Waterproofing historic builds

Specialists must collaborate when preserving historic buildings. Richard Walker of Peter Cox explains

Almost half a million buildings in the UK have heritage protection. This status is awarded to buildings of historical interest across the country, either for the role they’ve played in history or because they are architecturally significant.

The best way to keep them alive is to use them, as neglect and lack of maintenance can often lead to timber decay and structural issues. For this reason, it often makes sense to renovate the property and manage potential areas of damp ingress so it can be refurbished for commercial or residential use.

However, due to planning restrictions and listed building status architects and contractors will often have to ensure any listed building retains its original features. This can mean using renovation methods in keeping with the way the property was originally constructed, so the architectural and historic fabric and features of the building remain intact.

One of the most challenging aspects in any preservation project is the ‘damp proofing’ of underground structures. This will require the design skill of a CSSW Surveyor – Certificated Surveyor in Structural Waterproofing. These are professional surveyors who are qualified to design solutions and write specifications that will protect any building where the walls retain more than 600 mm of external ground in accordance with BS8102 – The Waterproofing of Structures Below Ground.

A collaborative approach
When it comes to restoring a protected building it’s important to find the balance between ensuring the structure is functional and free from damp and decay, while remaining aesthetically consistent with its original design. This can be a challenge on a restoration site and means architects and surveyors must work closely throughout the restoration process, to ensure the building is repaired in a way that is both in keeping with its original design, and safe for the occupants who will use it.

By ensuring regular design meetings take place, all parties can be confident that they are fully aware of the renovation process and the implications of their proposed changes on other disciplines.

Important waterproofing considerations
The type of building, its former and intended use must be carefully considered when thinking about waterproofing. A ‘risk based design’ approach must be adopted and the solution must be appropriate to the risk. A failure to adhere to these principles can lead to water ingress and the need for extensive repairs as a result.

Any below ground space that falls into Grade 3 for Residential and Commercial use with a high risk of water ingress should use at least two different types of waterproofing systems if it is to comply with BS8102 and the NHBC Technical Guidance Documents. These options are categorised as:

Type A: This provides a barrier against water ingress by fixing a waterproof material to the earth-retaining surface of the structure. This can be applied either internally or on the building’s exterior and is the most common Type for ‘new build’ applications.

Type B: Relies upon the design and materials incorporated into the external shell of the structure itself. A typical material that falls into this category would be reinforced water-resistant concrete.

Type C: This type is exclusively installed on the internal surface of the earth retaining walls. It works by controlling the flow of water ingress on the reverse side of the waterproof ‘cavity drain membrane’ sheeting by gravity, then collecting water ingress in drainage and sumps, then finally using a pump to evacuate the water ingress outside the building. This type of waterproofing is almost exclusively chosen for existing buildings as they do not impose any hydrostatic pressure on the building – therefore are more ‘sympathetic’ to the structure.

Each has its own benefits. For example, while Type C waterproofing will require ongoing maintenance, it is the only type, which is truly ‘maintainable’ and means it’s easier to identify and repair any leaks that might arise. However, Type A and B systems share one common characteristic – repairs are often expensive, disruptive and difficult to achieve.

We always endeavour to provide waterproofing solutions that work in conjunction with the overall design plan of the architect, however this is not always possible where ‘Dual Types’ are needed to fulfil the requirements of BS8102. To this end it is important that both architects and surveyors work with the client to ensure the best possible solution is achieved while at the same time ensuring warranty providers such as the NHBC are satisfied.

The challenges of restoration and renovation
Consultation between architects and surveyors is key. If waterproofing isn’t taken into consideration early in the design phase, major problems in the programme’s delivery can occur. This is why collaboration between specialists should happen as early on in the process as possible to ensure seamless completion of the job.

There is a wealth of free advice and design input available from manufacturers such as Delta Membranes and John Newton, as well as contractors like Peter Cox. These companies are able to provide robust solutions and where required design liability. It limits the architect firm’s risks when it comes to the below ground waterproofing.

It’s important to note that any new remedial work undertaken on a historic building must also be reversible. This means that any newly installed materials must be capable of being taken out and the building restored to its original state.

Furthermore, issues such as damp, dry rot and infestations of woodworm are more common in older buildings. If not well maintained, a building will suffer from moisture ingress and the risk of decay and insect attack will be more likely to occur as a result. Older properties are more susceptible due to their antiquated design, and this combined with the cost of maintenance can lead to damp and decay problems occurring.

Final thoughts
It is vital that architects understand the extent of any remedial works before final plans are submitted. A consultation meeting is an important first step in ensuring both parties are in aware of the remedial works that are required to the building. Moving forward as the project progresses, continued collaboration is important to guarantee that any other issues that come to light are tackled quickly with an agreed action.

Richard Walker is national technical and development manager at Peter Cox