Q. What are the main problems you tend to face when waterproofing a basement?
A. Water ingress into basements has historically been a problem in the UK, largely because adequate attention has not been paid to waterproofing design, together with a desire to cut costs to allow funds to be used in other areas of the building.
In response to a high level of claims for water ingress into basements, NHBC conducted a study into the main areas of failure. Summarising their findings in an article, NHBC wrote: “In our experience, the main causes of failed waterproofing are simple. Often, systems are poorly designed and/or poorly executed. At the heart of both these issues is a basic lack of appreciation that even the smallest defect can lead to problems that are both difficult to trace and expensive to repair”.
Q. How do water table levels affect the process?
A. In theory, the British Standard that covers basement waterproofing (BS 8102) allows for the use of less robust waterproofing systems in situations where there is a permanently low water table. However, the high cost of the hydrogeological surveys necessary to justify such a decision means that in practice, virtually all domestic basements are constructed on the assumption that the water table will reach the full height of the basement at some point during its design life. NHBC’s guidance on basement waterproofing (Chapter 5.4) advises to “generally assume exposure to the full height of water during the design life of the building”.
Q. Is tanking or cavity drain membranes the most effective option?
A. This used to be a big debate in the industry. However, today, both systems tend to be used together. NHBC Chapter 5.4 calls for the use of “combined systems” to minimise risk, and most commonly this entails a combination of tanking and cavity drainage.
The approach is to use tanking to keep as much water out of the basement as possible and use a cavity drainage membrane system to redirect any water that manages to seep in towards a sump and pump.
There are those that would argue that combined systems are over-engineered and that either tanking or cavity drainage used alone would provide sufficient protection against water ingress. However, the reason that warranty providers require the use of a combined system is that levels of claims have historically been high where reliance has been made on a single waterproofing system due to inevitable errors in design or installation.
Q. What is the typical step-by-step process for waterproofing a basement?
A. There is a five-stage process to a successful basement waterproofing project:
1. Design a resilient basement structure
Ideally, reinforced concrete in a simple shape with minimal service penetrations.
2. Choose a suitable waterproofing system
Choose a suitable outline waterproofing design suitable for the type of basement structure you wish to waterproof. Waterproofing product manufacturers can provide outline designs for different situations – e.g. piledwalls, refurbishment of existing basement, ‘dig out’ basement under existing house, etc.
3. Waterproofing design specialist
A Certificated Surveyor in Structural Waterproofing (CSSW)-qualified waterproofing design specialist should be employed to draw up a detailed design. You can either pay an independent basement design specialist to do this or work with a specialist basement waterproofing company that can provide a detailed design as part of their service.
4. Specialist waterproofing contractor
Although it can be tempting to carry out the basement waterproofing yourself, there are good reasons for using the services of a specialist waterproofing contractor. For example, they will have a good understanding of correct sequencing of works, experience in liaising with following trades to prevent damage or piercing of the waterproofing system, and an understanding of how to correctly prepare surfaces prior to the application of tanking systems. Above all, qualified and insured contractors are able to take liability for design and installation.
5. Handover and maintenance
Such a suitable contractor should be able to provide a handover package to the homeowner, providing evidence of the waterproofing system performance and a planned maintenance schedule. This might comprise photographic/video evidence of a ‘flood test’ of the system including the sump and pump as well as advice for following trades to avoid any damage before a final ‘wash out’ is carried out.
Detailed plans of the system should be provided showing the location of access points, pumps and alarm and control panels along with operator manuals and guarantee documents. There should be clear advice on the action to take in the event of an alarm activation.
It should be noted that an ongoing maintenance plan will likely be a fundamental condition of any insurance-backed guarantee, especially where a cavity drainage system forms part of the waterproofing design.
Q. How much does waterproofing a basement tend to cost per square metre?
A. Basement waterproofing costs vary widely, depending on the level of waterproofing required and the complexity of the basement to be waterproofed.
The outline designs provided by waterproofing product manufacturers can provide a good basis for an estimate of product costs, but for a more accurate quote, ask the manufacturer to put you in touch with a specialist waterproofing contractor that installs their systems. In most cases, they will be able to provide a ‘ballpark’ estimate on the understanding that an accurate quote can only be provided once the plans have been seen and the project discussed in detail.
Q. What waterproofing solutions should people look out for when carrying out this process?
A. Increasingly, environmental concerns are guiding people’s choices when designing and waterproofing basements. For example, a study carried out for the Royal Borough of Kensington and Chelsea found that embodied carbon in materials used for basement extensions was considerably higher than for above-ground extensions – largely due to the large quantity of concrete required. The study suggested that a 20% reduction in carbon could be achieved by using recycled coarse aggregate in the concrete.
In terms of waterproofing systems themselves, the scope for reducing carbon is much lower. However, considerable reductions in plastic use can be achieved by using cavity drainage membranes containing at least 50% recycled plastic.