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What Is Tanking? — Basement Waterproofing Explained
A plain-English guide to tanking and basement waterproofing: Type A, B, and C systems, when to use each, and BS 8102 guidance for UK construction.
Definition
Tanking is the process of making a below-ground structure waterproof to prevent water ingress. The term traditionally referred to applying a cementitious waterproof coating (tanking slurry) to basement walls and floors, but it is now used more broadly to describe any basement waterproofing system. The UK standard for basement waterproofing is BS 8102:2022 - Code of Practice for Protection of Below-Ground Structures Against Water from the Ground.
When Is It Used?
Tanking is required whenever a habitable or usable space is created below ground level. This includes converting existing cellars into living space, constructing new basements, building retaining walls with habitable space behind them, and protecting underground plant rooms or storage. The choice of waterproofing system depends on the grade of waterproofing required (BS 8102 defines grades 1-3 based on intended use) and the site's hydrostatic conditions.
Key Facts
- BS 8102:2022 classifies waterproofing into Type A (barrier), Type B (structural), and Type C (drained)
- Type C cavity drain membrane is the most popular system in UK basement conversions
- Cementitious tanking slurry (e.g., Sika-1, Vandex) is applied in 2-3 coats to achieve 2 mm minimum thickness
- Cavity drain membranes (e.g., Delta MS500, Platon) have 8 mm or 20 mm studded profiles
- A sump pump with battery backup is essential for Type C systems
- Waterproofing design should be carried out by a CSSW (Certificated Surveyor in Structural Waterproofing)
- PCA (Property Care Association) members offer insured guarantees for waterproofing work
- Building Regulations require basement rooms to have adequate means of escape, ventilation, and insulation
Related Calculators
Use the Concrete Volume Calculator for basement floor and wall quantities, or the U-Value Calculator for below-ground insulation design. The Basement Sump Calculator helps size pumps for cavity drain systems.
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Frequently Asked Questions
BS 8102 defines three types: Type A (barrier protection) uses a waterproof membrane or coating applied to the structure; Type B (structurally integral protection) relies on the concrete structure itself being waterproof, using waterproof concrete or additives; Type C (drained protection) uses a cavity drain membrane system that manages water entry by channelling it to a sump pump for discharge.
The choice depends on the water table level, ground conditions, and intended use. Type C (cavity drain) is the most commonly used system in the UK for basement conversions because it manages water rather than trying to stop it, making it more reliable in the long term. Type A is suitable where water pressure is low, and Type B is common in new-build construction. Many projects combine two types for added security.
Costs vary significantly by system and size. Cementitious tanking slurry (Type A) typically costs £40-£80 per m² applied. Cavity drain membrane systems (Type C) cost £80-£150 per m² including membrane, channels, sump, and pump. A complete basement conversion including waterproofing, insulation, and fit-out typically costs £1,500-£3,000 per m² of floor area.
Converting an existing cellar into habitable space generally does not require planning permission as it is usually classified as permitted development. However, you will need Building Regulations approval for structural work, fire safety, ventilation, insulation, and drainage. If you are excavating to create a new basement, planning permission is normally required, and a structural engineer must design the underpinning.
A sump pump is an electrically powered pump installed in a sump chamber (pit) at the lowest point of the basement. In Type C cavity drain systems, water collected behind the membrane is channelled to the sump and pumped out to a suitable discharge point (surface water drain, soakaway, or storm drain). A battery backup pump is strongly recommended in case of power failure. Sump pumps should be serviced annually.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.