Waterproofing Basics for Below-Grade Foundations

Damp-proofing and waterproofing are often used interchangeably in casual conversation, but they are different systems with different performance expectations, and confusing the two is a common source of wet-basement complaints after a build or renovation.

Damp-proofing -- typically a sprayed or troweled asphalt-based coating -- is designed to resist moisture migration through concrete under normal soil moisture conditions. It is not designed to hold back standing water or hydrostatic pressure. Many building codes accept damp-proofing as a minimum for foundations in well-drained conditions, but it is not adequate where the water table is high or drainage is poor.

True waterproofing systems -- sheet membranes, fluid-applied membranes, or bentonite panels -- are designed to resist actual hydrostatic pressure and standing water against the foundation wall. These are required (by code or by good practice) in conditions with a high water table, poor soil drainage, or below-grade habitable space where a wet basement is not an acceptable outcome.

Neither system replaces proper drainage. A functioning foundation drainage system -- weeping tile at the footing, connected to a sump pump or gravity outlet, surrounded by clean drainage stone, with the soil graded to slope away from the foundation at the surface -- does most of the actual work of keeping water away from the wall in the first place. Waterproofing membrane is the backup for water that reaches the wall despite good drainage, not a substitute for drainage that was never installed correctly.