Essentials for Underground Waterproofing

"Successful below-grade waterproofing requires more than simply selecting and installing a reputable product," says Brent Anderson, structural engineer and president of Brent Anderson Associates, Inc., a consulting firm in Bloomington, Minnesota.

Underground waterproofing systems require three interdependent lines of defense against water:

  • Careful site planning and landscaping to limit or control the sources of water
  • Backfilling or drainage techniques to reduce the amount of unavoidable moisture
  • Protective barrier system or membrane to eliminate the passage of water

By waterproofing, Anderson means, "the treatment of a structure to prevent passage of water." He says the term waterproofing is often confused with dampproofing. "Dampproofing actually connotes the prevention of dampness, but not full protection from water that may enter the structure.

"In the waterproofing industry, the owner often expects more than what can be done for a given cost and in a given time," Anderson says. Costly and complicated problems arise because of soil conditions, water tables, improperly cured concrete, improperly designed foundations, and poor workmanship.

In addition, groundwater conditions can change. It's very costly to do something about waterproofing later, says Anderson. That's why taking precautions at the beginning of a project is important. Designers, as well as owners, need to understand the complexity of the problem.

Most leaks occur at joints, at structural connections, or at shrinkage cracks in concrete. Anderson recommends the following guidelines for below-grade waterproofing:

  1. Place waterproofing treatment directly on the concrete or wood substrate.
  2. Do not place insulating materials between two waterproofing membranes. This causes water entrapment problems.
  3. Backfill with a granular material compacted to about 88 percent modified Proctor density. Many specs call out 95 percent modified Proctor density for backfill around walls. Experience has shown that using 88 percent induces less stress in walls during backfilling and achieves better drainage.
  4. Seal and caulk all flashings. Where possible, inspect and/or maintain annually.
  5. Be aware of surface temperature and curing requirements of concrete before a concrete product is specified. Many products require a 28-day cure for concrete before application. Water entrapment, lack of adhesion, and solvent evaporation are concerns with temperature and curing.

Edited by Lisa Schuetz

This article is based upon work supported by the University of Wisconsin–Madison Department of Engineering Professional Development. It is for general information and distribution. It is not intended to provide specific solutions or advice for specific circumstances, which should be sought from appropriate professionals.

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