The ground rules of waterproofing design

Designing a waterproofing scheme is no simple matter. With rainfall averages varying from a few hundred millimetres to several metres depending on where you are in the UK, and most of it undulating, it’s certainly no level playing field. Even when it is, you could still find your building sitting below the water line. Here, Niklas Ullfors, National Technical Manager for Visqueen, lays down some ground rules for specifying successful waterproofing solutions.

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ith the UK’s weather becoming more and more unpredictable, and taking in to account the local geology, it is imperative to have a varied range of robust waterproofing applications to meet any requirement.

While average annual rainfall across the country is 1.35m, some parts of Scotland and the East of England suffer a drenching of 3.0m and over.

In most cases this filtrates through the soil types until it reaches the natural water table and is far enough below the surface not to be troublesome. However, in places such as the fenlands in East Anglia, the water table is at or just below ground level and as a result, without an appropriate waterproofing scheme, a building could be sitting in water 100% of the time.

Terrain can also play a big part when selecting the appropriate waterproofing system. Few parts of the UK are flat and there isn’t a real option to have land remodelled, because it is expensive and a significant engineering exercise.

An alternative is to follow the natural gradient, with the front of the building at ground level and the rear partly below it, or vice versa. Some buildings are deliberately ‘buried’ to blend into the landscape and be energy efficient, while others feature a single or multi-storey basement. In all these cases, a robust waterproofing system will be needed.

Obviously, it’s essential the waterproofing design is done at the building design stage and dovetails into the overall design.

The British Standard code of practice for the theoretical design of waterproofing systems for a building or civil engineering structure is BS8102:2009. Introduced in 1990, it was updated in 2009. Whilst it is recognised as a guidance-only document, a number of recent legal cases have created a default system whereby it must be considered when designing any waterproofing scheme.

Know your ABCs

Waterproofing systems are split into Types A, B and C, and range from pre-applied and post-applied solutions, through to integral and drained systems. Depending on the amount of water encountered one or more types could be required. The most common are single and dual type applications. A waterproofing design with a combination of types lowers the risk for water ingress.

Type A

A ‘Type A’ system is bonded to the external face of the building or structure. It is completely bonded and known as a ‘Type A fully-bonded’ system.

When a fully-bonded system needs to go below a building then the product will be placed on the ground first and the concrete poured on top of it. This is known as a ‘pre-applied product’, whereas if for example, a liquid product is applied to the external face of a structure this is a ‘post-applied product’. Both are Type A systems as both are fully-bonded to the structure.

In the unlikely event the Type A system is damaged then water can enter through the damaged area. But tests have proved the water will stay localised to the damaged area. Conversely, if the system is not bonded to the structure then the water will track between the Type A product and the structure, and as a result water ingress will occur on a massive scale.

Type B

A Type B integral system is generally used when walls that are made from cast-on-site concrete and need to be made waterproof.

There are two ways to achieve this. Firstly, you can add a special additive, which is done at the concrete mixing plant. This is very common as it is the most efficient way to make concrete waterproof and is also the most cost effective.

Secondly, steel reinforcement can be added to the structure. This will keep the concrete taught and prevent any tiny crack appearing. If cracks do appear, then water will travel along them and enter the building.

Type C

A Type C system is completely different and often referred to as a ‘drained cavity system’ as it allows water to enter into the building through the structure itself. The water is then collected and managed away via a specialist drainage system. This system is very common if, for example, a building is being refurbished and water ingress has occurred or the building is undergoing a change of use.

Design considerations

Having considered external factors, a waterproofing designer also needs to consider the use of the proposed structure, what materials are going to be used and that the solution will remain intact for the in-service life of the building.

The environment and building use creates risk factors, which are categorised under BS8102:2009 into three grades.

Grade 1 has the lowest risk, a car park being a good example. Grade 2 would include a garage, plant room or workshop, whereas a domestic dwelling is a Grade 3 environment, needing to be the driest. Any water ingress or damp patches on the walls in the home are totally unacceptable.

Once all the background information and the risks are known, then we can start to consider the principles of the waterproofing design. To provide a working example I’m suggesting a domestic house with a basement in Cumbria in the North West of England.

The building is at risk from high levels of rainfall and, as it is a Grade 3 property, it needs to be dry at all times. At this initial stage we will be considering a dual waterproofing system. In this situation, a Type A + Type B system would be highly recommended and is a fairly common combination for a new build.

The Type B system will be an additive added to the concrete and the waterproof concrete will be poured on site to create the floor and walls of the basement. This is a simple way to build a structural waterproof wall.

The Type A system will be a pre-applied membrane (PAM) for use under the concrete floor slab and a post-applied self-adhesive membrane (SAM) for the walls. Being self-adhering, the SAM will stick onto the vertical waterproof concrete walls once they have cured.

Having been laid on the ground and the waterproof concrete poured in, the PAM will turn 90 degrees up the side of the poured concrete floor slab. It is at this point the self-adhesive SAM is joined onto the PAM. The Type A system is the PAM & SAM and the Type B system is the waterproof additive which goes into the pour concrete, thus creating a dual Type A and Type B waterproof system.

However, the job is not yet complete. Rainwater will filtrate through the soils and into the natural water table, at some point coming to bare against the below ground structure. While the installed waterproof scheme will resist the water, removing the water from baring against the structure will significantly reduce the risk from water ingress. This is achieved by using an external fluted or cuspated board. Connected to the base of the board is a horizontal perforated land drain pipe, which in turn is connected to the main surface water drainage system.

By taking account of the property’s location and risk grade, the dual waterproofing system, with an external water management system, is not only an effective way to minimise potential water for water ingress, but also meets the requirements of BS8102:2009.

So in summary, while the risk posed to buildings by water can be significant, particularly in a tricky location, designing a robust system need not be that complicated.

By choosing an appropriate system, or combined system that meets the British Standard, the waterproofing system designer can be confident of delivering a long-lasting solution.

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