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Waterproofing is an essential consideration for anyone who is building a basement which they intend to use for storage or habitable accommodation. Too often basement waterproofing is not given high enough priority and as a result a cheap and inappropriate basement waterproofing option is used to try and get inside an unrealistic budget. This often leads to failure and the resultant consequential losses can be disastrous for the owner of the building or property. New build basements are often designed and built from well graded and even 'waterproof concrete', concrete block or blocks with concrete infill. A well designed and built structure will generally form the primary resistance to water ingress as the concrete is too dense for water to pass through. However we must always be mindful that where there are construction joints there is risk of failure even if waterbars, hydrophilic/hydrophobic strips are well installed. With this borne in mind it is always a recommendation that as well as the structure itself, a secondary form of basement waterproofing is adopted even if the structure is new and/or built from 'waterproof concrete'. When the concrete construction itself is considered to be integrally waterproof this is described as type B form of basement waterproofing. Where the structure is not considered to be integrally waterproofing then we have 2 options for dealing with the potential for water ingress. One is to Tank it and the other is to waterproof it with a Cavity Drain Membrane. Tanking a basement means that a product is applied internally or externally and is designed to physically stop and hold back the water. This is referred to as Type A basement waterproofing. A Cavity Drain Membrane system is applied to the internal face of the earth retaining structure where it can accept the ingress of water depressurise it and manage it to a safe evacuation point. This is referred to as type C form of basement waterproofing. Tanking as a form of basement waterproofing to new structures has inherent risks. Whether you apply it internally or externally to the structure tanking systems need to be applied 100% defect free to be affective, if water is delivered under pressure against the structure. As dispute and High Court case between two companies Outwing Construction and Thomas Weatherald has set a precedent in the basement waterproofing industry and UK law. The ruling in the High Court of Justice was that it is not reasonable for a tanking or waterproofing system to be applied 100% defect free. What this means is that if you design for or apply a basement waterproofing system to

the structure which needs to be 100% free of defects to work and it fails you can be liable for the consequences of that failure. By their very nature all tanking systems need to be 100% defect free to work when tested by water under pressure from the ground. Most new build basement waterproofing designs that incorporate an external tanking system also include a land drain the purpose of which is to help de-water the surrounding ground areas and reduce the level of water which is delivered against the structure. There are often many associated problems with land drains used in basement waterproofing. A land drain should be positioned on the outside of the structure below the level of the internal slab so that it helps to relieve the volume of water to the full depth of the retained soil. Many times the position of the land drain is shown on a drawing or installed above the internal slab level and therefore could only ever be partially effective. 'The form and feasibility of remedial treatment' is a buzz phrase in the basement waterproofing industry. It means if there is a problem having the ability to get back to something to identify where and what the problem is and then rectify it. This is a problem with both external tanking and land drains as they are often buried under tonnes of earth. The size of the land drain is another issue as who can accurately predict the volume of water which could be expected to come to have at any time in the future? Also as stated the tanking system may rely heavily on the land drain and how can anyone guarantee it against defect or blockage in the future - again the form and feasibility of remedial treatment comes into question. The BS8102:2009 (British Standard code of practice for protection of below ground structures against water from the ground) recommends a maintainable basement waterproofing system. A buried external tanking system and land drain are not easily often maintainable. Building a strong basement structure to form the primary resistance against water is a good idea but where there are construction joints we must expect that water ingress could occur at some point. Therefore a more reliable form of secondary waterproofing other than the structure itself is a cavity drain membrane system or Type C basement waterproofing. Before the recent revision of the BS8102:2009 the previous code of practice BS8102:1990 stated that Type C form of construction and waterproofing was the most 'trouble free and effective'. Now the BS8102 says that Type A, B and C together is the most effective which probably goes without saying but as a stand alone basement waterproofing system type C cavity drain membrane is still more reliable that the others. Cavity drain membrane basement waterproofing systems take into consideration the following points: •Where earth is retained against a structure we must consider that water will enter the fabric of the building at some time in that structures life. •When water enters a structure and finds an air gap it depressurises and looses its potency. So the way a cavity drain membrane basement waterproofing systems like System 500 works is to provide air gaps and therefore depressurisation zones on the inside of the structure which relieve the water pressure. This is done firstly with an 8mm studded membrane applied to the walls. The

stud profile serves to hold the product slightly away from the structure to which it is applied. Any water entering through the walls is depressurised within the air gap provided by the studs of membrane, losses its potency and falls to the wall/floor junction. A perforated drainage conduit is situated at the base of the wall either in a recessed channel in the slab or on the slab with a layer of closed cell insulation. The drainage conduit serves 3 main functions. The first is that it collects and delivers water to an appropriate drainage outlet to be evacuated from the structure. The second is that it acts as a form work at the wall/floor junction which serves to maintain an air gap and therefore depressurisation zone at the point where water enters or ends up in most structures i.e. the wall/floor junction. Thirdly it allows future maintenance and testing of the system via rodding/inspection ports situated in the ring main of conduit and this in turn conforms to the recommendations in BS8102. Another feature of some suppliers drainage conduit is a separated fin design on the back face of each section. In some basement waterproofing designs where it is applicable this deals with the risk of condensation forming on the dry side of the membrane and also with the risk of imperfections (small tears/holes/rips in the membrane) by maintaining an open wall/floor junction. The drainage conduit is connected to either passive or powered means of evacuating the water. Passive means drained naturally by gravity to an existing drainage outlet, open elevation or designed in drainage. Powered is where none of the aforementioned options is available or feasible and a sump and pump is used. A floor membrane is applied across the floor area as a vapour barrier and linked to the drainage conduit or wall membrane to complete the basement waterproofing system. Almost any wall and floor finish required can be achieved on top of a Cavity Drain Membrane System. The advantages of using a cavity membrane system are:•There is often little or no preparation •They are flexible and can move with the structure. •They are not vapour permeable. •They are considered to be sympathetic to the structure and reversible •They can cater for less than adequate workmanship and interstitial condensation. •They can be maintainable. •They can be applied in less than ideal site conditions so will not hold up a project programme. •They can be tested prior to installing fixtures and fittings. •10 year insured guarantee's can be obtained when installed by a suitable specialist. •Full design liability can taken by certain specialist basement waterproofing contractors

Toby Champion is National Sales Manager at John Newton & Company, the leading provider of basement waterproofing and damp proofing solutions in the UK. John Newton & Company have provided basement waterproofing solutions for thousands of properties in the UK, including new build and listed buildings

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