Tanking describes the process of making a below-ground structure waterproof to prevent water ingress. Originally referring to cementitious waterproof coatings applied to basement walls and floors, the term now encompasses any basement waterproofing system. In the UK, the standard for basement waterproofing is BS 8102:2022 – Code of Practice for Protection of Below-Ground Structures Against Water from the Ground, which classifies systems into three types based on their waterproofing mechanism.

Tanking is required whenever a habitable or usable space is created below ground level. This includes converting existing cellars into living spaces, 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.

What Are the Three Types of Basement Waterproofing?

BS 8102 defines three types of waterproofing protection for below-ground structures:

  • Type A (Barrier protection) – A waterproof membrane or coating applied directly to the structure. Cementitious tanking slurry, such as Sika-1 or Vandex, is applied in 2–3 coats to achieve a minimum thickness of 2 mm.
  • Type B (Structurally integral protection) – The concrete structure itself is waterproof, using waterproof concrete or additives that reduce permeability.
  • Type C (Drained protection) – Cavity drain membranes manage water entry by channelling it to a sump pump for discharge. Membranes such as Delta MS500 and Platon feature 8 mm or 20 mm studded profiles.

Type C cavity drain membrane systems are the most popular choice for UK basement conversions. These systems accept that water may reach the structure but manage it through controlled drainage rather than attempting to create an absolute barrier.

How Does a Cavity Drain System Work?

In a Type C system, a studded membrane is fixed to the basement walls and floor, creating a small cavity between the membrane and the structure. Water that penetrates the substrate enters this cavity and is channelled via perimeter channels to a sump chamber installed at the lowest point of the basement. A sump pump automatically discharges collected water to a suitable location, such as a surface water drain or soakaway. A battery backup pump is essential in case of power failure, and sump pumps should be serviced annually.

This approach is often considered more reliable in the long term than barrier systems (Type A) because it does not rely on the membrane remaining perfectly intact. Localised failures in a barrier system can allow water to enter undetected, whereas cavity drain systems are designed to manage water ingress continuously.

When Should Cementitious Tanking Slurry Be Used?

Cementitious tanking slurry, a Type A system, is suitable where hydrostatic water pressure is low and the structure can provide a stable substrate for coating application. Products based on cement and polymer additives are applied in multiple coats, creating a waterproof barrier that adheres directly to masonry, concrete, or blockwork.

However, cementitious systems require the substrate to be sound, clean, and free from movement cracks. Any subsequent structural movement can compromise the barrier. For this reason, many projects combine two types of waterproofing for added security—for example, using a Type A barrier with a Type C cavity drain system as a backup.

What Are the Cost Considerations?

Costs vary significantly by system type and project size. Cementitious tanking slurry (Type A) typically costs £40–£80 per m² when professionally applied. Cavity drain membrane systems (Type C) cost £80–£150 per m² including membrane, drainage channels, sump chamber, and pump installation. A complete basement conversion including waterproofing, insulation, ventilation, 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 in the UK, as it is classified as permitted development. However, Building Regulations approval is mandatory for structural work, fire safety, ventilation, insulation, and drainage. If excavating to create a new basement, planning permission is normally required, and a structural engineer must design the underpinning.

Design and Quality Assurance

Waterproofing design should be carried out by a CSSW (Certificated Surveyor in Structural Waterproofing). Members of the Property Care Association (PCA) offer insured guarantees for waterproofing work, providing long-term protection for building owners. Proper specification and independent third-party inspection are critical to ensuring the system performs as intended over its design life.

Related to basement waterproofing, injection methods for retrospective sealing and adhesive and sealing systems offer complementary techniques for water ingress control in existing structures.