Basement Waterproofing Systems: Type A, B & C Explained

Basements and other below-ground areas are particularly vulnerable to water ingress because the walls and floors are in direct contact with the surrounding ground.

A successful waterproofing design therefore needs to do more than simply cover a damp wall. It must consider how water could reach the structure, the level of risk involved and what should happen if water gets through.

Basement waterproofing systems are commonly grouped into three categories: Type A, Type B and Type C. Each works in a different way, and in some projects more than one type of protection may be used.

Understanding the differences can help property owners see why the correct waterproofing system should be designed around the building rather than chosen as a one-size-fits-all solution.

Shield provides specialist basement waterproofing services for domestic and commercial properties across the UK.

Why Do Basements Need Waterproofing?

Below-ground structures are exposed to moisture in the surrounding soil.

The amount of water present can change considerably depending on rainfall, drainage conditions, groundwater levels and nearby construction.

Water may enter through:

• Basement walls
• Floors
• Wall-to-floor junctions
• Construction joints
• Cracks
• Service penetrations
• Defective waterproofing
• Gaps around pipes or other openings

Even a basement that has remained dry for years can sometimes develop problems when groundwater conditions change.

Our guide to water coming through basement walls explains some of the reasons previously dry basements can begin experiencing water ingress.

What Are Type A, Type B and Type C Waterproofing Systems?

The three categories describe different approaches to protecting below-ground structures from water.

In simple terms:

Type A attempts to stop water passing through the structure by using a barrier.

Type B relies on the structure itself being designed and constructed to resist water.

Type C accepts that some moisture may reach the inside of the structure and manages it safely using a drained cavity system.

The right approach depends on factors including the age and construction of the building, its intended use and the level of water risk.

What Is Type A Waterproofing?

Type A waterproofing is often described as barrier protection.

A waterproof material is applied to or incorporated within the walls and floors to prevent groundwater passing into the building.

This approach includes systems commonly referred to as tanking.

Materials can include waterproof renders, cementitious coatings, membranes and other specialist barrier products.

The principle is straightforward: create a continuous waterproof layer between the ground moisture and the internal space.

Where Is Type A Waterproofing Used?

Type A systems can be used in both new and existing structures.

They may be applied:

• Internally
• Externally
• To basement walls
• Across floors
• Around wall-to-floor junctions

The exact system depends on the construction and accessibility of the building.

External waterproofing can be particularly effective because it prevents water reaching the structure in the first place, although excavation around an existing building may not always be practical.

Internal tanking is therefore sometimes used when external access is limited.

What Are the Advantages of Type A Waterproofing?

A properly specified barrier system can provide effective protection and may be suitable for a variety of basement structures.

It can also sometimes be installed without the mechanical components associated with a drained system.

However, workmanship and preparation are extremely important.

The waterproof barrier needs to remain continuous across walls, floors, corners, joints and penetrations.

Any weak point can potentially allow water to bypass the system.

Why Can Basement Tanking Fail?

One challenge with a barrier system is that groundwater pressure acts against the waterproof layer.

If there is inadequate preparation, poor detailing, structural movement or a defect in the waterproofing, water may find a route through.

Common vulnerable areas include:

• Cracks
• Wall-to-floor junctions
• Service penetrations
• Changes in substrate
• Poorly prepared surfaces
• Areas of structural movement

Our article on why cellar tanking fails looks at some of these problems in more detail.

Tanking itself is not inherently unreliable; problems usually arise when the system is inappropriate for the structure or is badly designed, detailed or installed.

What Is Type B Waterproofing?

Type B waterproofing is known as structurally integral protection.

Instead of applying a separate internal waterproof barrier, the building structure itself is designed to resist water penetration.

This is most commonly associated with reinforced concrete basement construction.

Water resistance may be achieved through:

• Carefully designed concrete
• Controlled construction joints
• Waterstops
• Specialist joint systems
• Appropriate reinforcement
• Correct placement and curing of the concrete

Type B waterproofing is therefore generally more relevant to new-build basements and major construction projects than to an older cellar being waterproofed retrospectively.

Why Is Construction Quality Important with Type B Systems?

The waterproofing performance depends heavily on the quality of the structure.

Concrete needs to be correctly designed, placed and cured, while construction joints and penetrations require careful detailing.

Cracking also needs to be considered.

Minor cracking can occur naturally in concrete structures, but where cracks allow water ingress, specialist repairs such as resin injection may sometimes be appropriate following proper investigation.

A Type B system therefore needs to be considered as part of the overall structural design rather than as a product applied after construction.

What Is Type C Waterproofing?

Type C waterproofing is known as drained protection.

Rather than attempting to prevent every trace of moisture entering through the original structure, a cavity drain system manages any water that reaches the inside.

Typically, a studded waterproof membrane is installed on walls and sometimes floors.

Any water entering behind the membrane is directed into drainage channels and then safely removed.

Depending on the property, this may involve gravity drainage or a sump and pump system.

How Does a Cavity Drain Membrane System Work?

A cavity drain membrane creates a controlled space between the original basement structure and the finished internal surface.

If groundwater enters through the masonry or concrete, it remains behind the membrane rather than reaching the internal decoration.

Water is directed towards purpose-designed drainage channels.

From there it may flow:

• To a natural drainage point
• Into a sump chamber
• To a pumping system

The finished basement can then be constructed in front of the membrane.

Our behind-the-scenes cavity drain installation in Birmingham shows this type of waterproofing system in practice.

What Are the Advantages of Type C Waterproofing?

One major advantage is that the system is designed to manage water rather than rely entirely on resisting it.

This can make Type C particularly suitable for existing basements where groundwater pressure may vary.

It can also be less dependent on the original masonry being perfectly watertight.

However, a cavity drain system needs to be correctly designed and maintained.

Drainage channels must remain clear, and where pumps are required they need appropriate servicing and backup arrangements.

Does a Type C System Always Need a Pump?

No.

Some properties allow water to drain naturally by gravity.

In other basements, particularly where the drainage point is below the level of the external drainage system, a sump and pump may be required.

The survey and waterproofing design should establish where any collected water will go before the system is installed.

Simply fitting membrane without a properly planned drainage route does not provide a complete waterproofing solution.

Type A vs Type C Waterproofing

These are the two approaches homeowners are most likely to encounter when waterproofing an existing cellar or basement.

A Type A system aims to hold water back.

A Type C system aims to control and remove water that gets through.

Neither is automatically better.

The correct choice depends on the structure and the conditions surrounding it.

Our guide to tanking versus cavity drain systems provides a more detailed comparison of these two approaches.

Can More Than One Waterproofing System Be Used?

Yes.

Some basement designs use combined protection, incorporating two forms of waterproofing.

For example, a project could use a water-resistant concrete structure together with a cavity drain system.

The intention is to provide an additional level of protection if one element does not perform as expected.

Whether combined protection is appropriate depends on the risk, construction and intended use of the basement.

A habitable basement containing bedrooms or valuable finishes may warrant a different approach from a below-ground storage area.

Which Basement Waterproofing System Is Best?

There is no single answer.

The best system is the one that is suitable for the specific building and water risk.

A waterproofing specialist should consider factors such as:

• New-build or existing property
• Wall and floor construction
• Condition of masonry or concrete
• Evidence of existing water ingress
• Groundwater conditions
• External drainage
• Intended use of the basement
• Accessibility
• Structural movement
• Service penetrations
• Future maintenance requirements

The design should also take account of what would happen if one part of the waterproofing system failed.

Is Tanking the Same as Basement Waterproofing?

Not exactly.

Tanking is one method of basement waterproofing, normally falling within Type A barrier protection.

The wider term basement waterproofing can include Type A, Type B and Type C systems.

This distinction is important because property owners sometimes assume that waterproofing a basement simply means applying tanking slurry to the walls.

In reality, other approaches may be more suitable.

Our article on basement waterproofing versus tanking explains this in more detail.

What About Waterproof Paint?

Waterproof paints and coatings should not be confused with a properly designed structural waterproofing system.

A painted surface may temporarily disguise moisture staining but may not be capable of resisting groundwater pressure.

Where water is entering a basement, the source and level of risk need to be understood before deciding on treatment.

Simply painting over damp walls can sometimes result in blistering, peeling or moisture appearing elsewhere.

What Happens During a Basement Waterproofing Survey?

A professional basement waterproofing survey helps establish how water is entering the structure and which system may be appropriate.

The surveyor may consider:

• Wall construction
• Floor construction
• Existing waterproofing
• Cracks and joints
• Water staining
• Drainage
• Ground levels
• Previous flooding
• Sumps and pumps
• Existing membranes
• Intended use of the space

A survey is especially important where previous waterproofing has failed.

Shield’s guide to basement waterproofing surveys explains what owners can expect from the inspection.

What If an Existing Basement Waterproofing System Fails?

Waterproofing problems do not necessarily mean the whole system needs to be removed.

The first step should be identifying why the failure occurred.

Possible issues include:

• Blocked drainage channels
• Pump failure
• Defective membrane joints
• Damage to tanking
• Structural cracking
• Poor detailing around penetrations
• Inadequate drainage
• Changes in groundwater conditions

A specialist can then determine whether local repairs, pump work, drainage improvements or more substantial waterproofing work is required.

Waterproofing a Basement Before Conversion

If a basement is going to become a bedroom, office, living room or other usable area, the waterproofing should be considered before internal finishes are installed.

Problems are considerably harder and more expensive to rectify once walls, flooring, kitchens or bathrooms have been fitted.

The waterproofing design should therefore form part of the conversion planning from the outset.

This is particularly important where valuable finishes or electrical installations will be below ground.

Why Professional Design Matters

Basement waterproofing is not simply a decorating exercise.

The system has to cope with water potentially acting against a structure for many years.

Small details around joints, corners, pipes and floor junctions can determine whether the installation performs successfully.

Professional design also allows maintenance requirements to be considered from the beginning.

For example, drainage channels and sump pumps should remain accessible rather than being permanently concealed behind finished surfaces.

Type A, Type B or Type C: The Key Difference

The easiest way to remember the three approaches is:

• Type A – barrier protection: stops water passing through with a waterproof layer.
• Type B – structurally integral protection: the structure itself is designed to resist water.
• Type C – drained protection: manages water behind a cavity membrane and directs it safely away.

A professional waterproofing survey can establish which approach, or combination of approaches, is most appropriate for a particular basement.

Professional Basement Waterproofing

Shield Waterproofing & Preservation provides specialist basement and cellar waterproofing for existing buildings, conversions and new construction.

Our experienced waterproofing surveyors can assess water ingress, existing systems and the construction of the property before recommending the most suitable solution.

Depending on the project, this may include tanking, cavity drain membranes, drainage channels, sump and pump systems, resin injection and other specialist waterproofing measures.

The objective is not simply to hide damp walls but to provide a system designed around the building and the way the below-ground space will be used.

Need Advice About a Basement Waterproofing System?

If you have water entering a basement, are planning a conversion or are concerned that an existing waterproofing system has failed, arranging a survey is the best place to start.

A professional assessment can establish how the water is entering and whether Type A, Type B, Type C or combined waterproofing protection may be appropriate.

Contact Shield Waterproofing & Preservation to discuss your basement waterproofing requirements and arrange an inspection.