# Below Grade Waterproofing Details Below-grade waterproofing protects basement and sub-structure elements from groundwater ingress, hydrostatic pressure, and soil moisture. Failure of below-grade waterproofing is among the most costly and disruptive building defects to remediate, making correct specification and detailing at the design stage essential. The architect must select appropriate waterproofing systems based on water table levels, soil conditions, structural movement, and the intended use of below-grade spaces. --- ## Table of Contents - [Water Conditions and Classification](#water-conditions-and-classification) - [Waterproofing System Types](#waterproofing-system-types) - [External Membrane Systems](#external-membrane-systems) - [Integral Waterproofing](#integral-waterproofing) - [Cavity Drain Systems](#cavity-drain-systems) - [Waterstops and Construction Joints](#waterstops-and-construction-joints) - [Drainage and Protection](#drainage-and-protection) - [Design Considerations](#design-considerations) - [See Also](#see-also) --- ## Water Conditions and Classification ### BS 8102 Grade of Protection | Grade | Use | Description | |-------|-----|-------------| | **Grade 1** | Car parking, plant rooms | Some seepage and damp patches acceptable | | **Grade 2** | Workshops, storage | No water penetration; some damp tolerable | | **Grade 3** | Offices, retail, residential | Dry environment; controlled humidity | | **Grade 4** | Archives, data centres, electronics | Totally dry; controlled environment | ### Water Table Conditions | Condition | Description | Design Response | |-----------|-------------|----------------| | **No water table** | Soil moisture only, no hydrostatic pressure | Damp-proofing (tanking) sufficient | | **Seasonal water table** | Water table rises periodically | Full waterproofing with drainage | | **Permanent water table** | Constant hydrostatic pressure | Robust waterproofing, structural design for uplift | | **Perched water** | Trapped water above impermeable stratum | Drainage + waterproofing | --- ## Waterproofing System Types | Type | Approach | Application | |------|----------|-------------| | **Type A — Barrier** | External or internal membrane | Most common; rely on membrane integrity | | **Type B — Structurally Integral** | Waterproof concrete itself | Designed concrete mix + joint detailing | | **Type C — Drained** | Cavity drain membrane + sump pump | Internal drainage management | | **Combined** | Two or more types used together | Higher grades of protection (Grade 3–4) | BS 8102 recommends using **at least two types in combination** for Grade 3 and Grade 4 spaces. --- ## External Membrane Systems ### Pre-Applied Membranes | System | Material | Application | |--------|----------|-------------| | **HDPE sheet** | High-density polyethylene | Laid on blinding before pour; concrete bonds to membrane | | **Bentonite sheet** | Sodium bentonite clay between geotextiles | Self-healing clay swells on contact with water | | **Self-adhesive** | Modified bitumen sheet with adhesive backing | Applied to blinding or formwork before pour | | **Spray-applied** | Rubber-modified bitumen or polyurea | Sprayed onto blinding/formwork; seamless application | ### Post-Applied Membranes | System | Material | Application | |--------|----------|-------------| | **Torch-on bitumen** | Modified bitumen sheet, flame-bonded | Applied to cured concrete externally | | **Self-adhesive sheet** | Modified bitumen with peel-off backing | Applied to primed concrete surface | | **Liquid-applied** | Polyurethane, polyurea, or bitumen rubber | Brush/spray to cured concrete; seamless | | **Cementitious coating** | Polymer-modified cement | Trowel-applied, multiple coats | | **Crystalline** | Cementitious crystalline compound | Brush-applied; crystals grow into concrete capillaries | --- ## Integral Waterproofing Type B (structurally integral) waterproofing uses the concrete itself as the waterproof barrier: | Requirement | Specification | |-------------|--------------| | **Concrete grade** | Minimum C35/45 (typically C40/50) | | **Cement content** | Minimum 350 kg/m³ | | **w/c ratio** | Maximum 0.45 | | **Crack width** | Maximum 0.2mm (designed to BS EN 1992-3) | | **Admixtures** | Crystalline or hydrophilic admixtures to enhance watertightness | | **Curing** | Minimum 7 days wet curing | | **Cover** | Minimum 40–50mm depending on exposure | ### Joint Treatment | Joint Type | Treatment | |-----------|-----------| | Construction joints | Waterstops (hydrophilic or PVC), injection tubes | | Movement joints | Waterstop with movement capacity, sealant | | Day joints | Preparation, roughening, bonding agent, waterstop | --- ## Cavity Drain Systems Type C systems manage water that penetrates the structure by collecting and draining it: | Component | Function | |-----------|----------| | **Cavity drain membrane** | Studded HDPE sheet fixed to walls/floors creating drainage cavity | | **Floor channel** | Perimeter channel collecting water from wall membrane | | **Sump chamber** | Collection point for drained water | | **Sump pump** | Submersible pump evacuating collected water | | **Discharge pipe** | Routes pumped water to storm drain or soakaway | | **Battery backup / alarm** | Ensures operation during power failure | ### Advantages and Limitations | Advantage | Limitation | |-----------|-----------| | Can be installed internally (retrofit) | Relies on mechanical pump — maintenance critical | | Manages water rather than resisting it | Reduces internal room dimensions (25–30mm walls, 50–80mm floor) | | Tolerant of structural movement | Requires permanent drainage maintenance | | No external excavation needed for existing buildings | Energy cost of pumping | --- ## Waterstops and Construction Joints | Waterstop Type | Material | Application | |---------------|----------|-------------| | **Centre-bulb PVC** | PVC with central expansion bulb | Movement joints — accommodates structural movement | | **Flat dumbbell PVC** | PVC with flat profile | Construction joints — cast into concrete | | **Hydrophilic strip** | Bentonite or hydrophilic rubber | Construction joints — swells to seal against water | | **Injectable hose** | Perforated tube for grout injection | Construction joints — allows re-injection if leaks occur | | **Metal waterstop** | Stainless steel or copper strip | High-pressure situations, permanent installations | ### Construction Joint Detailing | Detail | Specification | |--------|--------------| | Joint preparation | Scabble or water-jet to expose aggregate | | Waterstop position | Centre of section; lapped at intersections | | Injection tubes | Secondary line of defence behind waterstop | | Kicker | 150mm concrete kicker cast with base slab; wall cast against kicker | --- ## Drainage and Protection | Component | Function | |-----------|----------| | **Drainage board** | Dimpled HDPE sheet directing water to land drain | | **Filter fabric** | Geotextile preventing soil fines clogging drainage | | **Land drain** | Perforated pipe at foundation level collecting perimeter water | | **Granular backfill** | Free-draining aggregate behind retaining walls | | **Protection board** | Sacrificial layer protecting membrane during backfilling | | **Sump and pump** | Collecting and discharging perimeter drainage water | ### Perimeter Drainage System 1. Perforated land drain at base of wall/footing, bedded in gravel 2. Gravel wrapped in geotextile filter 3. Drainage board against waterproof membrane 4. Protection board over membrane 5. Backfill with free-draining material 6. Discharge to storm system or soakaway with non-return valve --- ## Design Considerations | Factor | Impact on Design | |--------|-----------------| | **Water table level** | Determines hydrostatic pressure and system selection | | **Structural movement** | Joint design, membrane flexibility requirements | | **Access for maintenance** | Cavity drain systems need accessible sumps | | **Intended use** | Grade of protection determines system complexity | | **Ground conditions** | Aggressive soils (sulfates, contaminants) affect material selection | | **Uplift pressure** | Structural design must resist buoyancy (dead load or tension piles) | | **Service penetrations** | Every pipe, cable, and duct through waterproofing needs dedicated sealing | | **Phased construction** | Temporary works may be needed to manage water during construction | --- ## See Also - [[Waterproofing Systems and Details]] - [[Deep Foundation Systems]] - [[Shallow Foundation Design]] - [[Reinforced Concrete Design]] - [[Moisture Control in Buildings]] - [[Soil Mechanics for Architects]] --- #waterproofing #below-grade #basement #waterstop #cavity-drain #membrane #specifications