CPD 01 2023: Waterproofing techniques

This CPD sponsored by RIW explains the latest requirements on waterproofing structures such as basements as provided in the updated British Standard, BS 8102: 2022

Type A (barrier) water protection, as used here, is one of three types that can be used. The others are type B (structurally integral) and Type C (drained) systems – or else a combination

The code of practice for the protection of below-ground structures against water ingress; BS 8102: 2022, has recently been updated to reflect new practices and materials, provide increased clarity and ensure higher-quality waterproofing designs and installations.

This CPD module looks at why the guidance has been updated and what the latest edition means for waterproofing specialists, specifiers, contractors and end clients. Beyond outlining the changes, this module considers the implications and benefits of the updates.

Learning objectives:

  • Clarity on the role and purpose of a waterproofing design specialist
  • Awareness of updated grades of performance
  • Knowledge of updated guidance on protecting structures from water ingress
  • Understanding the need for greater collaboration within the design team

Overview of BS 8102: 2022 Protection of below ground structures against water ingress

BS 8102 covers all areas of protecting buildings from water including design philosophy, site evaluation
and the design of waterproofing systems as well as installation and maintenance. It also provides detailed guidance on implementing type A (barrier), type B (structurally integral) and type C (drained) systems, alongside combined solutions for addressing water ingress.

The guidance provided in the code of practice has evolved over the years from CP 102: 1973 to the latest edition published in March 2022 to reflect updates to best practice. The title of the document has also evolved from ‘Protection of below ground structures against water from the ground’ in 2009 to ‘Protection of below ground structures against water ingress’ in 2022 to recognise that it is not just ground water that the waterproofing strategy must protect against. Water ingress could also be a result of events such as flooding or water main leaks and the design of the protection should account for this.

Why update BS 8102?

British Standards codes of practice must reflect the most up-to-date guidance in that area. BS 8102 was last revised in 2009 and there have since been advances in waterproofing strategies, including new technologies and systems, and improved best practice approaches.

BS 8102 was also updated for better harmonisation with the current guidance provided in in other standards, particularly those that have also been revised since 2009, for example BS 8485: 2015: A1 2019, the code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings.

Role of waterproofing design specialist

The importance of including a waterproofing design specialist (WDS) in the design team was outlined in earlier versions of BS 8102. The 2022 edition reinforces this position. A WDS will ensure that the waterproofing strategy, as well as the chosen products and details, are appropriate for the project and comply with best practice.

BS 8102: 2022 offers further clarity on the qualifications and experience a WDS should have:

  • The industry-recognised certified surveyor in structural waterproofing (CSSW) is accepted as the minimum requirement.
  • The WDS should be experienced in a broad spectrum of waterproofing techniques, in addition to the type and size of the project in question.

The updated standard clearly states that the WDS is “appointed” as part of the design team, demonstrating that the WDS does not need to have a contractual obligation on the project. The appointment should be made as early as possible, although no later than the start of Stage 4 (as per the RIBA stage of works).

Grades of performance

One of the most important recommendations in BS 8102 is the need to agree the acceptable level of watertightness for the structure. The previous version outlined three grades of performance, with associated examples. However, BS 8102: 2022 has expanded this to four and removed the example end uses. This is to avoid restricting common end uses by tying them to associated grades, as in reality the process is much more fluid

GradePerformance level
1ASeepage and damp areas from internal and external areas are tolerable, where this does not impact on the proposed use of below-ground structure
1BNo seepage. Damp areas from internal and external sources are tolerable
2No seepage is acceptable. Damp areas as a result of air moisture or condensation are tolerable; measures might be required to manage water vapour and condensation
3No seepage or damp area is acceptable

As seepage and damp areas are mentioned prominently in the updated grades of performance, BS 8102: 2022 has clearly defined these:

  • Seepage – slow transmission of water through discrete pathways of a structure
  • Damp area – area which is slightly wet but no seepage.

Ingress from other sources

One of the most recognisable changes to BS 8102 is the document name, “Protection of below ground structures against water ingress”. Prior to the 2022 edition, waterproofing designers would only need to focus on below-ground sources of water ingress. With the new standards, water ingress from all sources must be considered.

One of the largest areas of concern, now covered in the BS, is the waterproofing of podium slabs, buried roofs and green roofs.

The BS has offered updated guidance on the waterproofing of these types of construction:

  • Substrate – roof slabs should be formed of reinforced concrete, as this substrate is the most reliable to waterproof. With alternative forms of construction, the potential movement/deflection and the impact on the chosen waterproofing system should be considered.
  • Falls – the roof slab should be laid to a fall of 1:80 to promote movement of water away from the structure towards suitable outlets. Where the slab cannot achieve this, an unbonded screed should be used. There should be no potential for water to pond or backfill.
  • Membrane choice – where the waterproofing layer is applied directly to the roof slab, this should be fully bonded. Opting for a loose-laid waterproofing system is only acceptable if permanent leak detection systems are employed.
  • Drainage – a drainage path should be provided directly above the waterproofing layer to promote drainage of water way from the waterproofing layer.
Figure 1 Waterproofing of green roofs, as shown here at the Macallan Distillery in the Scottish Highlands, are now included in the updated standard BS 8102: 2022

Waterproofing techniques

BS 8102: 2022 has updated its guidance on waterproofing techniques, and also what is considered acceptable for combined protection. Type A, type B and type C waterproofing techniques are still recognised, albeit with new guidance on acceptability.

Type A waterproofing

In the 2009 version, BS 8102 showed three possible locations for type A barrier membranes:

  • External (external face of structure)
  • Internal (inside face of structure)
  • Sandwiched (membrane in cavity, supported structurally if necessary).

The sandwiched approach is no longer recognised in the 2022 edition.

There is also further emphasis on the need for continuity with type A barriers, from the damp-proof course (DPC) to the below-ground structure. Particular reference is made for the need for continuity around structural elements (such as piled foundations).

Type B waterproofing

A key update affecting type B waterproofing was around penetrations, and the recommendation to specify cast-in penetrations due to the lower risk of failure. There is also an emphasis on waterstops, and the need for these to be continuous throughout the below-ground structure.

A further addition is the link between the desired grade of performance and the relevant Eurocode tightness clause, defining the required design crack width:

  • Grade 1A – tightness class 0 (to BS EN 1992 – 1 – 1)
  • Grade 1B – tightness class 1 (to BS EN 1992 – 3: 2006)
  • Grade 2 – further measures required
  • Grade 3 – further measures required.

Type C waterproofing

The premise behind type C waterproofing is that any seepage through the structure is safely collected and discharged. The structure itself must form the initial barrier, limiting seepage into the structure. In the 2009 edition it was stated the outer leaf of the structure must control water ingress, but the 2022 edition says any external element can control this.

As type C systems are often used in deep, multi-level basements, further guidance is provided on draining through intermediate floors and the required detailing here

At the British Museum extension, to counter the high water table in the four-storey basement, a combined waterproofing approach used an overall barrier layer of bentonite clay sheet waterproofing composite, plus cavity drainage using a flexible studded membrane

Combined protection

As in the 2009 edition, combined protection (often referenced as two forms of waterproofing, or as belt and braces) is still identified as an important risk reduction technique, and its implementation is recommended on either high-risk sites or where the consequences of water ingress are unacceptable.

Combined protection is where at least two forms of waterproofing are used in combination, so that if one system fails there is redundancy with the other system. In the 2009 this approach was only recognised if the two systems were different types of waterproofing (type A combined with type B or C, or types B and C combined, or else all three types together).

In the 2022 edition this is still valid, but a type A plus type A approach can also be followed provided then two systems have different performance characteristics, ensuring both systems do not fail due to a common cause.

Drive for collaboration

Another theme that is clear throughout BS 8102: 2022 is the need for collaboration at every stage of the project. Close collaboration among the design team is required from the earliest stages to ensure that the most appropriate and robust waterproofing solution is incorporated into the building’s design.

Fostering a close working relationship will also ensure a smooth build process and an issue-free handover to the client. Essentially, the aim should
be to collectively mitigate and manage risk throughout the project.

Phase 1 – design: At this stage the waterproofing specialist will seek to understand the client expectations and requirements and how this will influence the waterproofing strategy. They will also look at the site evaluation to produce a risk assessment and provide initial design guidance. The updated code of practice now includes a wider range of factors that may be identified during this phase as potentially affecting the waterproofing design. This includes the effects of climate change, burst water mains, flooding and defective sustainable urban drainage systems (SUDs) and sewers as well as ground gases and contaminants.

Phase 2 – constructability: Prior to work on site commencing, the waterproofing specialist should coordinate with the project delivery team to provide support, refine the proposed strategy and ensure the waterproofing can be implemented correctly as the project progresses. BS 8102: 2022 now includes a recommendation that service penetrations through type B systems should be cast insitu, and not installed after the structure has been formed, to reduce the risk of waterproofing issues.

Phase 3 – project delivery: Working with all parties involved in the construction, the waterproofing specialist will be able to develop a full installation programme, method statements and bespoke drawings, as well as providing toolbox talks to ensure operative best practice. They should also agree an appropriate inspection and test plan and undertake regular visits to provide quality assurance and check for non-conformity issues. One of the key updates in this area is the increased recognition that waterproofing systems need to be protected during all phases of the construction. This requires co-operation with all those working on site, including following trades, to ensure the systems are not damaged.

Warranties

While warranties and guarantees are an important part of the construction industry, in particular with waterproofing systems, in practice it can be difficult to rely on these alone. Often these documents can lack clarity and transparency, may use ambiguous language and have exclusions buried deep in the small print.  Mitigating and managing risk through the design, constructability and project delivery phases to project handover is therefore just as important as ensuring warranties and guarantees are in place.

Waterproofing is an essential part of the design of a building and BS 8102 forms part of a wider drive within the industry for closer collaboration among all those involved in a project. Appointing an experienced and dependable waterproofing specialist who can work closely with the design and project delivery teams throughout is essential.

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