CROSS Safety Report
Waterproofing exposed concrete slabs
This report is over 2 years old
This report highlights that water ingress into buildings continues to be a problem and discusses the role of the structural engineer when dealing with waterproofing of exposed concrete slabs.
It shows how this can be achieved by careful design and detailing of the reinforced concrete structure and by close collaboration of all parties involved.
Key Learning Outcomes
For construction professionals:
- A successful waterproof concrete slab can be achieved when there is close collaboration between the architect, structural engineer and hydraulic/drainage engineer in the design and by the contractor and their subcontractors during construction
- Where a concrete slab is exposed, ensure there are adequate falls and drainage outlets to remove the water as quickly and efficiently as possible
For structural design engineers:
- Pay careful attention to crack control in the design and detailing of reinforced concrete slabs
- There is good practical advice available, such as the Concrete Institute of Australia Current Practice Note CPN28 Watertight Concrete Structures
- Thin reinforced concrete slabs are not waterproof without special consideration
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Water ingress continues to be a major issue with many buildings, and although architects, or building designers, generally have responsibility for specifying waterproofing, the reporter notes that structural engineers have an important role especially when exposed concrete slabs are being used. The reporter’s experience is that some structural engineers mistakenly believe that thin suspended concrete slabs exposed to the weather are watertight or waterproof without some form of additional waterproofing.
For reinforced concrete slabs, even with a large amount of reinforcement, watertightness cannot be guaranteed without very special detailing and very specific attention paid to construction. Although post-tensioned slabs can be used to provide a water-resistant (watertight but not waterproof) slab, considerable design and detailing effort is required for the slab and the joints to minimise any leaks. Experience suggests that, even with the most careful design and construction, some local water staining and minor leaking from the underside of slabs is still possible.
For reinforced concrete slabs, watertightness cannot be guaranteed without special detailing and specific attention paid to construction
The reporter also notes that deflections of horizontal suspended slabs exposed to rain or water can lead to ponding of water, and this, in turn, may result in additional load, possible leaks and unsightly staining on the underside of the slab.
The NCC1 covers the requirements for damp- and weather-proofing in buildings. For non-habitable areas (such as car parks and some types of industrial buildings), slabs generally do not need to be weatherproof. The reporter notes that although this may not be a problem for such buildings, there is still the possibility of leaks and staining on the underside of exposed slabs. Often the visual appearance of such staining will cause significant concern, and this should be resolved and accepted by way of confirmation in writing at the time of design by all involved, including the developer, the client and preferably the final end-users. In considering this issue, all parties should agree on the method for identifying stains (e.g., at the time of crack inspection) and for determining non-compliant stains (i.e., of a certain measurable size). If certain stains are to be considered as non-compliant with the design or construction of a concrete structure, the parties should specify in writing which party is responsible for remediation.
Where watertightness and waterproofing are required for habitable areas, and for non-habitable areas if staining and some leaking from the underside of the slab are unacceptable, other measures need to be considered e.g., an applied waterproof membrane or a separate watertight roof constructed over the slab.
There may be situations when the ingress of water can lead to long term corrosion problems so interpretation of the NCC and whether a waterproof structure is required requires careful consideration as the consequences of reinforcement corrosion can be catastrophic.
The reporter draws attention to the Concrete Institute of Australia Practice Note CPN 28 on watertight concrete structures2 that provides a valuable resource on this topic for engineers, architects, building owners and construction managers.
 NCC Volume 1, Australian Building Codes Board, May 2019
2 Concrete Institute of Australia CPN 28 –Watertight Concrete Structures, 2005
Expert Panel Comments
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As the reporter notes, water ingress continues to be a major issue with buildings. For example, the 2019 report An Examination of Building Defects in Residential Multi-owned Properties by Johnston and Reid examined 212 building defect reports from New South Wales, Queensland and Victoria. These reports identified 3,227 building defects of which at least 20% were related to water ingress.
Water ingress may be the result of several factors and can involve any part of the building envelope. In many cases, it may not be considered a structural issue. However, this report deals with one situation that does involve the structural engineer, i.e. waterproofing of exposed concrete slabs.
An important question when dealing with exposed concrete slabs is to determine how watertight or waterproof the slab needs to be. Water ingress into habitable spaces is certainly a matter for health and safety; it may also result in corrosion of reinforcement and deterioration of other structural elements leading to distress and/or potential failure.
Water ingress may also result in corrosion of reinforcement and deterioration of other structural elements leading to distress and/or potential failure
NCC Section E Compliance
FP1.2 Preventing rainwater from entering buildings
Surface water, resulting from a storm having an average recurrence interval of 100 years must not enter the building.
A roof and external wall (including openings around windows and doors) must prevent the penetration of water that could cause (a) unhealthy or dangerous conditions, or loss of amenity for occupants; and (b) undue dampness or deterioration of building elements.
It then gives exemptions for certain classes of building where these requirements do not apply and that are “based on the belief that the use and safety levels of the exempted buildings will not be significantly diminished by surface water entering them.” Also, that “it is the responsibility of a building proponent to satisfy the appropriate authority that the exemption should apply”.
Design and detailing
As the reporter notes, it is possible to achieve a water-tight concrete slab, but this does require careful design, detailing and construction to be successful. To achieve a fully weatherproof design that is compliant with the NCC a layered approach is suggested as follows:
To achieve a fully weatherproof design that is compliant with the NCC a layered approach is suggested
1. If possible, prevent the water from reaching the concrete slab in the first instance, e.g., for some buildings it may be more cost-effective to provide a separate weathertight light steel roof over the concrete roof slab.
2. Where the concrete slab is exposed, remove the water as efficiently as possible by providing adequate falls to a correctly designed roof drainage system with outlets at its low points. This requires close collaboration between the structural engineer, architect and hydraulic/drainage engineer to ensure that the falls and drainage outlets take into consideration building movements and long-term deflections of the concrete slab.
3. Pay close attention to crack control, particularly from restraint to shrinkage and other building movement. The risk of cracking can be reduced by:
- Judicious location of service cores and other stiff building elements;
- The location of permanent and temporary movement joints;
- Provision of adequate and correctly detailed reinforcement;
- Use of post-tensioned concrete slabs;
- Correct concrete mix design, possibly with shrinkage-reducing admixtures.
4. Provision of a correctly specified, detailed and installed waterproof membrane.
5. Good construction and an understanding of the waterproofing requirements by the contractor and their subcontractors.
Where a waterproof membrane is to be applied, it should comply with AS4654 Parts 1 and 2 - Waterproofing membranes for external above-ground use. However, adequate falls must still be provided and attention paid to crack control, as in steps 2 and 3 above. The membrane should be installed by a competent and experienced specialist contractor to ensure it performs as intended. As the Masters Building Association of New South Wales website points out “waterproofing tops the list of the ‘10 most common defects’ in every State and Territory” and it has produced a series of Waterproofing Guides for use throughout the building industry and by training institutions.
Construct New South Wales has recently commissioned a course Waterproofing Design Principles to prevent water leaks and leaching in buildings. Although the course focuses on residential apartment buildings (class 2), the principles apply to all building types.
The Australian Institute of Waterproofing also provides technical advice and training and as a 2017 presentation to a Queensland Building and Construction Commission (QBCC) seminar on Preventing Waterproofing Defects concludes:
Successful waterproofing systems rely on:
- Design (compatibility chain)
- Specification (compatibility & installation)
- Membrane Application
- Adherence to specifications and standards
- Maintenance (other contractors)
- Common sense
To verify the membrane installation the specification should include the following hold points:
- Slab preparation
- Testing of membrane thickness (for liquid membranes)
- Corner and turnup preparation
Flood testing is also advisable for critical applications.
As noted above the NCC does allow some exemptions for certain classes of buildings involving non-habitable spaces, e.g., car parks and some industrial buildings, provided “the use and safety levels of the exempted buildings will not be significantly diminished by surface water entering them” and that “it is the responsibility of a building proponent to satisfy the appropriate authority that the exemption should apply”. This requires careful consideration at the design stage as water seeping through a concrete slab will not only cause unsightly staining, it may also damage car paintwork and other stored goods. One approach is to classify the end usage such as that in the UK Concrete Centre’s publication Basements: Waterproofing that can be used to agree the expected level of performance with all parties and for the contractor to have a clear level of delivery expected.
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