Basement Tanking Explained
Any structure built partially or fully below ground level is vulnerable to water ingress. Groundwater exerts constant pressure on basement walls, cellar walls, floors, and internal junctions, seeking weak points in masonry joints, floor-to-wall junctions, and service penetrations.
Think of pressing an empty ice cream tub, held flat, into a basin of water; the resistance is surprising. The principle is the same for basements. Although groundwater pressure is reduced by rock, clay, soil and other mineral elements, it still applies continuous force to existing structures below ground.
At depths of around 2.5 metres, hydrostatic pressure can reach 20–25kPa; sufficient to force moisture through unprotected masonry in a short period of time. Basement tanking is designed to control that pressure by forming a bonded barrier applied directly to the substrate, helping keep cellars dry and maintain a fully watertight basement.
The term “tanking” comes from the concept of creating a “negative tank”; a structure designed to keep water out rather than hold it in. In effect, a properly waterproofed basement behaves like an inverted water tank.
Recognised Basement Waterproofing Methods (BS8102:2022)
The waterproofing industry follows BS8102:2022 – Code of Practice for Protection of Below Ground Structures Against Water Ingress, which identifies three recognised waterproofing types.
Type A: Barrier Protection (Cementitious Tanking)
Type A systems include cementitious slurry coatings, bitumen coatings and liquid-applied waterproof membranes that bond directly to the substrate.
These systems:
- Create a continuous barrier seal
- Block water ingress by retaining water behind the structure
- Use specialist mortars and detailing at joints and service entries
Type A is commonly used internally to form a negative tank, particularly in refurbishment projects or where external excavation is impractical.
Typical applications:
- Existing basements in older properties
- Brick and stone-masonry
- Partial basements and garages
- Sites where external waterproofing is not feasible
Advantages:
- Cost-effective in many retrofit scenarios
- Versatile across a range of structures
- Suitable as part of dual waterproofing systems
Limitations:
- Does not relieve water pressure (only resists it)
- Requires thorough preparation
- Not ideal where structural movement is expected
Type B: Structurally Integral Waterproofing
Type B systems incorporate waterproofing directly into the structural design, typically through waterproof concrete and integrated joint sealing systems.
Typical applications:
- New build basements
- Larger engineered underground projects
Advantages:
- Waterproofing is integrated into the structure
- No internal membranes to damage during fit-out
Limitations:
- Highly dependent on design accuracy
- Rectification can be costly if errors occur
- Generally not suitable for retrofit projects
Type C: Drained Protection (Cavity Drain Systems)
Type C systems manage water rather than block it. Cavity drain membranes (often referred to as “egg box” membranes) allow water to enter behind the membrane, directing it into perimeter drainage channels and sump pump systems.
Typical applications:
- New build basements
- High water table sites
- Refurbishment projects with significant groundwater
Advantages:
- Relieves hydrostatic pressure
- Creates a dry internal environment
- Fast installation over large areas
Limitations:
- Higher system cost
- Requires pump power and maintenance
- Must be correctly designed by a competent specialist
When Basement Tanking Is the Right Solution
Basement tanking is typically specified where:
- Water ingress is occurring below ground level
- Hydrostatic pressure is present
- Persistent damp remains despite surface treatments
- A basement or cellar is being converted to habitable use
- Structural waterproofing is required, not just surface containment
Tanking should never be treated as decorative coating. Correct specification and assessment, often involving a qualified surveyor or waterproofing specialist, are critical to long-term success.
Do You Tank the Whole Basement?
Partial tanking is considered high risk. Water will often migrate around untreated areas and reappear elsewhere. A basement waterproofing system must be continuous, much like the hull of a ship, it cannot be “partly waterproof”.
Choosing the Right Basement Tanking System
System selection depends on:
- Wall construction
- Ground conditions and water table level
- Intended use of the space
- Risk assessment and long-term durability requirements
New builds often combine Type B and Type C systems. Refurbishment projects commonly use Type A or Type C depending on site conditions and groundwater levels.
Because the long-term integrity of fit-out works depends entirely on the waterproofing system, correct specification is essential. At Kingfisher Building Products, our technical team supports contractors and property owners in selecting and detailing appropriate tanking systems as part of a compliant, long-term waterproofing strategy.
Frequently Asked Questions
Is basement tanking suitable for habitable conversions?
Yes. When correctly designed and installed in accordance with best practice, basement tanking can support dry, safe living accommodation.
Can basement tanking stop active water ingress?
Yes. Properly specified systems are designed to resist ongoing groundwater pressure.
Does basement tanking require professional installation?
In most cases, yes. Structural waterproofing demands correct preparation, detailing, and curing to perform reliably.
Can a general builder install basement tanking?
While many waterproofing contractors began as general builders, structural waterproofing requires specific training and technical understanding. Manufacturer training and technical support are strongly recommended.
