Photo of large wall during below-grade waterproofing process.


Ensuring Building Resilience with Below-Grade Waterproofing

09 October 2023 Mallory Buckley

This article explores the critical factors, key considerations, and significance of below-grade waterproofing structures.


When it comes to constructing or renovating buildings with subterranean portions, one often overlooked but absolutely critical aspect is below-grade waterproofing. This continuous membrane system acts as a barrier, preventing water from infiltrating to the interior and should be integrated with the entirety of the building envelope.

Below-grade areas that may require waterproofing include plazas, planters, vaults, tunnels, below-grade building extensions, and foundation walls, as well as the ground floor slabs and elevator pits. This article explores the critical factors, key considerations, and significance of below-grade waterproofing structures.

Factors influencing below-grade waterproofing

  1. Site Conditions: Geographic location and climate play a significant role. Areas with heavy rainfall or a high-water table demand more robust waterproofing systems.
  2. Ground Water Level: The depth of the water table is a crucial factor. Lower water tables exert less hydrostatic pressure on the waterproofing system, while higher water tables require stronger defenses. Ground water quality and contaminants should also be considered.
  3. Soil Characteristics: The type of soil affects drainage and water penetration. Understanding the soil composition and presence of soil contaminants is vital for developing a successful waterproofing plan.
  4. Structural Foundation: The design and condition of the building’s foundations, whether deep or shallow, are critical. Proper integration of waterproofing with the foundation is essential.
  5. Construction Sequencing: Construction sequencing may influence items like backfilling and soil retention systems. Understanding the construction project’s timeline can guide the selection of waterproofing products with application methods appropriate for the sequencing.

Damp-proofing versus waterproofing

Damp-proofing and waterproofing—full waterproofing membranes—may sound similar, but they serve different purposes. Damp-proofing provides control against vapor diffusion; however, it is not intended to block the passage of water through a surface or structure in hydrostatic conditions. Full waterproofing membranes create a watertight barrier offering comprehensive protection against water intrusion, including in hydrostatic conditions. Therefore, full waterproofing membranes are required when a sub-grade space is within water table levels per the International Building Code.

Figure 1

Positive side vs. negative side waterproofing

Positive side waterproofing involves applying the membrane on the exterior of the structure to prevent water from entering. It is ideal for new structures and provides the first line of defense against water intrusion. By stopping water at the exterior, it prevents deterioration of the building’s foundation and structure.

Conversely, negative side waterproofing is implemented on the interior of a building, which makes it suitable for existing buildings where exterior access may be limited. While negative side waterproofing is a cost-effective retrofit option, it does not prevent initial water entry, may allow some seepage that must be managed, and may require ongoing maintenance.

The choice between the two approaches depends on factors like construction phase, site conditions, and project objectives, with each offering its distinct pros and cons.

Figure 2 v2

The consequences of inadequate below-grade waterproofing

Insufficient or improperly installed below-grade waterproofing can lead to several problems:

  • Water Intrusion: This can cause structural damage and create an environment conducive to biological growth.
  • Moisture Vapor Drive: Water vapor can permeate walls and floors, resulting in moisture-related issues, particularly in areas with sensitive finishes.
  • Temperature and Humidity Control: Inconsistent waterproofing can cause fluctuations in indoor temperature and humidity levels, impacting occupant comfort.


In conclusion, below-grade waterproofing is a critical investment in a building’s longevity and functionality. It safeguards against water intrusion, moisture-related problems, and energy inefficiency. To ensure the success of any construction or renovation project, it is essential to consider site-specific factors, choose the right waterproofing method, products, and detailing, and work with experienced professionals who understand the importance of a watertight below-grade envelope. By prioritizing below-grade waterproofing, building owners can achieve a dry, comfortable, and durable subterranean space that stands the test of time.

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