# Waterproofing Systems and Details Waterproofing is the provision of an impervious barrier to prevent water — whether from rain, groundwater, or internal wet areas — from penetrating the building fabric. Water ingress is the most common cause of building defects, responsible for the majority of post-completion complaints and insurance claims against architects. For the practicing architect, waterproofing is not a specialist afterthought but a fundamental design responsibility: the selection of waterproofing systems and the design of critical details determine whether the building remains dry and functional throughout its design life. --- ## Table of Contents - [Waterproofing Categories](#waterproofing-categories) - [Below-Ground Waterproofing](#below-ground-waterproofing) - [Flat Roof Waterproofing](#flat-roof-waterproofing) - [Wet Room Waterproofing](#wet-room-waterproofing) - [Joint Sealants](#joint-sealants) - [DPC and DPM Systems](#dpc-and-dpm-systems) - [Waterproofing Selection Guide](#waterproofing-selection-guide) - [Inspection and Testing](#inspection-and-testing) - [See Also](#see-also) --- ## Waterproofing Categories | Category | Water Source | Typical Approach | |----------|-------------|-----------------| | **Below-ground (tanking)** | Groundwater; hydrostatic pressure | Barrier, cavity drain, or structural waterproofing | | **Flat roof** | Rainwater ponding; driven rain | Membrane systems; liquid applied; metal | | **Pitched roof** | Rain; wind-driven rain; condensation | Tiles/slates with underlay; standing seam metal | | **Facade** | Driven rain; condensation | Rainscreen; barrier; cavity drainage | | **Wet rooms** | Internal water (showers, pools) | Tanking membranes; tile-backer; tanked drainage | | **Podium/terrace** | Rainwater above occupied space below | Inverted or warm roof detail with drainage mat | --- ## Below-Ground Waterproofing Below-ground waterproofing (basement waterproofing) is critical and difficult — access for remediation is extremely limited or impossible after construction. ### BS 8102 Classification | Type | System | Description | Application | |------|--------|------------|------------| | **Type A** | Barrier protection | External or internal applied membrane | New-build; accessible external face | | **Type B** | Structurally integral | Water-resistant concrete (RC to BS EN 1992-3) | Primarily structural with waterproof design | | **Type C** | Drained protection | Cavity drain membrane on internal face; managed drainage | Refurbishment; where external access impossible | **Best practice**: BS 8102 recommends combining two types (e.g., Type A + Type B, or Type B + Type C) for Grade 3 environments (habitable/commercial use). ### Grade of Protection (BS 8102) | Grade | Description | Typical Use | |-------|------------|------------| | **Grade 1** | Basic utility | Car parks; plant rooms (some water seepage tolerable) | | **Grade 2** | Better utility | Workshops; car parks (no water but damp patches tolerable) | | **Grade 3** | Habitable | Offices; residential; retail (dry environment required) | | **Grade 4** | Special | Archives; server rooms; galleries (controlled environment) | ### Barrier Systems (Type A) | Material | Application Method | Key Property | |----------|-------------------|-------------| | **Bituminous sheet membrane** | Torch-applied or self-adhesive to external face | Robust; proven; requires protection board | | **Bentonite clay membrane** | Fixed to external face; swells on contact with water | Self-healing; no primer; inorganic | | **Liquid-applied membrane** | Spray or roller to external/internal face | Seamless; follows complex geometry | | **Cementitious crystalline** | Brush-applied to internal or external concrete face | Penetrates concrete; self-healing crystals | ### Cavity Drain Systems (Type C) Cavity drain (or drained cavity) systems accept that water will reach the structure and manage it through a controlled drainage path: - **Studded membrane** (e.g., Delta, Platon) fixed to internal walls and floor - Water drains behind membrane into perimeter channel drain - Channel drain connects to sump with automatic pump - Pump must have alarm, battery backup, and maintenance contract - Internal finishes are built off the membrane — creating a drained cavity **Critical dependency**: Type C systems rely on the sump pump operating continuously. Power failure, pump failure, or blocked drains result in flooding. Specify dual pumps, battery backup, and high-water alarm. --- ## Flat Roof Waterproofing ### Membrane Types | System | Material | Life Expectancy | Application Method | Key Properties | |--------|---------|----------------|-------------------|---------------| | **Single-ply (PVC)** | Plasticised PVC (e.g., Sarnafil) | 25-40 years | Mechanically fixed or adhered | Lightweight; fast; hot-air welded seams | | **Single-ply (TPO/FPO)** | Thermoplastic polyolefin | 25-35 years | Mechanically fixed or adhered | Plasticiser-free; recyclable | | **Single-ply (EPDM)** | Synthetic rubber | 30-50 years | Adhered; taped/glued seams | Flexible; UV-resistant; single large sheets | | **Built-up felt (bituminous)** | Modified bitumen layers | 20-30 years | Torch-applied or pour-and-roll | Traditional; robust; heavier | | **Mastic asphalt** | Heated asphalt laid in-situ | 40-60+ years | Poured and trowelled (hot) | Seamless; extremely durable; heavy; skilled trade | | **Liquid-applied** | Polyurethane, PMMA, or GRP resin | 20-30 years | Roller or spray applied | Seamless; excellent for complex geometry; detail-heavy roofs | | **Green roof system** | Waterproofing + root barrier + drainage + substrate + vegetation | 40+ years (waterproofing) | Layered build-up | Biodiversity; SuDS; see [[Sustainable Drainage Design]] | | **Metal standing seam** | Zinc, copper, aluminium, stainless steel | 40-100+ years | Mechanically seamed joints | Premium; self-draining; low pitch possible | ### Flat Roof Construction Types | Type | Insulation Position | Waterproofing Position | Key Advantage | |------|-------------------|----------------------|---------------| | **Warm roof** | Above deck | Above insulation | No condensation risk; protects structure | | **Inverted roof** | Above waterproofing | Below insulation (on deck) | Protects membrane; XPS insulation only | | **Cold roof** | At ceiling level | On deck (no insulation above) | Simple; ventilated void; domestic pitched | | **Green roof** | As warm or inverted | Below growing medium | Stormwater; biodiversity; thermal mass | | **Blue roof** | As warm roof | As warm roof; controlled flow outlet | Stormwater attenuation on flat roofs | ### Critical Flat Roof Details | Detail | Key Requirements | |--------|-----------------| | **Upstand** | Waterproofing turned up minimum 150mm above finished roof level (300mm at door thresholds) | | **Outlet** | Sump or outlet recessed below membrane level; minimum 2 per roof area; secondary overflow | | **Penetration** | Pre-formed collar or liquid-applied detail; membrane dressed up penetration minimum 150mm | | **Edge/fascia** | Membrane dressed over or into edge trim; drip detail | | **Movement joint** | Upstand both sides; flexible bellows or proprietary expansion joint detail | | **Abutment (wall)** | Chase flashing into masonry; minimum 150mm above roof; DPC/counter-flashing | | **Falls** | Minimum 1:80 (1.25%) to outlets; 1:40 (2.5%) preferred; designed falls, not relying on deflection | --- ## Wet Room Waterproofing Internal wet areas (showers, bathrooms, swimming pools, commercial kitchens) require tanking to prevent water damage to adjacent structure and spaces: | System | Application | Standard | |--------|------------|---------| | **Liquid-applied membrane** (e.g., Mapei Mapelastic, Ardex S7) | Brush/roller-applied to substrate below tiles | BS 5385-4; manufacturer's specification | | **Sheet membrane** (e.g., Schlüter DITRA) | Mat bonded to substrate; tiles bonded to mat | Manufacturer's specification | | **Cementitious tanking** | Trowel-applied rigid waterproofing | Swimming pools; below-ground wet rooms | | **Preformed shower tray** | Factory-produced acrylic or resin tray | Residential; hotel; prefabricated pods | **Key wet room principles**: - Waterproofing must be continuous across floor and up walls to minimum 1,800mm (full height in showers; 300mm at non-wet walls) - All joints, corners, and pipe penetrations sealed with proprietary tape or collar - Falls to drain: minimum 1:80 (floor); 1:50 preferred for walk-in showers - Substrate must be stable, dry, and dimensionally sound before membrane application - Do not rely on tile grout as the waterproof layer — grout is not waterproof --- ## Joint Sealants Sealant joints accommodate movement between adjacent building components while maintaining weathertightness: ### Sealant Types | Type | Movement Capability | Durability | Application | |------|-------------------|-----------|------------| | **Silicone (neutral cure)** | ±25-50% | 20-30 years | Curtain wall; glazing; non-porous substrates | | **Silicone (acetoxy)** | ±20-25% | 15-20 years | Glazing; not for metals or cementitious | | **Polyurethane** | ±25% | 15-25 years | Concrete joints; porous substrates | | **Polysulphide** | ±25% | 20-30 years | Concrete; masonry; immersed joints | | **Hybrid (MS polymer)** | ±25% | 20-25 years | General purpose; paintable; versatile | | **Acrylic** | ±12.5% | 10-15 years | Internal only; paintable; low movement | | **Fire-rated intumescent** | ±10-20% | Project life | Fire-rated joints; penetration seals | ### Joint Design Rules | Parameter | Guideline | |-----------|----------| | **Width** | Minimum 10mm; typically 15-25mm; sized for expected movement | | **Depth** | Width-to-depth ratio 2:1 (ideal); minimum depth 6mm | | **Bond** | Two-sided adhesion only (not three-sided — use backer rod) | | **Backer rod** | Closed-cell polyethylene; 25% larger than joint width | | **Primer** | Always use manufacturer's recommended primer for the substrate | | **Surface preparation** | Clean, dry, free of dust and contaminants | | **Application temperature** | Per manufacturer's data (typically 5-35°C) | | **Tooling** | Concave profile for positive contact with both substrates | --- ## DPC and DPM Systems ### Damp-Proof Course (DPC) A horizontal barrier in masonry walls to prevent rising damp: | Type | Material | Application | |------|---------|------------| | **Polymer DPC** | Polyethylene or polypropylene sheet | Standard for new masonry construction | | **Bitumen felt DPC** | Bitumen-impregnated hessian or polyester | Traditional; still used | | **Slate DPC** | Two courses of engineering slate | Heritage conservation; traditional detail | | **Lead DPC** | Code 4 lead sheet | Heritage; complex geometry | | **Chemical DPC (injection)** | Silicone or siloxane cream injected into mortar course | Retrofit for existing walls without DPC | **DPC position**: Minimum 150mm above external ground level. In cavity walls, the DPC bridges both leaves with a cavity tray to direct any moisture outward through weep holes. ### Damp-Proof Membrane (DPM) A horizontal or vertical membrane to prevent moisture from the ground entering the building: - **Polythene DPM** (1200 gauge / 300μm minimum): Below ground floor slabs; lapped 300mm and taped - **Liquid-applied DPM**: For complex geometries or existing slabs - **Gas membrane**: Combined DPM and radon/methane barrier for contaminated sites --- ## Waterproofing Selection Guide | Application | Recommended System | Key Factor | |------------|-------------------|-----------| | Basement (new-build, habitable) | Type B (structural) + Type A (external membrane) | Dual protection; Grade 3 | | Basement (refurbishment) | Type C (cavity drain) + sump pump | No external access; managed drainage | | Flat roof (standard) | Single-ply PVC or TPO | Cost; speed; warranty | | Flat roof (long-life / premium) | Mastic asphalt or standing seam metal | Durability; whole-life cost | | Flat roof (complex geometry) | Liquid-applied (PMMA or PU) | Seamless; adaptable | | Podium deck / plaza | Inverted roof with root-resistant membrane | Protected membrane; trafficable | | Shower / bathroom | Liquid-applied tanking under tiles | Continuous; accessible | | Swimming pool | Cementitious tanking or PVC liner | Hydrostatic pressure resistance | --- ## Inspection and Testing | Test | Application | Standard | |------|------------|---------| | **Flood testing** | Flat roofs; podium decks; wet rooms | Hold water 24-48 hours; observe for leaks | | **Spray testing** | Curtain walls; windows; cladding | AAMA 501.2; hose testing per CWCT | | **Electronic leak detection (ELD)** | Single-ply and liquid-applied membranes | Low-voltage or high-voltage scanning | | **Adhesion testing** | Liquid-applied membranes | Pull-off test per manufacturer's requirements | | **Visual inspection** | All waterproofing installations | During application; at laps; at details | | **Air test (drainage)** | Below-ground drainage pipes | BS EN 1610; 100mm water gauge for 5 minutes | **Timing**: Waterproofing must be tested and approved before it is concealed by subsequent construction. For flat roofs, flood testing before ballast or paving installation. For basements, inspection before backfilling. --- ## See Also - [[Architectural Detailing Principles]] - [[Building Envelope Fundamentals]] - [[Roof Systems and Design]] - [[Moisture Control in Buildings]] - [[Heritage Conservation Principles]] - [[Concrete Properties and Mix Design]] --- #waterproofing #detailing #roofing #basement #sealants #dpc #membranes