Below Grade Waterproofing

Below-grade waterproofing protects foundation walls and slabs from groundwater intrusion. The system selection depends on hydrostatic pressure conditions, soil characteristics, and interior space usage requirements.

Damp Proofing vs Waterproofing

Damp proofing resists moisture vapor and minor dampness but cannot withstand hydrostatic pressure. Applied as spray-on or rolled coatings (bituminous emulsions, polymer-modified cementitious materials), damp proofing is suitable only for sites with good drainage where the water table remains below the foundation depth year-round.

Waterproofing provides a continuous, impermeable barrier that resists hydrostatic pressure from groundwater. Required when:

• Water table may rise above foundation base
• Interior spaces will be occupied or climate-controlled
• Soil has poor drainage characteristics (clay, silt)
• Foundation extends more than 3 feet below grade
• Site lacks adequate positive drainage away from building

Hydrostatic Pressure Conditions

Hydrostatic pressure develops when groundwater accumulates against foundation walls without drainage relief.

Pressure calculation: P = γw × h

Where:

• P = hydrostatic pressure (psf)
• γw = unit weight of water (62.4 lb/ft³)
• h = height of water column above measurement point (ft)

Example: 8-foot water column = 62.4 × 8 = 499 psf (3.47 psi)

Design considerations:

• Water table elevation during maximum seasonal rise
• Perched water conditions from low-permeability soil layers
• Surface water infiltration patterns
• Drainage system capacity and reliability
• Foundation wall structural capacity to resist lateral loads

High hydrostatic pressure (>5 feet of head) requires robust waterproofing systems with redundancy, drainage relief, and structural foundation design to resist bending moments.

Waterproof Membrane Systems

Sheet Membranes

Self-adhered rubberized asphalt:

• Composition: Rubberized asphalt compound on polyethylene carrier with release liner
• Thickness: 40-60 mils typical
• Application: Adheres directly to primed concrete/masonry
• Advantages: Self-sealing around penetrations, cold-applied, immediate protection
• Limitations: Surface prep critical, temperature-sensitive installation, moderate cost
• Products: W.R. Grace Bituthene 4000, Carlisle CCW-705

Self-adhered HDPE:

• Composition: High-density polyethylene sheet with aggressive adhesive
• Thickness: 30-45 mils HDPE + adhesive layer
• Application: Applied to primed surface, mechanically fastened at top
• Advantages: Excellent puncture resistance, dimensional stability, UV-resistant during construction
• Limitations: Requires proper surface moisture content, limited self-sealing
• Products: Henry Blueskin WP200, W.R. Grace Preprufe Plus

Bentonite sheets:

• Composition: Sodium bentonite clay between geotextile and HDPE layers
• Thickness: Bentonite layer 15-20 lb/100 ft² (compressed to ~4mm)
• Application: Mechanically fastened or adhered panels with overlapped seams
• Advantages: Self-healing, tolerates damp substrates, low installed cost
• Limitations: Requires hydration and confinement pressure, not suitable for flowing water, contaminated water reduces swelling
• Products: Voltex DS, Cetco Hydrotite

EPDM/TPO sheets:

• Composition: Ethylene propylene diene monomer or thermoplastic polyolefin
• Thickness: 45-60 mils typical for waterproofing
• Application: Mechanically fastened or adhered with heat-welded seams
• Advantages: Long-term durability, chemical resistance, factory-controlled seams
• Limitations: Seaming requires skilled labor, vulnerable to puncture during backfill
• Products: Firestone RubberGard, Carlisle Sure-Weld TPO

PVC sheets:

• Composition: Polyvinyl chloride with plasticizers
• Thickness: 30-60 mils
• Application: Mechanically fastened with heat-welded or solvent-welded seams
• Advantages: Excellent chemical resistance, heat-weldable seams, root-resistant
• Products: Sika Sarnafil, Soprema PVC

Fluid-Applied Membranes

Polymer-modified asphalt:

• Composition: Asphalt emulsion with polymer modifiers (SBR, acrylic)
• Application: Spray, trowel, or squeegee in multiple coats
• Dry film thickness: 60-120 mils total (2-3 coats)
• Advantages: Seamless application, conforms to complex geometries, self-terminating at penetrations
• Limitations: Cure time required before backfill, thickness control critical, weather-dependent application
• Products: Tremco TREMproof 250GC, W.R. Grace Bituthene Liquid Membrane

Polyurethane coatings:

• Composition: Single or two-component polyurethane
• Application: Spray or roller applied
• Dry film thickness: 40-80 mils (multiple coats)
• Advantages: Excellent elongation (>300%), cold-weather application, fast cure
• Limitations: Moisture-sensitive during cure, requires reinforcing fabric at transitions, higher cost
• Products: BASF MasterSeal NP 150, Tremco Watchdog NP-1

Cementitious crystalline:

• Composition: Portland cement, silica sand, active chemicals
• Application: Brush, spray, or trowel applied to dampened substrate
• Thickness: 1/16 to 1/8 inch typical
• Advantages: Bonds to concrete, self-healing through continued crystalline growth, applicable to negative-side applications
• Limitations: Requires moisture for activation, rigid (no crack-bridging), limited to concrete substrates
• Products: Xypex Concentrate, Kryton Krystol T1

Spray-applied polyurea/polyurethane:

• Composition: Fast-reacting two-component spray system
• Application: Plural-component spray equipment
• Dry film thickness: 60-125 mils in single pass
• Advantages: Rapid application, monolithic membrane, immediate protection, excellent elongation
• Limitations: Specialized equipment required, skilled applicators necessary, surface prep critical
• Products: VersaFlex, SWD Urethane, Polyguard RapidHyde

Sheet vs Fluid-Applied Comparison

Protection Boards

Protection boards shield waterproofing membranes from damage during backfill operations and throughout building life.

High-density polyethylene (HDPE) boards:

• Thickness: 1/4 to 1/2 inch
• Compressive strength: 3,000-5,000 psi
• Application: Mechanically fastened to wall or adhered to membrane
• Advantages: High impact resistance, dimensionally stable, lightweight
• Products: Cetco Watchdog Protecto Board, Tremco Protecto Board

Expanded polystyrene (EPS) boards:

• Thickness: 1 to 2 inches
• Compressive strength: 10-60 psi (varies by density)
• Application: Adhered or mechanically fastened
• Advantages: Combined insulation and protection, cost-effective
• Limitations: Lower impact resistance than HDPE
• Products: Standard EPS insulation boards

Extruded polystyrene (XPS) boards:

• Thickness: 1 to 2 inches
• Compressive strength: 15-60 psi
• Application: Adhered to wall or membrane
• Advantages: Higher compressive strength than EPS, moisture-resistant
• Products: Owens Corning Foamular, Dow Styrofoam

Asphaltic fiberboard:

• Thickness: 1/2 to 3/4 inch
• Density: 18-22 lb/ft³
• Application: Mechanically fastened
• Advantages: Conformable, traditional material
• Limitations: Absorbs moisture, degrades over time, largely obsolete

Geocomposite drainage boards:

• Composition: Drainage core (dimpled plastic, geotextile fabric) bonded to filter fabric
• Thickness: 1/4 to 1/2 inch core
• Function: Combined protection and drainage
• Advantages: Single-component system, high drainage capacity
• Products: Tremco Watchdog Drain Sheet, Cetco Voltex Drain

Termination Details

Proper terminations prevent water infiltration at system boundaries and transitions.

Top-of-wall termination:

• Extend membrane 6-12 inches above final grade
• Mechanical termination bar anchored to wall
• Sealant bead between membrane and bar
• Flashing or cant strip to protect exposed edge
• Metal flashing extended over termination and integrated with wall drainage plane

Footing transition:

• Continue membrane onto footing top surface minimum 12 inches
• Seal to footing with compatible mastic or sealant
• Bentonite systems require confinement at footing (mud slab or membrane overlap)

Penetrations (pipes, conduits):

• Sheet systems: Cut membrane, seal with mastic, apply boot or patch
• Fluid-applied: Build up thickness around penetration, self-terminating
• Waterstops at pipe penetrations through structural wall
• Maintain minimum 6-inch clearance between penetrations for sealing

Cold joints in concrete:

• Waterstops installed during concrete placement
• Membrane adhered over waterstop
• Additional sealant or mastic at joint
• Crystalline systems applied to joint with focus treatment

Inside/outside corners:

• Reinforce with additional membrane strip or fabric
• Radius corners where possible (min 1-inch radius)
• Fluid-applied systems: Additional coat at corners
• Sheet systems: Prefabricated corner pieces recommended

Quality Control

Pre-application inspection:

• Substrate surface profile (CSP 1-2 for most systems)
• Substrate moisture content (<5% for adhesion-dependent systems)
• Temperature and weather conditions within manufacturer specifications
• Curing time verification for concrete (typically 14-28 days)
• Surface cleanliness (no oil, dirt, laitance, form release agents)

During application:

• Thickness verification (wet film gauges for fluid-applied)
• Overlap dimensions (6-inch minimum typical for sheets)
• Adhesion testing (random pull tests for self-adhered systems)
• Seam integrity (visual inspection, test cuts)
• Coverage rates and material consumption tracking
• Documentation with photographs at all penetrations and transitions

Post-application inspection:

• Holiday detection (electrical conductance testing for conductive membranes)
• Flood testing (when feasible before backfill)
• Seam probe testing
• Dry film thickness verification (destructive core samples for fluid-applied)
• Protection board installation before backfill authorization

Flood testing protocol:

• Applicable for horizontal surfaces and enclosed sections
• Maintain water level minimum 2 inches above highest point for 24-72 hours
• Inspect interior surfaces for moisture
• Mark and repair any leaks, retest repaired areas

Backfill requirements:

• Controlled backfill in 12-inch lifts maximum
• Free of rocks, debris, and sharp objects
• Compaction to specified density without damaging protection board
• Adequate foundation drainage system operational before final backfill

Product Comparison

Cost factors relative to basic damp proofing (1.0x = ~$2-3/ft²)

System Selection Criteria

For low hydrostatic pressure (<3 ft head):

• Self-adhered rubberized asphalt with drainage board
• Bentonite sheets (if groundwater clean)
• Polymer-modified asphalt fluid-applied

For high hydrostatic pressure (>8 ft head):

• Self-adhered HDPE with redundant drainage
• Spray-applied polyurea with reinforcement
• Bentonite with secondary drainage plane
• Consider double membrane systems for critical applications

For occupied below-grade spaces:

• Premium sheet systems (EPDM, HDPE)
• Fluid polyurethane with high elongation
• Include vapor barrier on interior if required
• Dehumidification system for interior climate control

For difficult substrates:

• Fluid-applied systems conform to irregular surfaces
• Cementitious crystalline for damp concrete
• Mechanically fastened systems for low-strength masonry