What is BS8102?
BS8102 is a British Standard for the design of waterproofing systems for basements : BS8102: 2009 Code of practice for protection of structures against water from the ground. All good companies, with qualified surveyors (C.S.S.W. – Certified Surveyor in Structural Water-proofing), will refer to this standard early on in discussions with you. Those that don’t should be avoided!
Any specification you are offered should state clearly if BS8102 has been considered. Those that do, require the surveyor to design a waterproofing system that can be expected to withstand entry of groundwater into your basement. Note: this does not normally include water that arises from leaking drains – although the system might well deal with that too.
Those systems that do not consider the requirements of BS8102 are unlikely to come with any guarantee against water entry from the ground or the guarantee may be questionable. In most cases, these conversions have excluded the drainage part of the system or the drainage system may be substandard, often to save cost.
To avoid confusion, the best way to refer to these treatments is to refer to them as “waterproofing”. This can then be sub-divided into tanking and cavity drainage membrane (CDM) systems.
Understanding the principles involved in waterproofing a structure allows the risk of failure of the system applied to be reduced to an acceptable level. Furthermore, with forethought, systems can be used where, should problems occur, remedial / repair work can be carried out without excessive disruption.
Basement/cellar waterproofing is the application of a covering to masonry usually, but not necessarily, below ground, to prevent or control penetrating dampness or water ingress. The masonry can be treated from outside or inside, but most waterproofing to existing buildings is carried out from inside, as access to the outside is not possible or practical. Waterproofing and the types of systems in use is a complex subject. We cover only the basic details here. Each project will have its own conditions and requirements and therefore require specific expert advice.
The type of waterproofing chosen depends on the conditions which it is trying to deal with. This is the control of water vapour, or a little liquid water or considerable amounts of water. The use to which the area will be put is also important. Both tanking and CDM systems can deal with all of the above, but the performance of each system has different results and different effects on the structure to which it is being applied. This is such an important feature that in some cases only one of the system types will be suitable. Choice of the wrong system could result in structural failure of the building components. This is another reason why expert advice is needed.
Grades of Waterproofing Protection
British Standard BS8102 : 2009 – Code of Practise for Protection of Structures Against Water from the Ground defines performance levels for the dryness of buildings in three grades as follows:-
|Grade||Basement Usage||Performance Level|
|1||Car parking, plant rooms (excluding electrical equipment), workshops||Some water seepage and damp areas tolerable dependent on the intended use. Local drainage might be necessary to deal with seepage.|
|2||Plant rooms and workshops requiring a drier environment (than grade 1), storage areas||No water penetration acceptable. Damp areas tolerable; ventilation might be required.|
|3||Ventilated residential and commercial areas, including offices, restaurants, leisure centres etc.||
No water penetration acceptable.
Ventilation, dehumidification or air conditioning necessary, appropriate to the intended use.
In most cases, a Grade 3 (Dry Environment) level of protection is required for habitable use or for storage of anything that could be damaged by moisture.
Types of Waterproofing
British Standard BS8102 : 2009 – Code of Practise for Protection of Structures Against Water from the Ground defines different types of waterproofing system. There are 3 recognised waterproofing types:-
|B||Structurally Integral Protection|
It is generally considered best practice to install dual systems which comprise of 2 different types of waterproofing as this provides a failsafe in case the primary waterproofing barrier fails. When converting an existing basement this is not usually practical.
Tanking (Type A - Barrier Protection)
Type A Tanking can consist of:-
- Cement based coatings and multi-coat render systems
- Liquid applied bituminous & synthetic rubber coatings
- Externally acting liquid injected resins, installed from inside the structure, acting from the outside
- Plastic, bituminous and rubber membranes
Method 1 requires a high level of original surface preparation as these coatings are designed to stick or key to the walls being tanked. If they fail to key then water under hydrostatic pressure (penetrating dampness below ground) can force the material to debond from the original surface and break the tanked surface. Some variants are particularly well suited for new concrete and as such work very well on new build structures.
Method 2 is a particularly useful as liquid applied membranes are often more flexible than other forms. Large areas can also be covered in a time efficient manner using spray application. These systems are usually suited to external use as waterproofing barriers but may also be used internally as vapour barriers.
Method 3 is a niche product that is usually selected for tanking failure repairs of say movement or casting joints.
Method 4 is a common method of tanking for new build properties, often in conjunction with other forms of waterproofing.
Tanking (Type B - Structurally Integral Protection)
Type B tanking is usually waterproof concrete. This is concrete strictly manufactured using specific grades of sand correctly mixed with the correct ratio of cement and water. Often there are admixes introduced to the concrete to aid its waterproofing properties.
Type B tanking systems will be formed by casting the concrete in stages. The construction joints will incorporate a hydrophilic water bar. This is a type of seal that will swell in contact with water thus sealing the joint tightly so that water cannot enter.
Type B waterproof concrete systems are almost certainly exclusive to new build.
Cavity Drainage Systems (Type C - Drained Protection)
A cavity drainage system manages water ingress using membranes and drainage.
The membranes used are specifically designed for this purpose. They are formed using extruded high-density polyethene, sometimes coated with thermically bonded polypropylene mesh to aid with the adhesion of finishes. They have a profile similar to an egg box or the sole of a football boot and are often referred to as studded or dimpled sheet membranes. The membrane is loosely mechanically fixed to walls using sealed nylon plugs.
The purpose of the membrane is to form a cavity between the membrane and wall/floor. This cavity allows groundwater to freely run under the influence of gravity towards the base of the wall into a planned drainage system. The air gap also allows some degree of breathability behind the membrane whilst working as a vapour barrier, limiting moisture vapour from reaching the room.
There are 2 methods of providing drainage for cavity drainage systems.
Method 1 – Perimeter Drainage
This method involves a prefabricated void forming channel to be placed around the perimeter of the basement, usually at the floor wall junction. In larger basements or basements with a complex circumference, additional channelling may be installed to link runs of perimeter drain.
The channel will lead to a removal point such as an external drain or sump chamber.
Method 2 – Modular Drainage
This method involves the placement of a 110mm drainage network into the floor slab with an opening created in the floor at centres of around 1 every 12 square metres. This method allows water to run below the floor membrane where the drainage holes act as a plug hole.
The drainage network will lead to a removal point such as an external drain or sump chamber.
With either system access to inspect and service the drainage channels must be allowed for in the design.
Sump & Pump Systems
When water cannot be taken away from a basement safely and reliably using gravity drainage then a pump station will be required.
This will normally comprise of a prefabricated plastic sum chamber sunk into the floor of the basement. Within the sump chamber, a pump arrangement is then installed. This is an automatically float activated system with a high water alarm.
Again, component failure should always be allowed for in the design. It has now become the norm and considered best practice for pump systems to comprise of 2 pumps (a primary and backup) as well as a battery back up (uninterrupted power supply – UPS) system. This means that if the first pump fails then the second pump will activate either by mains power or in the event of a power failure then the UPS.
Pump systems technology has rapidly developed in recent years. It is now easily possible to monitor pump system remotely and as well as receiving notifications in case a major component fails.
As a pump system is a mechanical system it should be serviced to ensure its functional service life is maintained.