Basement Tanking Explained

What is basement tanking?

Basement tanking refers to waterproofing methods used to prevent water ingress into below-ground rooms. Unlike above-ground damp, basements face hydrostatic pressure from groundwater in the surrounding soil, which can force water through walls and floors even when no visible cracks exist.

Tanking is required when converting a cellar or basement to habitable use (living space, bedroom, home office) because Building Regulations demand dry, healthy internal environments. It is also used in new-build basements to prevent water damage to structure and finishes.

The term "tanking" originally referred to creating a waterproof box (like a tank) within the structure, but modern systems include barrier, structural, and cavity drainage methods.

BS 8102: the standard for basement waterproofing

All basement waterproofing in the UK must comply with BS 8102:2009 (Code of practice for protection of below ground structures against water from the ground). This standard defines three types of waterproofing system and sets design and performance requirements.

Building Control will require:

• Waterproofing design by a qualified specialist (Property Care Association, CSSW, or equivalent).
• Drawings showing system type, sump location, drainage paths, and ventilation.
• Product specifications and British Board of Agrément (BBA) certificates.
• Installation by an approved contractor with a written guarantee (typically 10-20 years).
• Completion certificate confirming installation to BS 8102 standards.

DIY tanking rarely meets these requirements and will fail Building Control inspection.

Type A: Barrier waterproofing (external tanking)

How it works

A waterproof membrane is applied to the external face of basement walls and under the floor slab before backfilling. This forms a physical barrier that prevents water from entering the structure. The membrane is typically a bituminous sheet, liquid-applied coating, or bentonite clay layer.

Water is stopped at the outer surface, so the structure remains dry and no internal drainage is needed.

When to use Type A

• New-build basements where external access is available during construction.
• Major excavations where the basement exterior can be exposed.
• Sites with high water tables or aggressive ground conditions (sulfates, contaminants).

Advantages

• Protects the structure from water contact, reducing long-term deterioration.
• No loss of internal space (membrane is outside the building).
• Effective in high groundwater conditions when properly designed.

Disadvantages

• Expensive (£100-200 per m²) due to excavation, shoring, and backfilling.
• Not feasible for existing basements unless major external works are planned.
• Vulnerable to damage during backfilling or from root penetration.
• Difficult to repair if the membrane is punctured after construction.

Common materials

• Bituminous membranes: Sheet or liquid-applied. Durable but can be damaged during backfilling.
• Bentonite clay mats: Self-healing clay layer that swells when wet. Effective but requires precise installation.
• Liquid membranes: Spray-applied polyurethane or acrylic coatings. Flexible and joint-free.

Type B: Structural waterproofing (cementitious tanking)

How it works

A water-resistant cementitious coating or additive is applied to the internal surface of walls and floor. The coating is typically 10-20mm thick and includes waterproofing admixtures (crystalline additives or polymer modifiers) that block capillary pathways in the concrete or masonry.

The structure itself becomes water-resistant. No cavity or drainage is created. The coating is applied directly to the substrate and can be plastered or rendered over.

When to use Type B

• Low-grade environments (no standing groundwater, occasional dampness only).
• Basements with good existing structure (no major cracks or movement).
• Projects where space is tight and cavity systems would be too intrusive.

Advantages

• Lower cost than Type A or C (£60-100 per m²).
• Minimal loss of internal space (10-20mm coating).
• Can be applied to walls and floors in one continuous layer.

Disadvantages

• Relies on structural integrity. Cracks from settlement or movement will compromise waterproofing.
• Not suitable for high groundwater or aggressive water conditions.
• Difficult to repair if waterproofing fails (requires stripping and recoating).
• No drainage path for water that penetrates the coating.

Common products

• Sika Watertight Concrete: Admixture added to concrete during pouring. Effective for new construction.
• Sovereign Chemicals Aquatex: Cementitious slurry applied to existing walls.
• Vandex BB75: Crystalline waterproofing system that penetrates concrete and seals capillaries.

Type C: Drained cavity system (cavity drain membranes)

How it works

A dimpled plastic membrane is fixed to internal walls and floor, creating an air gap (typically 8-20mm) between the structure and the habitable space. Water penetrates the structure but is channeled behind the membrane to a sump pump via perimeter drainage channels.

The membrane studs create a physical barrier that prevents water from entering the room. Water flows down the back of the membrane, is collected in a floor channel, and is pumped out via a sump system.

A vapour-control layer is often applied over the membrane, followed by battens and plasterboard (walls) or insulation and screed (floors).

When to use Type C

• Retrofit basement conversions where external access is not available.
• High groundwater or sites with persistent water ingress.
• Basements with structural cracks or movement that would compromise Type B systems.
• Projects requiring a reliable, maintainable system with visible drainage paths.

Advantages

• Most reliable for retrofit applications. Does not depend on structural integrity.
• Allows water to drain safely without damaging the structure.
• Can be inspected and maintained (access to sump and drainage channels).
• Suitable for high groundwater and aggressive conditions.

Disadvantages

• Requires a functioning sump pump (risk of flooding if pump fails). Backup pumps and alarms are recommended.
• Loss of internal space (50-100mm on walls, 100-150mm on floors).
• Higher cost than Type B (£80-150 per m²).
• Requires Building Control approval for sump location and pump specification.

Common systems

• Delta Membranes: Industry leader. Dimpled HDPE membrane with comprehensive system components (channels, sumps, vapour barriers).
• Platon: Canadian system. Similar to Delta, widely used in UK.
• Oldroyd: UK manufacturer. Range of membranes for walls and floors.

Installation process for Type C systems

1. Prepare the substrate

Remove any loose render, plaster, or finishes down to bare masonry or concrete. Repair major cracks or voids with mortar. The substrate does not need to be perfectly smooth because the membrane spans minor irregularities.

2. Install floor drainage channel

A perimeter drainage channel is installed around the floor edge to collect water flowing down the walls. The channel connects to a sump chamber (typically located in a corner or low point).

3. Fix wall membranes

Dimpled membrane sheets are fixed to walls using mechanical fixings (plugs and washers) or adhesive strips. Sheets are lapped at joints to maintain continuity. The membrane extends from floor level to above anticipated water level (typically full height).

4. Install floor membrane

Membrane is laid over the floor slab, lapping with the wall membrane to create a continuous cavity. Water drains from walls, across the floor, and into the perimeter channel.

5. Install sump and pump

A sump chamber (plastic box, typically 300-500mm diameter) is installed below floor level. Water from the drainage channels flows into the sump. A submersible pump lifts water to external drainage or soakaway.

BS 8102 requires:

• Dual pumps (primary + backup) for habitable basements.
• High-water alarm to warn if pumps fail.
• Battery backup for pumps in case of power failure.

6. Apply vapour barrier

A vapour-control layer (typically polythene sheet) is applied over the membrane to prevent humid basement air from condensing on the cold structure behind the membrane.

7. Build stud walls and screed floor

Timber or metal stud walls are built in front of the membrane. Insulation is added between studs, then plasterboard and skim. Floor screed (75-100mm) is laid over floor membrane with insulation beneath.

How much does basement tanking cost?

Type A (external barrier)

£100-200 per m² including excavation, membrane, backfilling. A small basement (20m² floor, 40m² walls): £6,000-12,000. Only feasible during new construction or major external works.

Type B (cementitious)

£60-100 per m² for coating only. A small basement (60m² total surface area): £3,600-6,000. Add £2,000-4,000 for plastering, decorating, and finishes.

Type C (cavity drain membrane)

£80-150 per m² for membrane, sump, pump, drainage, and finishes. A small basement (20m² floor, 40m² walls): £5,000-9,000 including stud walls, screed, and decoration.

Additional costs

• Specialist design and survey: £500-1,500.
• Building Control fees: £300-800.
• Structural repairs (underpinning, crack stitching): £3,000-10,000+.
• Ventilation and dehumidification: £500-2,000.

Do you need Building Control approval?

Yes, if the basement is being converted to habitable use (bedroom, living room, home office) or if structural changes are made. Building Regulations Part C (Resistance to moisture) requires waterproofing systems to be designed and installed to BS 8102 standards.

You will need:

• Full Plans application showing waterproofing system, sump location, drainage, ventilation, and escape routes.
• Structural calculations (if floors, walls, or openings are altered).
• Product specifications with BBA certificates.
• Installation by an approved contractor with certification.

Non-habitable use (storage, plant room) may not require full Building Control approval, but waterproofing is still recommended to protect the structure.

Can you DIY basement tanking?

Not if you need Building Control approval. BS 8102 requires professional design and installation by approved contractors with written guarantees. DIY systems rarely meet these standards.

For non-habitable cellars (storage only), DIY cavity drain membranes are available. However, incorrect installation (poor lapping, blocked drainage channels, inadequate sump sizing) can lead to flooding and structural damage.

If you proceed with DIY:

• Use a reputable system (Delta, Platon, Oldroyd) with full installation instructions.
• Install dual pumps and a high-water alarm.
• Ensure drainage channels slope toward the sump (minimum 1:100 gradient).
• Lap all joints by at least 100mm.
• Test the sump and pump before backfilling or finishing.

Maintenance and long-term performance

Type A (external barrier)

Requires no routine maintenance if installed correctly. However, membrane damage (root penetration, puncture during landscaping) is difficult to detect and repair without excavation.

Type B (cementitious)

No maintenance required. However, structural movement or cracks will compromise waterproofing. Regular inspection for cracks is recommended, especially in the first 5 years after installation.

Type C (cavity drain)

Requires annual sump pump testing and cleaning. Check that:

• Pumps activate when water level rises.
• Backup pump operates if primary pump fails.
• High-water alarm sounds if both pumps fail.
• Drainage channels are clear of debris.

Most contractors offer annual maintenance contracts (£150-300) that include pump testing, cleaning, and guarantee renewal.

Sources

• BS 8102:2009: Code of practice for protection of below ground structures against water from the ground
• Building Regulations Approved Document C (2022): Resistance to moisture
• Property Care Association: Code of Practice for Below-ground Waterproofing (2023)
• BRE Report BR 528: Basement construction and waterproofing (2012)