Waterproofing below-grade foundations is straightforward in principle but difficulties can come up during application. This article focuses on issues related to waterproofing foundations in below-water-table construction that is close to the property line, although most of the principles apply equally to other below-grade structures.

Above: Property-line below-grade construction maximizes utilization of a site by building structures adjacent to existing buildings and streets.

Most of the problems that arise in waterproofing are due to one or more of the following:

The project team can solve these problems by considering four important aspects of design and installation:

The proactive approach, therefore, starts with the owner's performance expectations. Often the conversation with the owner is limited to the basic level of performance. The owner does not recognize that “no water entry” requires a design with more than the minimum number of details and some level of redundancy. Owners must be educated so that they can define the expected performance level and accept the construction costs needed to achieve it.

Acceptable performance of the waterproofing system should take into account the building's use and location. Minimal seepage may be acceptable in some cases, such as in subterranean parking garages, while in others, such as laboratories, hospitals, and libraries, absolutely no water entry can be tolerated.

Effective below-grade waterproofing requires that some redundancy be designed into the system, especially when buildings are built into the water table. Redundant design elements can include multiple layers of waterproofing membranes or drainage systems, which can be either interior water management systems or exterior dewatering systems.

Buildings constructed above the water table can more easily achieve high performance levels. For these projects there is usually a permanent exterior dewatering system to remove water that would exert pressure against the exterior wall, which reduces the wall thickness and quantity of reinforcing steel. In these designs there is a built-in redundancy of a primary exterior drainage system with a secondary layer of waterproofing.

The designer is responsible for reviewing the risks and benefits of various designs and allowing the owner to choose the appropriate design approach. In below-water-table construction, using a single layer of waterproofing to result in no interior leaks would require near perfection in both the design and installation, which is impractical to attain or expect. Therefore, depending on the agreed upon level of performance, designers must consider the use of a waterproof system in combination with a drainage system.