Commercial Basement — BelowGrade External Waterproofing

Project Context

A new-build commercial development in an urban environment includes a two-storey basement car park and plant room. The basement is constructed below the water table (estimated 2m+ below finished ground level). The design specification requires BS8102 Protection Class 2 (some seepage acceptable, but no structural damage or operational impairment).

The structural engineer specifies a positive-side (external) liquid waterproofing system applied to the concrete walls and floor slab before backfill. This provides a continuous, seamless barrier against hydrostatic pressure — superior to cavity drain or tanking systems when properly applied.

BS8102 Specification

The specification calls for:

• • BS8102 Class 2 protection (positive-side external waterproofing system).
• • Continuous liquid rubber membrane applied to external concrete surfaces (walls and floor slab).
• • Minimum nominal dry film thickness: 1.5mm (typically applied in two coats of 750µm each).
• • All construction joints, penetrations and wall-to-slab junctions detailed and sealed.
• • Primer and base coat applied to clean concrete; top coat applied after curing of base.
• • Third-party inspection and test certificates required (adhesion, thickness, continuity).

System Selection: BelowGrade

BelowGrade liquid rubber is the specified system because:

• Designed for hydrostatic pressure: formulated to resist water pressure from below-grade applications.
• Seamless application: hand-applied to create a continuous membrane with no joints or seams.
• Flexible and robust: accommodates minor substrate movement and construction tolerances.
• Compatible with concrete: forms a chemical bond to clean concrete surfaces.
• Fully specified: Part 3 certification and performance data supporting BS8102 Class 2.
• Rapid application: allows phased basement construction without extended programme delays.

Application Protocol

Once the concrete structural frame and floor slab are poured and cured, the waterproofing phase begins:

• • Concrete surfaces (walls and slab) inspected, swept and power-washed to remove dust, laitance and contamination.
• • All construction joints, cracks and penetrations (service ducts, etc.) identified and prepped.
• • Concrete allowed to dry — minimum 7 days post-pour (or moisture testing if accelerated timeline required).
• • Specialist primer applied to all concrete surfaces (ensures BelowGrade bond and moisture control).
• • Base coat of BelowGrade applied by hand, working systematically across all wall and slab areas.
• • Construction joints reinforced with specialist tape or mesh strip, then top-coated.
• • Final top coat applied to achieve nominal 1.5mm dry film thickness.
• • Third-party inspector attends to verify adhesion, thickness and continuity via holiday testing.
• • Certificate of compliance issued; backfill can then proceed once BelowGrade has fully cured (typically 7–14 days depending on conditions).

Construction Joint & Penetration Detail

Special care is taken at locations of higher risk:

• • Wall-to-slab joints: Reinforced with specialist fabric tape bedded in primer and BelowGrade, then top-coated.
• • Construction joints in walls: All joints sealed with specialist sealant before waterproofing; reinforced tape applied.
• • Pipe penetrations: Boot-detailed with reinforced membrane collar, fully sealed against water ingress.
• • Floor drain sumps: Fully sealed and detailed to allow water collection without structural exposure.