New performance-based guidance from CIBSE brings research, full-scale testing and advanced modelling together to address the distinctive drainage challenges created by tall buildings. As buildings rise higher, the engineering systems serving them must respond to conditions that become increasingly complex with height. Sanitary drainage is no exception.
Published in December 2025, CIBSE TM70: Tall Building Drainage Design establishes a new evidence-based framework for the design and assessment of sanitary drainage systems in tall buildings. It represents a significant step forward for public health engineering, replacing design assumptions derived largely from lower-rise buildings with guidance developed specifically around the pressures, airflow, and transient conditions found in taller structures.
Why was new guidance needed?
Many established drainage standards were developed primarily for low- and medium-rise buildings. Their recommendations are often based on historical experience, steady-state calculations, and the extrapolation of earlier research. Tall buildings behave differently. When water is discharged into a tall drainage stack, it interacts with the air inside the pipework. This can produce positive and negative pressure fluctuations throughout the system. As a building becomes taller, airflow resistance increases and these pressure fluctuations can become more significant.
If they are not adequately controlled, pressure fluctuations can deplete or displace the water held in fixture traps. This water seal is the barrier preventing foul air and potentially harmful pathogens from entering occupied spaces. Maintaining the seal is therefore fundamental to the sanitary integrity of a building.
TM70 addresses the question at the heart of this challenge: do the design assumptions inherited from lower-rise buildings adequately represent how drainage systems perform at height? The research behind the guidance indicates that conventional approaches do not always capture the dynamic conditions found in tall buildings. TM70 consequently moves the industry towards a more informed, risk-based method of design.
From precedent to performance
TM70 was developed through collaboration between consultants, academic researchers, professional engineers and industry partners. The involvement of manufacturers helped connect theoretical research with the practical realities of system manufacture, installation and construction.
Its recommendations draw on laboratory research, full-scale physical testing and advanced numerical modelling. Testing at the National Lift Tower in Northampton allowed drainage and ventilation systems to be examined in a true vertical environment. The facility contains what its operator describes as the world’s tallest drainage-testing installation. This physical research was supported by more than 3,000 computational simulations using the AIRNET model. AIRNET simulates the interaction between air and water, including the pressure dynamics created as discharges travel through a drainage network. Comparing its predictions with experimental results enabled researchers to validate the model and explore a much broader range of configurations than physical testing alone would allow.
Crucially, the research examined system performance boundaries rather than simply confirming that standard arrangements worked under ideal conditions. Understanding how and when a system approaches failure gives designers a stronger basis for assessing risk, selecting ventilation strategies and creating more resilient systems.
A globally relevant framework
TM70 does not replace national regulations or established drainage standards. Instead, it provides additional guidance for the conditions that arise in tall buildings and can be used alongside the relevant statutory and technical requirements for a project.
The publication draws on major international standards, including BS EN 12056, DIN 1986-100, AS/NZS 3500.2. This alignment gives the methodology relevance beyond the UK and allows its principles to be applied across international projects.
The guidance covers areas including:
- flow-rate estimation and pipe sizing
- the effect of building height on system performance
- drainage ventilation configurations
- pressure equalisation
- the treatment of stack offsets
- stub-stack connections
- risk-based ventilation sizing
This consistent framework gives engineers a way to evaluate drainage strategies against defined performance requirements, rather than relying solely on prescriptive configurations.
Creating space for innovation
The move towards performance-based design is particularly important. A prescriptive approach can unintentionally limit designers to systems and arrangements already described in an established code. A performance-based approach instead defines the outcome that the system must achieve. Designers can then demonstrate, through calculation, modelling, testing or supporting evidence, that a proposed solution satisfies those requirements.
This creates greater scope for innovation while maintaining a clear focus on safety and reliability. New drainage and ventilation systems can be assessed on their demonstrated performance rather than excluded simply because they are not represented in older guidance. It also encourages closer collaboration between designers, manufacturers and researchers. Manufacturers can contribute practical knowledge of how systems are produced and installed, while testing and modelling provide an independent evidence base for assessing how those systems behave.
Applying TM70 at Parkgate Street
IN2 is applying the principles of TM70 to the drainage design for Block A of the Parkgate Street development in Dublin. At approximately 90 metres and 30 storeys, the building falls within TM70’s definition of a tall building. The guidance is being used alongside Ireland’s Technical Guidance Document H – Drainage and Waste Water Disposal and I.S. EN 12056-2. The proposed strategy uses dedicated stacks for the principal fixture groups within each apartment. This avoids offsets within the building core and simplifies the main stack installations.
Secondary ventilation is provided alongside the primary drainage stacks, with cross-relief vents at regular intervals. These measures provide additional routes for air movement and pressure equalisation, reducing the risk that transient pressures could compromise fixture trap seals. The lower floors are served using stub stacks connected at a suitable distance from the main stack. This separates these fixtures from the turbulent connection zone at the base of the taller drainage system.
The result is a design informed not simply by precedent, but by a clearer understanding of how airflow, water flow and pressure interact throughout a tall building.
Designing for resilience
TM70 represents an important evolution in drainage engineering. By combining full-scale testing, validated modelling and practical industry knowledge, it gives engineers an evidence-based way to address risks that established low- and medium-rise design methods may not fully capture. It also changes the design conversation. Instead of asking only whether a drainage system follows a familiar arrangement, engineers can ask whether it has been demonstrated to perform safely under the conditions it will experience. For IN2, this approach aligns with our wider commitment to technically rigorous, performance-led design. As taller and more complex buildings continue to reshape our cities, TM70 provides the industry with a stronger foundation for creating drainage systems that are safe, resilient, constructable and ready for the demands of modern high-rise development.