The BS8102 2009 Code of Practice for Protection of Structures Against Water from the Ground suggests that waterproofing designers consider a combination of waterproofing systems for retained structures where the likelihood of leakage is high or the consequences of leaks is unacceptable. Waterproofing designers are advised to consider the following:
- It is important to note that the success of externally applied Type A membranes depends on the highest quality of workmanship. The quality of the concrete surface, especially with bonded membranes such as bitumen sheeting, is critical and is not easy to achieve – IStructEdocument for ‘Design and construction of deep basements’.
- The judgement in the overview of the High Court case ‘Outwing vs.Weatherald’ agrees that ‘it is not reasonable or realistic to expect a bonded sheet membrane to be applied without any defects at all’. Therefore if you incorporate a system which needs to be 100% defect free to work in your design and it fails you may be accountable.
- ‘Leaks caused by defects in external membranes are practically impossible to locate and repair, since the water invariably enters the structure internally through cracks or other vulnerable points, such as any movement joints, at some distance from the external defect’ – IStructEdocument for ‘Design and construction of deep basements’.
- Because consideration should be given to the ‘form and feasibility of remedial work’ which means if there is a problem, the ability to get back to the structure to diagnose and determine the cause and location and then rectify the problem. – BS8102:2009
- Because consideration should be given to providing a ‘maintainable’ waterproofing system. – BS8102:2009
What is Combined Waterproofing?
The three types of waterproofing defined by the BS8102:2009 are defined into very clear Categories of Structure:
- Type A – Barrier Protection
The Structure has no integral protection against water and relies on a membrane to be introduced internally or externally to prevent the ingress of water into the inner building fabric. Examples of barrier membranes applied externally to the “positive” side are bonded sheet membranes, sodium bentonite matting, liquid applied coatings (cementitious/bitumen or polymer). Internally similar approaches can be adopted to hold water back on the “negative” side of the below ground structure such as cementitious coatings, bitumen or polymer based liquids applied via spray, trowel, brush or roller.
- Type B – Integral Protection
The structure itself is constructed to be integrally waterproof and the primary resistance against water ingress. These type of structures will be of high grade concrete, usually with more reinforcement fabric introduced to reduce risk of shrinkage cracks and as such water ingress. In addition, these forms of structure will have waterbars (hydrophilic) which swell on contact with water, metal which form an actual physical barrier to protect the vulnerable construction joints from the risk of water penetration, or injection waterbars that have the capability to inject grout to porous and poorly compacted joints as well as resins for post construction leak sealing.
- Type C – Drained Protection
It is accepted that water could enter the building and an internal cavity is provided to depressurise and manage the water. Type C systems include HDPE dimpled sheets with maintainable internal drainage to guide water via gravity to open elevations or to be removed by mechanical means.
When are “Combined” Waterproofing Systems Required?
Waterproofing Design Specialists are now tasked to recommend a “combined “waterproofing system approach where:
- the likelihood of leaking is high
- the consequences of leakage is unacceptable
- additional vapour checks are necessary for a system where unacceptable water vapour transmission could otherwise occur.
With this in mind the possible combinations of waterproofing are Type A,B & C, Type A & B, Type A & C or Type B & C. Usually a combination of 2 forms of waterproofing is adequate when designing a habitable space requiring a completely dry internal environment, defined as ‘Grade 3′ within BS8102:2009. Whichever combination of waterproofing is chosen to achieve the Grade 3 environment, in most cases the safest combination will include a Type C internal cavity drain membrane system as one of the forms of waterproofing. The choice of the other system is largely dictated by the type of structure.
Waterproofing To All Environmental Grades With Newton Systems
Many Newton systems can be combined with each other, to provide waterproofing to all environmental grades regardless of the severity of risk and the clients exacting expectations, ensuring that professional specifiers and specialist waterproofing contractors have the products to successfully waterproof both commercial and domestic projects, first time, every time. Our Systems cover internal and external waterproofing of New-Build and Refurbishment Basements including HDPE sheet and seamless liquid waterproofing membranes for the waterproofing of podium decks, car parks, balconies, flat roofs & terraces and swimming pools. In addition, Newton also provide a comprehensive range of submersible pumps & pumping systems for clean, grey and foul water.
Who Should Design and Install The Combined System?
Newton as always recommend that the combined waterproofing system be installed by one of our registered contractors (NSBC Newton Specialist Basement Contractors). They are as technically qualified and trained by Newton in the design and installation of all of our waterproofing products. We recommend that they are involved as early as possible, preferably as the design specialist, as they will ultimately be responsible for both the design and the installation, allowing them to give your client meaningful and insured guarantees.
Most of our contractors hold design liability insurance allowing you to delegate the full design liability for the waterproofing aspect of the project to them. The industry accepted term of guarantee is 10 years and the optional underwritten insured guarantee gives the client/end user peace of mind that the guarantee will be worth something to them if the issuing company were not in a position to honour the guarantee. In some instances the guarantee may be extended at the end of the initial term following an inspection and proof that any on-going maintenance requirements have been fulfilled.
Our contractors are approved installers of a number of waterproofing products and as such will, without bias, put forward a design using the system or systems which they feel will be the most suitable, trouble free and effective in the given circumstances.
This article featured in the October 2014 edition of Concrete Magazine. Download Article