Below-Grade Waterproofing

Water is the single greatest threat to building longevity. Below-grade waterproofing isn’t optional — it’s a fundamental requirement for any structure with components in contact with soil.

Dampproofing vs. Waterproofing

These terms are often used interchangeably, but they describe different levels of protection.

Dampproofing is a moisture-resistant coating (usually asphalt-based) that resists moisture in damp soil but cannot withstand hydrostatic pressure. It’s the minimum code requirement for habitable basements in many jurisdictions, but it is not waterproofing.

Waterproofing is a membrane system that bridges cracks and resists water under hydrostatic pressure. It’s essential when:

The water table is near or above the foundation level
Soil has poor drainage (clay-heavy)
The space will be finished or used for storage
Local experience shows chronic moisture problems

Waterproofing Methods

Fluid-Applied Membranes

Sprayed or rolled onto foundation walls, fluid-applied membranes cure to form a seamless, flexible barrier.

Pros: No seams, self-healing around minor cracks, easy to apply on complex geometries
Cons: Requires careful surface prep and consistent mil thickness

Sheet Membranes

Pre-manufactured rubberized asphalt sheets are adhered to the foundation wall.

Pros: Consistent thickness, factory quality control
Cons: Seams are potential failure points, harder to apply on irregular surfaces

Bentonite Clay Systems

Sodium bentonite clay panels or sheets swell when wet, creating a self-sealing barrier.

Pros: Self-healing, effective in wet conditions, can be applied in cold weather
Cons: Requires confinement (backfill) to function, not suitable for all soil types

Drainage is Half the Battle

Even the best waterproofing membrane will fail without a proper drainage system. Water management is a system, not a single product.

Essential drainage components:

Foundation drain (footing drain) — Perforated pipe at the base of the footing, wrapped in filter fabric, bedded in gravel
Drainage board — Dimpled HDPE sheet installed over the membrane to channel water downward and protect the membrane from backfill damage
Gravel backfill — Free-draining aggregate in the zone adjacent to the foundation
Surface grading — Ground should slope away from the foundation at a minimum 5% grade for the first 10 feet
Gutters and downspouts — Direct roof water away from the foundation, discharging at least 6 feet from the building

Interior Backup Systems

For high-risk situations or existing buildings, interior drainage provides a secondary defense:

Interior French drain — Channel around the inside perimeter at the slab edge
Sump pump — Collects and discharges water to daylight or storm sewer
Vapor barrier on slab — Minimum 10-mil polyethylene under interior slabs

Common Failures

Most waterproofing failures aren’t material failures — they’re installation failures:

Membrane not extended below the footing line
Inadequate overlap at seams
Punctured membrane during backfill (no protection board)
Clogged footing drains from missing filter fabric
Negative grading directing surface water toward the building

Invest Early, Save Later

Waterproofing is far cheaper to install during construction than to retrofit after the fact. Budget for quality materials and careful installation. A waterproofing failure doesn’t just mean a wet basement — it means mold, structural damage, and a major hit to property value.