Below-ground structures face waterproofing conditions that are fundamentally different from those at roof or wall level. Groundwater does not follow a seasonal dry period in the same way that roof-level moisture does. Hydrostatic pressure acts continuously against the structure and its membrane. Thermal and structural movement opens and closes construction joints repeatedly across the service life of the building. And critically — once the structure is backfilled, most of the applied membrane is permanently inaccessible. Detection of a below-ground failure typically requires internal investigation and often re-excavation. Remediation costs routinely exceed the original waterproofing budget several times over.
WATERPROOFING APPROACHES — TYPE A, TYPE B AND TYPE C
BS 8102 describes three approaches to the protection of below-ground structures from water ingress. These approaches may be used individually or in combination, depending on the structure, ground conditions, water table and the required grade of internal environment.
Type A — Barrier Protection. A waterproofing barrier is applied to the structure to resist water ingress. This includes liquid-applied membranes, sheet membranes, mastic asphalt tanking and cementitious coatings applied to the structure face. BelowGrade may form the applied barrier membrane within a Type A barrier-protection design, subject to the complete project waterproofing strategy, specification and manufacturer confirmation.
Type B — Structurally Integral Protection. The structure itself is designed and constructed as the waterproofing element — typically reinforced concrete designed to a watertight specification. Type B relies on the quality, mix design, cover, compaction, construction joint design and workmanship of the concrete. A liquid-applied product cannot substitute for or replicate Type B performance — it is a different approach, not a coating for failed Type B concrete.
Type C — Drained Protection. A cavity-drain system allows water to enter a controlled void within the building envelope and directs it to drainage or pumped collection. Type C is often used for remedial work on existing structures where external access is not available. It is a fundamentally different strategy from a barrier membrane — it manages ingress rather than excluding it.
These three approaches must not be confused. A product reference to BS 8102 does not make the product itself a complete waterproofing design. The project designer must determine the appropriate protection strategy — including whether a single approach is sufficient or whether a combination is required — before any material is specified. Maintainability, repairability and long-term access for inspection must also be considered.
STRUCTURES AND APPLICATIONS
The following below-ground structure types each have specific considerations for waterproofing design, preparation and detailing:
Basement walls (cast concrete or masonry retaining) — primary application for external positive-side membrane. Membrane applied to the retained face before backfill. All construction joints, wall-to-slab junctions, tie holes and service penetrations require specific detailing.
Basement slabs (ground-bearing or suspended) — membrane applied to the underside or blinding before slab construction, or to the top surface before screed in appropriate circumstances. Falls and drainage to sumps must be confirmed as part of the design.
Foundation walls — poured concrete or masonry foundation sections below finished ground level. Access is often limited once the superstructure commences — coordination with the construction programme is critical.
Retaining walls — free-standing or basement-forming concrete or masonry walls retaining earth. External membrane required on the retained face. Protection board required before backfilling against the retained face. Drainage layer behind the retaining face may be a structural design requirement independent of the waterproofing strategy.
Lift pits — confined, low-clearance structures below ground slab level. Geometry creates complex detailing requirements at base, wall corners and lift-guide penetrations. Access for external application may be limited.
Service pits, plant rooms and tank chambers below ground — similar to basements in structural terms; drainage and chemical environment may impose additional constraints on membrane selection.
Podium structures (buried roof decks, plaza decks and landscaped decks over structure) — combination of below-ground waterproofing and inverted-roof or trafficked-deck considerations. Protection from root penetration, drainage-board installation and imposed loading must all be addressed at design stage.
Underground car parks — ground-slab level and below. May combine Type A external waterproofing at walls with Type A or B at slab level, and a separate traffic-rated membrane at the deck surface. These are different specification requirements and must not be conflated.
Tunnels and culverts — external membrane application requires access and programme coordination with civil works. Detail requirements at construction joints are intensive.
EXTERNAL POSITIVE-SIDE WATERPROOFING
Positive-side waterproofing — the membrane applied to the face of the structure that is exposed to the water source — is the preferred approach for new-build and refurbishment work where external access is available. The membrane is placed on the water side, where hydrostatic pressure acts to press it into the substrate rather than attempting to lift it away. This gives the membrane a mechanical advantage under water pressure that negative-side (internally applied) systems do not have.
External positive-side application requires excavation access to the complete structure face to be waterproofed. Access and backfill sequencing are critical. The programme window for membrane application — between exposure of the structure face and commencement of backfilling — must be sufficient to allow full preparation, primer cure, membrane application, detail completion and protection-board installation. Shortening this window to accelerate backfill is one of the most common contributors to below-ground waterproofing failure.
A completed external membrane may be difficult or impossible to repair after backfilling. Inspection before covering is not optional — it is the last opportunity to identify and repair any defect before access is permanently lost.
Do not describe an internally applied coating or cavity-drain system as positive-side waterproofing. Positive-side means the water-face of the structure, applied before the structure is buried.
SUBSTRATE PREPARATION
Below-ground concrete and masonry substrates require preparation before membrane application. Generic preparation — a pressure wash and a coat — is insufficient in this application. Required steps include:
Sound substrate confirmation — the concrete or masonry must be structurally sound and stable. Active structural movement must be resolved before any membrane specification. Failing or friable surface material must be removed.
Laitance removal and surface profile — surface laitance (weak surface layer) must be removed by grit blasting, light scabbling or grinding to expose the aggregate face. Smooth-formed concrete is insufficiently porous for reliable adhesion without preparation.
Concrete defect repair — honeycombing (voids in the concrete from inadequate compaction), blow-holes, bug-holes, tie-rod holes and any voiding must be filled and reproofed before membrane application. The membrane cannot bridge significant voids.
Construction joint preparation — each construction joint must be identified, assessed and treated. Existing sealants must be confirmed compatible; failed sealants must be removed and replaced.
Contamination removal — release agents, shuttering oils, curing compounds, efflorescence, and any existing coatings or prior waterproofing treatments must be identified and removed. Each contaminant affects adhesion differently.
Corner preparation — internal and external arrises at wall-to-slab junctions, corners and transitions should be formed to the radius or chamfer specified in the approved build-up. Sharp right-angle arrises concentrate stress in the membrane at corners and reduce coverage thickness.
Moisture condition — the substrate moisture condition must be within the application limits for the specific product and build-up at the time of application, not at the time of survey. Below-ground substrates can gain moisture rapidly from ground conditions. A moisture check carried out days before application may not reflect the application-day condition.
Primer — primer requirements depend on the substrate type, condition and the approved build-up. Primer must not be omitted where specified.
JOINTS, PENETRATIONS AND INTERFACES
The field membrane applied to the main structure face does not, by itself, resolve the critical details. Each of the following requires specific treatment within the approved specification, confirmed against manufacturer documentation:
Construction joints — the primary leak path in below-ground concrete structures. Require reinforcement, sealant, bandage strip or specific joint detail as part of the approved build-up. Field spray alone does not resolve construction joints.
Movement joints — designed to permit relative movement between structural sections. Require an appropriate flexible joint detail that accommodates the design movement without transferring load to the membrane. A membrane bridged over an unresolved movement joint will fail at that joint.
Day joints — construction sequence discontinuities. Require treatment as construction joints.
Wall-to-slab junctions — a transition point that experiences differential movement and is typically the most water-vulnerable detail in the structure. Cannot be resolved by increasing membrane thickness.
Pipe penetrations and service ducts — each penetration represents a potential membrane discontinuity. Requires collar sealing, bonded flashing or purpose-designed sleeve as confirmed in the approved build-up.
Pile caps and ground beams — interfaces at transitions in substrate type and geometry. Require specific detailing.
Tie holes — formed by formwork tie bolts in cast concrete. Must be filled and reproofed before membrane application.
Terminations — the top and bottom edges of the membrane must be correctly terminated. An unterminated edge allows water to track behind the membrane from the edge.
Interfaces with other systems — where the below-ground membrane meets a damp-proof course, a roof waterproofing system, a drainage layer, an insulation board or another applied treatment, compatibility must be confirmed and the interface detailed in the approved specification.
PROTECTION BEFORE BACKFILLING
A completed membrane must be inspected and protected before backfilling commences. This is not an advisory step — it is a specification and warranty requirement.
Protection is required because backfill materials, drainage boards, insulation, sharp aggregate, compaction plant, reinforcement and access traffic can all damage the membrane. Damage that occurs after application and before protection is installed may not be visible after backfilling and will not be discovered until water ingress occurs — potentially years later.
The protection build-up may include: a protection board compatible with the membrane; drainage layer materials; geotextile; and insulation where specified. Each element must be confirmed compatible with the membrane and with the overall waterproofing and drainage design.
Before protection is installed, the membrane must be inspected — ideally by a qualified person — and any defects or areas of insufficient coverage identified and repaired. A photographic record of the completed membrane before protection is evidence for quality assurance and warranty purposes.
Backfilling must not commence until: membrane inspection is complete; any required repairs are made and confirmed sound; protection boards are installed; and the membrane has achieved the required cure.
Controlled backfilling means using approved fill materials, maintaining the required stand-off from the membrane face during initial lifts, and not compacting directly against the membrane without a protection layer.
DRAINAGE AND GROUNDWATER
Waterproofing and drainage are related but distinct design considerations. A below-ground membrane does not remove the need for groundwater assessment or drainage design.
Groundwater level — the seasonal high water table determines the maximum hydrostatic head on the structure and membrane. This must be established from ground investigation records, borehole data or piezometer monitoring — not from a visual inspection at a single survey date.
Hydrostatic pressure — the pressure of groundwater against the below-ground structure increases with depth. A membrane that resists hydrostatic pressure must be fully bonded to a substrate that is stable against that same pressure. Unbonded or partially bonded membranes can be displaced by hydrostatic pressure.
Temporary dewatering — on new-build projects, the excavation may be dewatered during construction. The membrane must be designed for the permanent condition (without dewatering) not only the construction condition.
Permanent land drainage — a drainage layer and perimeter drain system may be required to manage groundwater around the structure. This reduces the hydrostatic head on the membrane and is often a structural design requirement independent of the waterproofing strategy.
Sumps and pumps — below-ground drainage may be directed to a sump and pump system. Pump systems require maintenance and may fail. Where the structure cannot tolerate any flooding, pump redundancy may be required.
Drainage discharge — the drainage design must account for where collected groundwater is discharged. This is typically a civil and structural engineering matter, not a waterproofing product decision.
The membrane is one element of the below-ground waterproofing strategy. It does not replace groundwater assessment, drainage design, structural design or construction quality management.
BELOWGRADE SYSTEM
BelowGrade liquid rubber is a two-component spray-applied waterproofing membrane principally used for below-ground applications. It is cold-applied, water-based, VOC-free and solvent-free. The cured membrane is seamless, fully bonded to the substrate and permanently flexible.
The following properties are stated in or consistent with the reviewed BelowGrade TDS:
— Two-component spray-applied system. Cold spray application for field areas. Hand application (brush or roller) for detailing, penetrations and transitions. Minimum application temperature: 5°C. Substrate must be dry, unfrozen and uncontaminated.
— 850% elongation to ASTM D638. The cured membrane can extend to accommodate substrate movement within the detailing limits of the approved build-up.
— Membrane build: approximately 1.35 kg/m² at approximately 1 mm for an airtight seal; approximately 2.7 kg/m² for a nominal 2 mm watertight membrane. The required thickness and consumption for a specific project must be confirmed in the approved specification — these nominal figures are not automatically the requirement for every build-up or detail.
— The spray reaction forms the membrane rapidly. Touch-dry is typically around one minute at 20°C. This is not full adhesion, full cure or readiness for backfilling. Full adhesion to the substrate develops within approximately 48 hours; actual timing depends on temperature and humidity. Protection and insulation installation should not be treated as permitted immediately after touch-dry — timing must be confirmed against application conditions and manufacturer guidance.
— Spray application can support efficient installation across suitable prepared areas. Actual project output depends on preparation, detailing, access, environmental conditions and sequencing — not on the application rate alone.
— Any warranty position is project-specific and depends on the substrate, preparation, complete system build-up, application records and manufacturer acceptance. Relevant manufacturer documentation can be reviewed as part of the project-specific specification process.
Substrates for which adhesion test data are available in the reviewed TDS include: concrete, limestone, wood, PVC, zinc and aluminium. On below-ground projects, the primary substrates are concrete and suitable masonry or mineral substrates where confirmed. Other materials — including interfaces at penetrations and transitions — must have their suitability, primer requirements and preparation confirmed per project in the approved specification. Adhesion test data do not by themselves establish that every listed material is suitable as a complete below-ground waterproofing substrate in all conditions. Substrate use, primer, preparation and interface detailing must be confirmed per project.
BelowGrade may form the applied barrier membrane within a Type A barrier-protection design under BS 8102:2022, subject to the complete project waterproofing strategy, design and specification. The product is not itself the complete design, does not independently satisfy all Type A requirements, and does not carry BS 8102 certification or product approval. The project may require more than one form of protection. A suitably qualified waterproofing designer should determine the protection strategy. Maintainability, repairability and risk must be considered.
The complete project build-up — including preparation, primer, joint detailing, membrane consumption, terminations, protection and drainage interfaces — must be confirmed in the approved project specification. BelowGrade is the field membrane component of that build-up. It is not, by itself, a complete below-ground waterproofing design.
REFURBISHMENT AND REMEDIAL WORK
New-build external waterproofing — membrane applied to the external face of the structure before backfill as part of the construction programme. This is the primary application for BelowGrade.
Existing structure with external excavation access — where excavation can be carried out to expose the existing structure face, external membrane application is possible on an existing below-ground structure. The programme must account for excavation, substrate assessment, preparation of the existing surface, any prior waterproofing removal, application, protection and re-backfilling.
Existing structure without external access — where external excavation is impractical, an external positive-side membrane cannot be applied. In this condition, the appropriate routes are: Type C cavity-drain system installed internally; internal barrier coatings where conditions permit; crack injection; or a combination of approaches. External liquid waterproofing cannot be retrofitted where excavation or access is impractical.
Localised repair — where a discrete area of a previously waterproofed structure has failed, localised repair may be possible if the failure zone is identifiable and accessible. The failure mechanism must be confirmed before repair specification.
QUALITY ASSURANCE
Below-ground waterproofing has a particularly high consequence of failure. Quality assurance must be planned, not retrospective.
Required quality assurance steps include: confirmation that the approved substrate condition has been achieved before application begins; recording of environmental conditions (temperature, humidity, substrate moisture) at the start of each application period; batch and delivery records; equipment settings for spray equipment; consumption monitoring (material used per measured area); wet-film check during application where applicable; detailed inspection of all joints, penetrations and interface details during and after application; photographic records of the applied membrane before protection is installed; inspection of any damage after protection board installation; and signed-off handover records before backfill commences.
Warranty requires full compliance. Warranty is not automatic on completion of application — it requires documented evidence of compliant installation.
