Extending a residential building underground by adding a basement has many benefits, including increased living and storage space, privacy, and property value. With above-ground expansion often limited by neighbouring properties and planning restrictions, maximising square footage by digging down is rising in popularity.
This is especially true in rich areas such as London, where research published in the Urban Geography journal revealed that over 7,000 basements were granted planning approval in Greater London between 2008 and 2019. The majority were just one storey, but 14.2% had two or three storeys, with 2.3% classed as ‘mega’ basements.
Who wouldn’t want extra space below ground for a home office, private gym, swimming pool, home cinema, wine cellar, garage… or even just more bedrooms? As attractive as these subterranean residential developments may seem, however, the costs can be even higher if below-ground structures aren’t properly protected against water ingress.
Groundwater and underground streams pose a huge flood risk to basement structures if the proper measures aren’t taken to waterproof them. In a reflection of the growing demand for underground structures, the British Standard for ‘protection of below ground structures against water’ was revised and updated in 2022.
As well as overhauling how the waterproofing industry works, these changes affect anyone who builds or owns an underground structure. So, if you’ve added a basement under your property, or you’re planning to, here’s what you should know about the new waterproofing standards.
The leading code of practice has evolved from the limited guidance of the early 1970s, but hadn’t been updated since 2009’s standard for ‘Protection of below ground structures against water from the ground’. The new guidance of BS 8102:2022 recognises the risks from other water sources, and establishes a wider scope for ‘Protection of below ground structures against water ingress’.
British Standard 8102 provides recommendations for preventing the entry of water into structures below ground level, covering groundwater risk assessment, waterproof construction, barrier materials, and drainage options. The 2022 updates reflect the latest developments in waterproofing knowledge and technology to achieve structurally integral, waterproof underground spaces.
The new guidance also aims to reduce risks in other areas – including fire protection, safety, and life cycles – and in harmony with other recently updated codes of practice, such as the standards for protection against methane and carbon dioxide ground gases in new buildings.
While the previous version was primarily concerned with groundwater ingress, the biggest change to BS 8102 is its consideration of protection against all types of water ingress. The new version looks into ingress from a range of possible sources, and clarifies standards for specific structures like buried roofs, podium decks, and tunnels.
The updated BS 8102:2022 advises on the design and installation of waterproofing systems, amends the grading for waterproof protection, and includes new guidance on complying with fire safety and gas membrane testing regulations for underground structures.
The updated standards recognise the importance of successful waterproofing in preventing the failure of an underground construction project, recommending that a specialist design team be appointed as early as possible in the planning process.
Having a dedicated responsible party to oversee the waterproofing design and approve its detailing ensures high-quality work, reducing the risks of problems with unfit materials or incorrect implementation at subsequent stages of the project.
The 2022 amendment states that waterproofing designs at or below ground level should be continuous to protect against water coming up against any part of the structure during its lifetime, with the damp-proof course extended to 150mm above external ground level.
It also reinforces that the water protection design should be appropriate for the intended purpose – such as Grade 3 for habitable spaces, Grade 2 for non-habitable areas, and Grade 1 for light wells. Designers should also consider sustainability and remedial contingencies.
Previous versions of waterproofing grading involved four grades in 1990 and the removal of the fourth grade (archive storage) in 2009. The 2022 version has split the first grade into two categories, and removed the examples of associated uses to prevent over-reliance on them.
Here are the new grades for waterproofing underground structures from 2022 onwards:
The updated standards also clarify the definitions of seepage and damp areas, as follows:
These amendments emphasise the importance of agreeing on the acceptable level of watertightness for a structure through proper design consideration and consultation with the client, rather than tying common uses to associated grades.
In addition to revising grades of waterproofing, BS 8102: 2022 reviews waterproofing protection techniques. The previously determined Type A, Type B, and Type C are still accepted, but with updates reflecting improved knowledge and methods.
Barrier waterproofing, often called tanking, applies waterproof materials to walls and floor slabs to prevent groundwater ingress.
The 2022 guidance no longer recognises the ‘sandwich’ approach, emphasising continuity with barriers around structural elements instead.
Structurally integral waterproofing relies on the design and material quality of the structure’s ‘water resistant’ shell.
There are new recommendations for cast-in penetrations and continuous water-stops, as well as matching the waterproofing grade to Eurocode tightness clauses for crack widths.
Drainage (a common method for retrofitting a waterproofing system) is used to collect and discharge seepage behind cavity walls to a sump, typically powered by a pumping station.
BS8102:2022 recommends that Type C cavity drainage membranes should have a maintenance schedule for regular inspections, recording details to prove the system is functioning correctly.
Combined protection uses at least two types of waterproofing to achieve the acceptable level of protection. This reduces higher risks of water ingress by providing a ‘back-up’ system if the other fails, and is usually implemented where ingress would have unacceptable consequences.
Previously, the combined approach could only be used if it involved different forms of waterproofing, such as Type A with Type B, or all three types used together. However, it’s now possible to combine multiple forms of the same type of waterproofing, as long as these systems don’t have the same performance characteristics that could lead to failure from a common cause.
As the latest BS 8102 editioncame into effect last year, waterproofing manufacturers, architects, and contractors should all be trained in how to follow the new guidance and achieve the best possible results for their clients and the safety of their properties.
The updated code of practice encourages the mitigation and management of water ingress risks in structures partially or completely below ground level. For further peace of mind, part of the liaison that’s encouraged between contractors and clients from the earliest stages should involve the implementation of a building warranty.
This type of structural warranty involves additional risk assessments and start-to-finish regular inspections of the construction process, providing further peace of mind that any mistakes or problems won’t go unnoticed or unresolved.
If you are building a basement and looking for a building warranty quote for your underground structural renovations, enquire about our ABC+ Warranty today to find out whether our cover would be suitable for your project.
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