Yorkshire Dampcourse home page :: Basement Tanking and Waterproofing

Basement Tanking and Waterproofing

There is a British Standard to help with the design of water proofing systems for basements : BS8102: 2009 Code of practise 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 it is to BS8102 or not. Those that are, require the surveyor to design a water proofing system that can be expected to withstand entry of ground water 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 not to BS8102 are unlikely to come with any guarantee against water entry from the ground, and will have no protection against flooding. In most cases these conversions have excluded the drainage part of the system (often to save cost) and so can only be described as dry lining systems. There is more on this below.

To avoid confusion, the best way to refer to these treatments for existing basement / cellars, is to refer to "water proofing". This can then be sub-divided into tanking and cavity drainage membrane (CDM) systems. There are other methods of tanking for new-build structures that are not covered here. Tanking encompasses structures that include membranes incorporated into the structure, either outside, inside or sandwiched between masonry components. Tanking also includes waterproofing conveyed by waterproof reinforced concrete structures, which are defined by specific British Standards, specifically relating to the concrete. CDM systems always include a membrane, always include a sump / pump or gravity drainage system and are always based on allowing water to enter the building structure and then for it to be removed in a controlled way by the drainage system. The membranes are there to direct water into the drainage system and to offer a dry surface for internal finishes. A CDM system without drainage can not be a CDM system by definition, and is referred to as a dry lining system.

Tanking is the term often used to refer to the waterproofing of basements / cellars. In fact, tanking is defined as a continuous and completely waterproof membrane applied over a masonry surface, preventing completely the entry of liquid water under hydrostatic (water) pressure.

Basement / cellar waterproofing is often carried out using CDM systems or dry lining systems. The CDM systems allow water to enter behind the membrane and directs the water to sumps and conventional gravity drainage, or to active drainage systems, such as  sumps with pumps. Dry lining systems cover walls (and maybe floors), without the provision of any sump / pump or gravity drainage, on the assumption that liquid entry of water will not occur at any time in the future. This is a risky assumption as changing weather patterns in the UK mean that cellars that have been converted without sump / pump or gravity drainage, risk flooding. This is because no membrane system can be expected to hold back a head of water. The water has to be allowed to drain somewhere.

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.  

Methods of Waterproofing Existing Basements / Cellars

  1. Cement coatings and multi-coat render systems (tanking)
  2. Epoxy resins and bituminous coatings (liquid applied membranes - also tanking), applied inside the structure
  3. Externally acting liquid epoxy resins, installed from inside the structure, acting from the outside - these are new systems
  4. Cavity drainage membranes (Type C, drained protection, as defined by BS8102 : 2009)

Both 1 and 2  require 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. For this reason this company chooses to use mainly cavity drainage membranes for its waterproofing work. The exception to this is in circumstances where these cavity drainage membranes can not be applied. Here we use cement tanking products. We do not use any of the other methods in 1 and 2 above.

Recently we have examined the systems in 3. above, provided by MC Chemicals. We consider their range of products for use on specific projects that justify that approach.

We are asked frequently not to include a sump / pump drainage system in our work, usually because the client is happy with the level of flood risk associated with having no drainage. This is a cost / risk assessment by the client. In these cases we are installing a dry lining system only, and such systems are outside of, and do not comply with, BS8102: 2009.

Cement tanking is applied to prepared surfaces like plaster, either with a brush or trowel, to form a layer of material through which water should not move. This may be plastered over to give the dry finished internal surface.

Cavity Drainage Membranes

Cavity drainage membranes are formed from high density polyethylene. They have a profile similar to the sole of a football boot and are often called studded membranes. When the studs are up against the wall an air gap of about 5 litres per square metre is formed between the membrane and the wall (depending on the profile of the studs). This may or may not be ventilated, but will allow any free water to run behind it, under the influence of gravity. The membrane is loosely mechanically fixed to original sound wall surfaces, usually with treated battens or other framing systems, on to which insulation and plasterboard can be fixed. In some circumstances the battens can be omitted and plasterboard and membrane fixed directly using special plastic plugs. There are also versions including polypropylene mesh welded to the membrane, onto which plaster can be applied directly.

With regard to internal finishes, the object of waterproofing, by whichever method, is to put a layer of waterproof material between the damp wall or floor and the internal  finish. This will prevent the inner material from being in direct contact with wet masonry. None of the systems eliminate the occurrence of water vapour in a treated area, so consideration needs to be given to controlling / reducing relative humidity using ventilation, air conditioning, dehumidification or some other method.

Recently Building Regulations have been updated to require that internal finishes provide improved levels of thermal insulation and sound proofing in some circumstances. In these cases we are able to apply insulated plaster boards and other insulation to walls and floors, as well as the range of sound reducing plaster boards that are available.

Other finishes are possible depending on the final use of the converted area. These include membrane only, plywood, timber panelling and other wall boarding materials. The finish chosen will be governed by the use to which the converted area is put.

Area use specification

In simple terms the specification for structural waterproofing against use is shown below.

Storage Low specification
Utility/Kitchen/Basic office Higher specification
Bedroom/Dining area/Lounge Formation of habitable room - highest specification

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 patches tolerable
2 Workshops and plant rooms requiring drier environment, retail storage areas No water penetration but moisture vapour tolerable
3 Ventilated residential and working areas, including offices, restaurants, leisure centres etc. Dry environment. Humidity control required

 

Within BS8102, grades 2,and 3 require the same application of CDM systems in each case. What varies is the amount of air humidity control; escalating up from, say, air bricks, forced electric ventilation, heat exchange ventilation, dehumidification and total air conditioning. The drier the environment required the more air conditioning is needed.

Conversion considerations

We have found the requirements of our clients to be very varied and each basement must be treated on its own merits given the individual circumstances found. Consideration must be given to the following:-

Does the area flood? Sump and pump required. Risk vs. cost
How much will it cost?

Type of area to be formed and what is there at the moment

Budget - how much do we have to spend?

Other connected items

Foundations -  will they become exposed ?

Floors - damp ? - level ? - final floor finish ?

Other walls e.g. partition walls

Internal joinery - doors, architraves, skirting boards etc.

Ceilings - hiding pipe and wire runs ? - head height ?

Windows and external doors - natural light and ventilation

Services (plumbing, drainage and electrical systems)

Ground levels, soil type and position of drains outside

Sound and thermal insulation

Heating, ventilation and condensation control

Planning, Building and Environmental Health Regulations

Fire escapes and alarms,  and security alarms

 

We can prepare an individual report and specification for the damp proofing of each basement we inspect. Sometimes we advise clients that their ideas are not possible, or that they are only possible to one of the lower specifications. The British Standard -  BS8102 : 2009  - sets the standards for such works, but often we find that the basement or client's requirements / circumstances do not allow or require for such systems to be installed. It is usually down to use (and therefore specification) versus cost. However, we do try to carry out our assessment of the conversion  to the British Standard, and use as many of its practises within our recommendations. Our starting point for the assessment is always with the British Standard.

Most recent Building Regulations, and BS8102, require designers of water proofing systems to consider contamination of sites by ground gases. Radon and other ground gases, such as methane, are now more recognised as contributors to health and safety in buildings – and especially basements. Those designing basement systems must consider the possibility of ground gases in any design, and this is now a requirement of Building Regulations and Codes of Practise. A desk based on-line search can be done to locate the risk of radon at your property. We automatically offer this for you as part of our design service for basements. The outcome of any radon search will categorise the ground around the property as requiring no action, or a basic radon barrier system, or a full positive ventilation radon control system. The latter two require the introduction of addition materials and installation techniques to create a system to provide the level of protection designated. The positive ventilation system requires additional components to force air to move behind membranes to extract ground gases, such as radon. These systems have additional costs over and above standard basement water proofing, but the costs can be controlled by designing in ground gas control as an integral part of the water proofing system. The UK maps identifying areas of high radon contamination can be seen at UKRadon.org website.

Getting it Right

The law places a duty of care on us as experts to provide the fullest information to our clients. This places a burden onto our surveyors and the company, particularly in the current litigious climate. As a result of this our reports and specifications may contain one or more of the paragraphs below. They are repeated here to help clarify our approach to various items which are critical to the success of any work. One of the main points of concern for us is to make sure the client understands the risk of future flooding. A basement that has "never flooded" to the knowledge of the client, might flood tomorrow. The paragraphs below provide a warning about the risks of flooding when a drainage system is not included in the specification. We want our clients to be able to make an informed decision of the risk taken when they chose not to include a sump and pump in their specification (and therefore take the specification outside of BS8102 : 2009).

Paragraphs that we use standard in our correspondence are given below:-

British Standard

"The client should note that there is a British Standard for the design of waterproofing systems for both new and existing basements - BS8102 : 2009 The Protection of Structures against Water from the Ground. The system specified in this report is not to this standard. However, we have chosen elements from the standard, choosing system and material components to provide a satisfactory degree of water tightness (as defined by the standard), suit the client’s cost/budget and fulfil the client’s performance requirements. Please note that a system fully complying with BS8102 : 2009 is available and has a higher cost."

Flooding

"The client should note that the work we propose will not prevent the cellar/basement from flooding, (e.g. from leaking drains, burst pipes, changes to the existing water table height) should such circumstances arise. The membrane application we will carry out deals with penetrating vapour dampness from high ground levels and not the physical liquid entry of water under hydrostatic pressure. Only a higher specification system using a weather-lapped cavity drainage membrane, linked into sub-floor drainage or a sump/pump system, will overcome such a situation. This has not been allowed for in this quotation. Recent case law has suggested that specifiers of underground waterproofing systems need to specify these on the assumption that the primary waterproof barrier will be less than adequate. This means that our specification would need to be at a level above that given in this report, and the result of this would be a system that may be outside the range of the client’s budget. A higher specification for such a waterproofing system is available to the client at their request. Such systems cost more. Please discuss this with this office."

Electrical and Plumbing Systems

"Where membrane dry-lining is to be applied to walls the existing plumbing and electrical installation (sockets and light switches) will require modification due to the thickness of the new finish.  This must be carried out by the client’s own plumber or electrician (a “first fix”) prior to the arrival of our operators on site."

We normally recommend that electrical wire runs are made behind the plaster board surface to hide the wires and that plumbing pipe runs are made on the surface of the walls for future access. This means that the electrical first fix should include for the running of wires to the individual electrical points, without connection to the supply. This is followed by a second electrical fix, when the back boxes and face plates are installed. The first fix for plumbing should allow for the introduction of pipes into the basement area terminated at isolation valves, with the system being extended from these points as part of the second fix when the plaster boards and skim plastering has been completed.

Condensation control

"In the areas treated there may still be significant levels of background water vapour after our work, and in the future, which needs to be controlled to avoid condensation. Some of this condensation may occur on internal finished surfaces and be easy to detect and deal with. However, a more difficult problem is that caused by interstitial condensation. This occurs when condensation takes place behind wall linings. This is harder to detect, and the first signs of this may be damp patches on the floor, where water has run down the studded membrane inside the waterproofing system. The system is not designed to deal with this problem, so the client needs to ensure that condensation is prevented by providing a satisfactory balance between background heating and ventilation. High levels of relative humidity must not be allowed to develop in these areas and this will require the use of forced electric ventilation to areas of moisture production (e.g. kitchens and bathrooms – part of the Building Regulations). In some situations it might become necessary for the client to introduce dehumidification, or other forms of condensation control, into a newly treated area to forcibly reduce humidity levels. The need for this may not become known until the water proofing system has had time to "settle". Relative humidity in these areas must be reduced to an acceptable level by ventilation, air conditioning, dehumidification or other means. Further advice on condensation control is available from this office."

Planning, Building and Environmental Health Regulations

"There are regulations concerning the conversion of dwellings, including basements, under Planning, Building and Environmental Health controls. These relate to all properties. The client should satisfy themselves that this work complies with such regulations. If there is any doubt concerning this, the relevant authority should be consulted. In particular, new Building Regulations called “Part L” (concerning thermal insulation), “Part E” (concerning sound insulation) and “Part F” (concerning ventilation) have recently been adopted. We can advise further on this, if required."

Recently, as part of a complete re-appraisal of Building Control Regulations, new requirements have been introduced by the authorities. Our experience is that the new regulations are interpreted and enforced by building control officers in differing ways. The two main new areas are those of thermal (heat) insulation (known as the part L regulations) and sound insulation (known as the part E regulations). The regulations set out minimum criteria for the thermal insulation of walls, floors and ceilings. The sound regulations deal with the reduction of sound "nuisance" between properties and especially into bedrooms from adjoining rooms, even in the same property.

Whatever emphasis the local building control officer places on these building regulations, the conversion of a basement is a good opportunity to upgrade the thermal and sound insulation of the areas converted, and other adjacent areas. This improves the overall comfort of the property and in the longer term will save money by reducing heating costs, and also improve the property's energy efficiency rating. This is now and important feature of the  new HIPS seller's pack.

Floor / Wall interfaces (treatments to walls only)

"It will not be possible within this specification to link the wall membrane to any floor membrane (if existing). This will leave a weak point at the floor/wall interface, where moisture may enter. The solution to this would be the full replacement of the floor, incorporating a damp proof membrane beneath the new flooring concrete. The damp proof membrane should be lapped up the walls by a minimum of 400 mm. Alternatively, if the floor is sound (see below), studded membrane could be extended off the walls across the floor, topped with a floating (optionally insulated) chipboard deck. This additional work is not included in this quotation.

The existing solid floor may appear sound, but the client should be aware that without a damp proof membrane, the floor concrete may be liable to sulphate attack and heave in the future. We can provide a supplementary specification and quotation for the formation of a new concrete floor incorporating a damp proof membrane.  For the work in this specification, we will proceed on the basis that the client wishes to use the existing floor construction and that the risk of future floor heave has been considered, but a new floor not chosen."

Survey and Inspection

The current recognised basement water proofing qualification is C.S.S.W. - Certified Surveyor in Structural Water proofing. All projects that include structural water proofing should be overseen by someone with that level of qualification.

There is only one satisfactory way of determining if a basement/cellar areas is suitable for conversion and that is by consulting qualified experts. The expert should be invited to view the area so that a full investigation of the potential for the area can be made. The person who is going to be the end-user, client, bill payer or who has a direct interest in the conversion MUST be present. This is so that the exact client requirements are established, and especially so that the risks of flooding can be fully discussed and understood. It is no good taking a decision to leave out a sump and pump system to save money, and then be standing in inches of water years on. It is even more unacceptable to be in the same position and not to have known that it might happen. This problem is the most frequent cause for litigation between the client and the installer - and this is to be expected. After all, the client wanted a dry and usable basement!

Categories
Case Study - basement water proofing This is a case study of a basement room that was a converted to a lounge - and VERY damp.

 
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