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Type C Waterproofing

The use of type C systems as a method of structural waterproofing has become the commonest form of waterproofing system used in retrofit and existing basements and increasingly is being fitted in new build basements. Their popularity as a type of waterproofing system is a result of the fact that systems are relatively quick to install and have a greater margin for defects.

What exactly is type C protection?

Type C is defined by BS8102:2022 (Code of practice for protection of below ground structures against water ingress) as drained protection. The structure itself provides primary resistance against water ingress and incorporated drained cavity within the basement structure collects any leakage.

Typically, seepage is removed via a mechanical sump pump system, or occasionally by gravity to low ground or drains externally where properties are formed into sloping sites. A third option is to use pumped back-flow protection devices where connection to drains is not safe. In all cases, consideration should be given to the point at which water discharges, understanding that the effectiveness of the system is reliant on removal of water, so an appraisal of this factor is required.

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When Type C might not be used 

Type C is applied in both existing and new build construction and its greater tolerance for errors in workmanship make it a very attractive option. However their continued success relies on continued maintenance. 

The nature of the systems means that the structure must provide the primary form of resistance to the ingress of moisture and they may not be suitable for convoluted shapes and flat soffits.

Other types of waterproofing >>

How does Type C waterproofing work?

Type C systems do not provide a hydrostatic barrier but provide protection by means of water management. The external basement wall must provide enough PRIMARY resistance to water ingress to ensure the cavity accepts only a controlled amount of water or dampness.

Type C construction relies on water being resisted by the structure and any water that penetrates being collected in a cavity formed between the external wall and an internal lining/wall. There is permanent reliance on this cavity to collect groundwater seepage and direct it to a suitable discharge point, e.g. drains or a sump for removal by gravity drainage or mechanical pumping. The amount of free water entering the cavity will depend on the size of the structure, on the volume of external water and its hydrostatic pressure, and on the resistance of the structure itself to water ingress.

Type C pumped systems should be engineered to cope with worst-case water ingress. If drainage capacity is exceeded, this may result in dampness or flooding. Type C systems are only designed to control and manage minor leakage and seepage into a structure.

The use of Cavity Drainage Membranes for Type C Protection

Cavity Drainage Membranes (CDM) are vacuum formed high-density polyethylene or polypropylene sheet materials with moulded studs for application internally to below ground structures, on internal walls, sloped ceilings and floors.  The fixed wall and ceiling membranes are used to support dry lining or may be plastered (depending on the type of membrane), whilst the floor membranes may be screeded or overlaid with wooden flooring.

Cavity drainage systems will not usually put the substrate into tension, but they will need to be adequately drained in accordance with the manufacturers’ instructions to either natural drainage or pumping points. Cavity drainage systems cannot withstand any hydrostatic pressure. Mechanical equipment such as pumps will require routine maintenance and the system will fail if there is a breakdown of the power supply. All drainage channels that form part of the system should also be subject to regular routine maintenance in order to ensure the free flow of water to drainage points. It should be determined that the drainage outlet has the capacity to the additional flow.

PCA - Type C Waterproofing

Limitations of Type C waterproofing systems

  • May not be suitable for application to convoluted shapes and flat soffits.  
  • The forming of drainage channels or rebates into structural or reinforced floor slabs may have unforeseen consequences. It is therefore important to consider the structural implications of this action. If there is any doubt about the ability of a structure to absorb the changing stresses caused by waterproofing, the guidance of a structural engineer should be sought.

Maintenance of Type C systems

The need to service and maintain the drainage elements of a Type C Waterproofing system is paramount to its long term success. As cavity drainage systems rely on free drainage usually in association with sumps, perimeter drains and mechanical pumping devices it is essential that these elements are regularly maintained to ensure their long term effectiveness.

Immediately after the installation of a drainage system, the drainage channels and sumps MUST be cleared out and tested. Pumping devices must be checked, tested and properly commissioned. 

In accordance with the Code of Practice BS 8102:2022, they need to be maintained. This has been helped significantly with the introduction of perimeter drainage channels and inspection ports, so as to make the drainage aspects maintainable and help to prevent blockages caused by the existence of free lime for example.

BS8102 2022 Document >>

Issues of ‘free lime’ in cavity drainage systems

In most new construction, retrofit and also in refurbishment basements (where the floor has been replaced), there is a high risk of free lime and /or mineral salts leaching from the concrete walls and floors. In retrofit, this is particularly prevalent where “dry pack” is used at the top of the underpinning.

As free lime leaches from the new construction by groundwater ingress it deposits itself within the drainage cavity, (behind and underneath membranes) and particularly within the sump chamber and around the sump pumps. This can potentially cause sump pump failure and therefore failure of the type C system.

The impact of free lime within the system will greatly increase the frequency of maintenance over the first 3 – 5 years, but especially within the first six months, reducing the interval to weeks in some instances, thus increasing both the costs of maintaining the system and also putting the system under undue risk.

In order to minimise the risk of free lime impacting on the system, an “anti-lime” coating should be applied to the new concrete in all cases.

Service Intervals for Type C systems

Type C systems should be inspected and serviced at least annually, but in some circumstances the period between services may be considerably shorter. Site conditions, design, materials, machinery used and the implications of any system failure will all play a part when deciding on the frequency and scope of service visits. 

It is advisable to schedule the first service visit within three months of the system being commissioned. This will highlight any problems, allow for the removal of debris that may have been deposited in the drainage channels during the construction phase and allow the service engineer to assess the risk posed by free lime and silt that may be washed in-to the system.  

If uncertainty exists as to the frequency of service visits then it is always advisable to take a cautious approach. It is better to schedule more visits than may be necessary and prevent a failure rather than leave things too long and risk a flood. If it is decided that visits are occurring too often then the maintenance schedule can be amended (see note in service and maintenance section above).  

Type A Waterproofing

Historically known as “Tanking”, type A waterproofing provides protection (a barrier) against groundwater ingress by applying a waterproof material (such as cement) to the internal or external walls and floor slab of a basement or underground structure forming a barrier between the structure and any groundwater present.

More about Type A Waterproofing >> 

Type B Waterproofing

Type B protection is where the structure itself is constructed as an integral water resistant shell. This relies heavily on the design and materials incorporated into the external shell of the structure, as well as the quality of the workmanship contributes to the success of Type B Systems.

More about Type B Waterproofing >>

Additional technical documents you can view

For those interested, there is a variety of structural waterproofing related 'Codes of Practice', Technical Documents, Guidance Notes via our Technical Document Library.  Simply click on the button below to view the library.  Documents of interest include:

  • Code of Practice for Waterproofing of Existing Below Ground Structures
  • Insulation in Basements guidance note
  • Best Practice Guidance: Continuity of Waterproofing Systems
  • Best Practice Guidance: Podium Decks and Buried Roofs

Visit the library >>

Training courses that might be of interest

Want to learn more about structural waterproofing

For those interested in learning more about structural waterproofing and BS8102 requirements; there is a variety of PCA training options for surveying professionals as well as technical/trade professionals. 

Use the search tool below to find available waterproofing related training courses or simply go to our training & qualifications section.  Alternatively, if you want to chat to someone, contract our training team on 01480 400 000 or contact them online.

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Types of Waterproofing - A,B & C

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