Waterproofing your basement: what do you need to know?

Nicholas Donnithorne FLS FRES, UK Technical Services Manager, Rentokil Property Care

Converting your basement or cellar into a liveable space could be a wise move in today’s property market. Halifax recently reported house price growth dropping to a five-year low, and it will inevitably have left some homeowners wondering whether they are better off staying put and improving the property they’re in rather than selling.

Building or expanding a basement can be a great way of creating more space, as well as adding more value to the property. The Hidden Geography of Residential Basement Developments found at least 4,650 basements were granted planning permission in Kensington and Chelsea, and the West End of London between 2008 and 2017. But it’s not only the pools, gyms and cinemas being built in central London – many homeowners up and down the country are also considering adding extra storage, a wine cellar or games room downstairs.

In recent years the government has been working to make self-building and improvement a more realistic prospect, setting itself a target to facilitate 20,000 self-build and custom-build homes by 2020. Under these new regulations, basement conversions and new builds can be an attractive and far more cost-effective solution to generate extra space.

Waterproofing is a critical part of any basement project and very often, it’s the difference between success and failure. The Chartered Institute of Building states that 75% of building failure is the result of water ingress – with failures leading to structural problems, mould, corrosion and other moisture-related issues such as dry rot.

The National House-Building Council (NHBC) consistently reviews its guidance and requirements around structural waterproofing. Its published standards make it clear that any waterproofing project must include early input from a waterproofing design specialist.

There are three grades of protection when it comes to waterproofing, which are based on the intended use of the basement. Homes typically fall into Grade 3 protection, which means they must maintain a completely dry environment with adequate ventilation. This level of protection will ensure damp cannot penetrate the living area of the building. It must always be remembered, that once a basement is water tight, any moisture created by occupancy needs removal by suitable ventilation, otherwise condensation can become an issue.

We always recommend getting a Property Care Association (PCA) qualified Certificated Surveyor in Structural Waterproofing (CSSW) involved in any new-build or conversion project. Not only is it the best way to minimise risk, but they will help you to understand the costs of waterproofing – which can represent a significant proportion of the total costs.  This is particularly true if the property requires structural support to accept a new basement or update an existing basement.

Types of waterproofing

In any below ground space that falls into Grade 3 and/or high-risk environments, two different types of waterproofing systems are recommended, in-line with the NHBC’s Chapter 5.4 (2015) standard. The three types of waterproofing systems defined by BS8102:2009 ‘Code of Practice for protection of below ground structures against water from the ground’ are:

Type A: This provides a barrier against groundwater and water ingress by surrounding the external walls, floor (and roof) with a waterproof material – literally creating a barrier between water and the structure. This can be applied either internally or externally to the concrete/masonry structure.

Type B: Relies upon the design and materials incorporated into the reinforced concrete structure itself. It is important that all joint detailing is robust and the concrete correctly compacted, as leaks later are very difficult to fix.

Type C: The masonry structure provides the primary resistance to water ingress. Type C systems use a cavity allowing groundwater seepage to reach the drain, while internal surfaces are protected by a cavity drain membrane, with all joints and junctions correctly lapped and sealed. It involves adequately disposing of water, either by gravity, or through a sump and pump to a suitable outlet. This must be designed with adequate access points for servicing and maintenance, and a one-way valve to prevent backflow.

It is also advisable to consider two pumps, Battery back-up and alarms, as the cost of flooding, should there be faults or power cuts, outweigh the expenditure and provide peace of mind.

Each of these types of waterproofing has its own advantages and disadvantages. For example, while Type C waterproofing will require ongoing maintenance, it is much easier with this system to identify and fix any problems that might arise. However, all systems share one common characteristic in that retro-fitting to a completed project is extremely expensive, disruptive and difficult, if not impossible in some cases, so planning your waterproofing requirements at the outset is always the best course of action.

Final words

Given the complexity of waterproofing a building, a design specialist should always be involved as early as possible. Getting the process wrong can lead to the building’s structural integrity being compromised and therefore cost you more in repair and damages later on in the building’s life. Best advice is to get in touch with a waterproofing expert at the earliest stage, which will then leave you free to get on with the exciting bit – the design and décor of your new space.