing membrane may resist moisture ingress for years, but the structural load from trapped water remains. The wall is still being loaded; you just won't see the water coming through.

The correct sequence is: drainage first, waterproofing second (if required). Never waterproof as a substitute for drainage.

2. When Is Waterproofing Required?

Waterproofing a retaining wall is typically specified when:

• The retained face of the wall is adjacent to a habitable space (basement, below-ground room, undercroft)
• The wall is a below-ground structure that must remain dry for building performance reasons
• The retained soil is chemically aggressive and the membrane protects the wall structure
• Specific appearance requirements exist for the exposed face

For purely external retaining walls with no building envelope implications, drainage — not waterproofing — is the primary design requirement.

3. Waterproofing Products for Retaining Walls

Common systems used on retaining wall retained faces:

• Sheet-applied membranes (torch-on bituminous or self-adhesive) — robust, suitable for below-ground applications, require protection board before backfill
• Liquid-applied membranes (polyurethane, cementitious) — good for complex geometry, performance depends heavily on application quality
• Crystalline waterproofing (Xypex, Kryton type) — penetrates concrete and seals cracks by crystal growth; suitable for concrete walls, not masonry
• Drainage composite boards (dimple mats) — technically a drainage product but also provides some separation between soil and wall face

4. Application Requirements

Regardless of product type, correct application requires:

• Clean, sound, and dry substrate at time of application
• Primer applied as specified by manufacturer (most systems require this)
• Laps and terminations correctly detailed — the most common point of failure
• Protection board installed before backfill (for sheet and liquid membranes)
• Drainage board or ag-drain still required behind the waterproofing

Membrane waterproofing is a specialist trade. For walls adjacent to habitable spaces, always use a licensed waterproofer and get the specification in writing from the engineer or architect.

Drainage design for retaining walls →