Below-grade expansion joint waterproofing: A starter guide for designers

This article was authored by Esper Chao, PE, Project Consultant, Simpson Gumpertz & Heger; and Justin deWolfe, PE, Associate Principal, Simpson Gumpertz & Heger.

Expansion joints play a critical role in building design, especially for underground construction. Structural engineers often require below-grade expansion joints to accommodate the independent movement of adjacent elements such as basement foundations, tunnels, parking garages, or utility vaults. Unlike above-grade systems, below-grade waterproofing and expansion joint design faces higher risks of water intrusion due to hydrostatic pressure, soil movement, and runoff conditions. Because these systems are buried behind concrete and soil, repairs are costly and disruptive, making early planning and proper product selection essential.

This article explores the factors influencing below-grade expansion joint waterproofing design, compares common system options, and offers best practices for architects, engineers, and contractors to improve performance on commercial, institutional, and infrastructure projects.

Understanding Site Conditions

The first step in below-grade waterproofing design is understanding groundwater and soil pressures. Foundations built below the water table are exposed to significant hydrostatic forces, which vary by location and season. Determining the design water table elevation requires a geotechnical report that details soil properties, groundwater conditions, and potential settlement.

Based on this data, project teams should provide structural protection behind the expansion joint gap. This protection resists soil pressure while still allowing for movement. Recommendations in the geotechnical report—such as foundation drainage systems, settlement allowances, and waterproofing strategies—should be integrated into the overall expansion joint design.

Structural Engineering Considerations

Because expansion joints are primarily a structural requirement, coordination with the structural engineer is critical. Key questions to resolve early in design include:

• How much movement is anticipated (horizontal, vertical, and shear)?

• What is the expected gap width, and will joint fillers be used?

• Can the below-grade expansion joint be eliminated altogether if movement is negligible?

If the joint can be eliminated and the foundation doweled together, the waterproofing design becomes far simpler and leakage risks are reduced. Where joints are required, the system must be sized to accommodate the anticipated movement. Manufacturers publish movement capabilities, usually as a percentage of their relaxed dimension, but joints rarely compress to zero. This requires careful coordination between the structural engineer, architect, and product manufacturer.

Occupancy and Risk Levels

The use of space adjacent to a below-grade expansion joint should also inform the waterproofing approach.

• Low-risk spaces (e.g., parking garages, utility rooms): cost-effective waterproofing may be acceptable.

• High-risk spaces (e.g., labs, data centers, operating rooms, or storage areas for sensitive materials): require robust, redundant systems and possibly relocation away from the joint.

By understanding occupancy risk, project teams can align waterproofing investments with performance expectations.

Construction Methods: Positive-Side, Blind-Side, and Negative-Side

Waterproofing installation methods greatly influence system performance. Three common approaches are:

• Positive-side waterproofing. Installed after concrete placement, on the exterior surface. Offers the best access for surface preparation and detailing but requires over-excavation.

• Blind-side waterproofing. Installed against shoring before concrete is placed. Common in zero-lot-line projects but difficult to repair and reliant on proper concrete consolidation.

• Negative-side waterproofing. Installed on the interior face of the foundation. Offers better control than blind-side but requires careful detailing to tie into adjacent membranes.

Each approach requires different detailing and product compatibility. Early coordination between contractors, waterproofing installers, and design teams is essential. Preconstruction meetings, QA/QC inspections, and experienced subcontractors reduce the risk of failures during installation.

Expansion Joint System Options

Expansion joint systems (EJS) are the first line of defense against below-grade water intrusion. Unlike joint fillers, which are not watertight, EJS products must integrate with surrounding waterproofing membranes. Common options include:

Sheet Membrane Bellows

Made from strips of sheet waterproofing membrane, these joints are cost-effective and field-fabricated. However, they lack published performance data, have limited movement capacity, and are prone to field splice failures. Best suited for low-risk conditions above the water table.

Vulcanized Rubber EJS

Factory-fabricated rolls of vulcanized rubber with reinforced flanges. Highly flexible, nearly 100% compressible, and reliable for long-term waterproofing. Installation requires precise field measurements and can delay projects if adjustments are needed. Works best for positive- or negative-side applications.

Thermoplastic EJS

Extruded thermoplastic membranes designed specifically for blind-side applications. They provide strong hydrostatic resistance and can be hot-air welded to adjacent thermoplastic membranes. Factory welds are more reliable than field welds, but limited tolerance can complicate installation.

Clamped Reinforced Rubber EJS

The most robust option, featuring fabric-reinforced rubber clamped to the structure with steel plates. Ideal for tunnels, hospitals, transit facilities, and other high-risk infrastructure projects. Highly durable but expensive, with long lead times and custom detailing requirements.

When selecting a system, evaluate movement capability, hydrostatic resistance, durability, ease of integration with adjacent waterproofing, and long-term maintenance considerations.

Secondary Expansion Joint Waterproofing Options

For high-risk occupancies or critical infrastructure, redundant systems provide an added layer of protection. Secondary strategies include:

• Installing two EJS systems in series.

• Using compressible foam joints designed for buried conditions.

• Injecting chemical grout into reinforcing foam blocks.

• Integrating PVC waterstops with thermoplastic membranes.

• Installing drainage systems tied to sump pumps or stormwater connections.

• Designing interior leakage collection troughs with sump pumps or electronic monitoring.

These systems should not replace the primary joint system but can mitigate leakage if the primary system fails.

Troubleshooting and Long-Term Maintenance

No below-grade system can guarantee a zero-leakage service life. Seasonal groundwater fluctuations, construction damage, and structural movement all present long-term risks. Best practices include:

• Planning for remedial grout injection or interior waterproofing.

• Leaving unfinished interiors where possible for easier access.

• Providing concealed troughs or drains to capture leaks and divert water.

• Adding electronic leak detection in high-risk occupancies.

Conclusion: Best Practices for AEC Professionals

For architects, engineers, and contractors, success in below-grade expansion joint waterproofing comes down to four key practices:

• Design: Assess groundwater, soil pressures, structural movement, and occupancy use. Eliminate unnecessary joints where possible.

• Construction: Coordinate with all trades, choose the right installation method, and require qualified waterproofing subcontractors.

• Product Selection: Match EJS products to site conditions and occupancies, and consider proven systems with manufacturer support.

• Redundancy and Maintenance: Add secondary systems for critical facilities and plan for future leak detection and repairs.

By aligning structural engineering, geotechnical data, and waterproofing best practices, AEC professionals can deliver durable, watertight foundations that support long-term building and infrastructure performance.