Cavity Drain Membrane systems – Type C systems under BS 8102:2022
If you have been considering waterproofing your basement, it is quite probable that you will have seen BS 8102: 2022 being mentioned. This is the British standard, which outlines the code of practice for protecting structures below ground from water ingress. BS 8102:2022 breaks down basements into specific grades of water-tightness, along with the types (systems) of waterproofing into 3 types, which are; Barrier Protection, Integral protection and drained protection.
BS 8102:2022 defines 4 grades of water-tightness which should be considered in a below ground structure. These are:
Grade 1a: For the purposes of BS 8102:2022, seepage and damp areas as a result of internal and external moisture are tolerable, where this does not impact on the use of the below ground level structure. Internal drainage may be necessary to deal with water seepage.
Grade 1b: for the purposes of BS 8102:2022 no seepage or water penetration from internal or external sources is tolerable.
Grade 2: for the purposes of BS8102:2022, No water seepage is acceptable. Damp areas as a result of internal moisture / condensation are tolerable, depending on use of the area. Measures may be required to manage the production of water vapour / condensation etc.
Grade 3: for the purposes of BS 8102:2022, no water ingress, or damp areas are acceptable. Ventilation, dehumidification, heating and, or air-conditioning is likely to be necessary; appropriate for the intended use of the space.
BS 8102: also defines 3 types of water-resistant construction as;
Type A – Barrier (Membrane)
This is where a structure is constructed from concrete or masonry and offering no protection against the ingress of water or water vapour by the nature of its design. Protection is therefore totally dependent on a continuous barrier system applied to the structure. This would either be applied on the external (positive) side, or the internal (negative) side using waterproof renders etc.
B – Structurally integral
In this scenario, the structure will be designed and constructed in reinforced or prestressed concrete either to BS EN 1992-1-1:2004 and BS EN 1992-2:2005 [to minimize water penetration], or to BS EN 1992-3:2006 [to prevent water penetration]. However, dependent on the chosen grade of basement use, transmission of water vapour (positive to negative sides) may not be wholly prevented.
C – Drained Protection
Generally, in new-build basement structures, the structure will be formed using structural concrete to minimize the ingress of water. Cavity drained membrane systems are also installed as retro-fit (into a refurbishment project) or as part of a new-build project and is generally considered the most effective and trouble free when specified and installed correctly.
In the following, we will deal primarily with Type C (drained protection) systems:
Firstly, if you are considering converting your basement, cellar or sub-pavement vault it is worth arranging for a CSSW (Certified Surveyor in Structural Waterproofing) qualified surveyor to visit the property so that your requirements can be properly assessed.
Any moisture which does find its way into the basement is managed and channelled, collected and discharged via a sump / pump unit. Vapour transmission may be prevented by ventilation of the cavity created by the cavity membrane system. It is extremely important that the system is maintained i.e. the drain channels are flushed through and the sump / pump units are serviced at the required intervals.
Most membrane systems are likely to include four main components, which include: Cavity Drain Membrane materials to the floors, walls and, in places, soffits of the basement, Drainage Channels set into the perimeter of the floor slab, Pump Systems connected to the drainage Channels, plus Control Systems, which give warnings and high-water level alarms. Depending on the situation, the type of structure and design of the systems, there may also be a need to install additional, barrier type membrane to walls, floor slabs and soffits. This will then be deemed a Type A and Type C waterproofing system in compliance with NHBC requirements.
Cavity Drain Membranes and Drainage
It is important that the installed cavity drain membrane (CDM) systems are maintainable. By which, we mean that the perimeter drain channels are accessible in places, where jetting points are installed, to allow for the perimeter drain channels to be “flushed through”. Also, the sump / pump units must be accessible for service engineers to access and test the pumps on a regular basis. Such servicing may be once or twice annually, depending on the water flow from the drainage systems into the sump chamber(s). Flushing through of the drainage channels is usually the responsibility of the property owner. A separate contract to inspect and maintain the sump pumps, alarm systems etc. should be set in place with a sump / pump servicing contractor.
Due to the way in which a cavity drain membrane system works, it is generally thought of as the lower risk option (in the right environment), when considering basement waterproofing. This is because it is essentially managing the water ingress which may be occurring, as opposed to trying to stop, and possibly diverting the water.
The use of cavity drain membranes does not allow pressure to build up against the internal construction. The air gap – between the wall / floor substrate and the membrane materials – allows the substrate to be vapour permeable and does not increase pressure on the structure which may be the case with a barrier waterproof system.
Pumps and Control Systems
Once the water has been captured by the cavity drain membranes and drainage channels, it needs to be safely removed. Whichever sump chamber and pump system is installed, it should be designed to effectively pump water from the sump chamber out and into ground drainage systems.
Some advantages of installing a cavity drained membrane system?
The cavity drained system offers certified basement waterproofing. It efficiently captures and removes water from behind the membrane on the walls, floor and soffit into the drainage systems, which is then pumped out from the sump chamber. This creates a dry and protected internal environment.
Its versatility makes it suitable for various projects, including new-builds, refurbishments, and historic buildings. One of the reasons that cavity drained membrane system(s) are preferred in historic buildings, is because the membrane system leaves the historic substrate intact. Whereas a rendered system (Type A – barrier protection) would seal and potentially damage the surfaces of the substrate.
The system may also provide a degree of gas proofing capabilities for below-ground spaces. With compatible ancillary products and efficient water removal using pumps, the CDM System is a reliable and maintainable solution.
As it is often possible to insulate in the space between the membrane materials and the internal plasterboard, it also creates a warmer environment than a render / plaster waterproof system.
Once the membrane has been installed, wall surfaces can be dry lined or plastered directly (onto meshed membrane) and floors can be screeded, or a floating dry board flooring system installed. This allows for internal decorative finishes to be installed much quicker than would be the case with a render / plaster system.
A fully installed system will be guaranteed by the installer for a Ten-Year period and can also be protected with an optional Ten-Year warranty, from a separate third-party insurer.
Some things to be aware of when considering installation of a cavity membrane system:
Probably the most common one we come across, is the builder / person who are not specialists in this type of work but tell the client they can purchase the materials and install the system at a lower cost. It is these instances where we probably come across the most system failures. The areas where failures are most likely, tend to be the joints between wall and floors, or wall and soffits, or other junctions where drain channels meet. Often, where this is the case, repairs are not possible and it means large-scale replacement of the system is the only option.
Is the cavity membrane system, coupled with subsequent floor build-up, extra thickness on internal walls etc. going to be suitable for the basement in your property. It is an important consideration, as the build up on the floor membrane can be around 100mm and the wall membrane / battening and plasterboard on the walls can be 50-60mm.
Can the water drain from a discharge pipe connected to the sump chamber, into a suitable drain system?
If you would like to discuss your requirements with one of our qualified Surveyors, or would like to book an appointment for an inspection of the basement, please contact us on:
Tel: 01276 66466
