Tanking

Tanking is the process of waterproofing below-ground structures — basements, cellars, and underground rooms — to prevent water from the surrounding soil entering the living space. Unlike a damp-proof course, which deals with rising damp in above-ground walls, tanking creates a waterproof barrier on all surfaces that are in contact with the ground: walls, floors, and sometimes the junction between the two.

Why tanking is needed

Below-ground structures face water pressure (called hydrostatic pressure) from all directions. Soil holds moisture, and that moisture pushes against basement walls and floors. Without effective tanking, you’ll experience:

• Damp or wet walls and floors
• Mould growth
• Musty smells
• Damage to stored belongings
• Structural deterioration over time

Types of tanking

Cementitious tanking (internal)

A multi-coat waterproof slurry is applied to the inside surfaces of walls and floors. This is the most common method for domestic basements:

• Relatively affordable
• Can be applied to existing walls
• Requires careful preparation — the surface must be clean and sound
• Cannot tolerate movement (may crack if the building settles)

Membrane tanking (internal)

A dimpled or studded membrane is fixed to the walls, creating a cavity that allows any water ingress to drain to a sump pump:

• Manages water rather than trying to stop it completely
• More tolerant of building movement
• Requires a sump pump and drainage channel
• The cavity means you lose a small amount of floor space

External tanking

A waterproof membrane is applied to the outside of the basement walls. This is the most effective method but:

• Requires excavating around the building — expensive and disruptive
• Usually only practical during new construction or major structural work
• Stops water before it reaches the structure

When tanking comes up in renovation

Tanking is relevant when you’re:

• Converting a cellar or basement into a usable living space
• Addressing persistent damp in below-ground rooms
• Building a new basement extension (increasingly popular in urban areas where lateral space is limited)

Tips for homeowners

• Get a specialist survey — basement waterproofing is a specialist discipline. A general builder may not have the right expertise
• Expect significant cost — proper tanking is not cheap, but cutting corners leads to expensive failures
• Consider a sump pump — even with tanking, a sump pump provides a safety net if water pressure overwhelms the membrane
• Check building regulations — converting a basement to habitable space requires building control approval for ventilation, fire escape, ceiling height, and structural adequacy
• Budget generously — basement projects are notorious for uncovering surprises, so keep a healthy contingency