What is tanking in construction?

Tanking is the application of a continuous waterproof barrier to the surfaces of a below-ground structure - typically a basement or cellar - to resist the entry of water under hydrostatic pressure from the surrounding ground. It is classified as Type A (barrier) waterproofing under BS 8102. Products include cementitious tanking slurries, bituminous membranes, and crystalline waterproofing compounds.

What is the difference between tanking and a cavity drain membrane?

Tanking (Type A waterproofing) aims to stop water entering the structure altogether by applying a barrier to the walls and floor. A cavity drain membrane (Type C waterproofing) accepts that water will get in and instead manages it - channelling any ingress to a sump and pump system. Type C is generally more reliable for existing structures where the substrate is uneven or cracked.

Can tanking be applied internally?

Yes, internal tanking is common in basement conversions where external access is not possible. However, internal tanking must resist hydrostatic pressure pushing the membrane away from the wall, which requires the system to be well keyed to the substrate and any floor-wall junction carefully detailed. For retrofitting, cavity drain membranes are often more reliable than internal tanking slurry alone.

What grade of waterproofing does a basement need?

BS 8102:2022 defines four grades of waterproofing performance for below-ground structures. Grade 1 allows minor damp and seepage, suitable for car parks and plant rooms. Grade 2 allows no water ingress, suitable for workshops. Grade 3 (dry) is required for habitable spaces such as living rooms and bedrooms. Grade 4 is used for archives and highly sensitive environments.

Tanking - Construction Glossary

Definition

Tanking is the application of a continuous waterproof barrier to the walls and floor of a below-ground structure - typically a basement or cellar - to resist water ingress under hydrostatic pressure from the surrounding ground. Under BS 8102, it is classified as Type A (barrier) waterproofing. Products include cementitious slurries, bituminous coatings, crystalline waterproofing compounds, and bonded sheet membranes.

In practice

Tanking is most effective when applied externally to a new structure before backfilling, where the hydrostatic pressure pushes the membrane against the substrate rather than away from it. Internal tanking is used in retrofit situations where external access is not possible - typically cementitious slurry applied in two or three coats to prepared masonry, with all angles and junctions reinforced with waterproofing tape or fillet.

The critical weakness of any tanking system is the floor-wall junction and any penetrations (pipes, cables). These must be carefully detailed - a continuous cove fillet at the floor-wall junction prevents bridging and stress concentration. For existing structures with cracked or uneven masonry, a Type C cavity drain membrane system is often more reliable because it manages water rather than trying to exclude it entirely.

Building Regulations

Relevant PartPart C - Resistance to moisture

Key standardBS 8102:2022 - Protection of below-ground structures

ClassificationType A - Barrier waterproofing

Habitable basementGrade 3 (dry environment) required

BS 8102:2022 is the key standard for waterproofing below-ground structures in the UK. It recommends that for any habitable below-ground space, a minimum of two complementary waterproofing types should be used (e.g. Type A tanking combined with Type C drainage) - a dual-protection approach. Building Control will assess compliance with Approved Document C for any basement conversion or new below-ground habitable room.

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