1:19:11
Industry Partners Series: surface drainage in public spaces and building thresholds
Engineers Australia
Overview
This video discusses the critical updates to surface drainage design in public spaces and building thresholds, focusing on the revised NCC 2025 and AS/NZS 3500.3:2025 standards. It highlights the shift from isolated drainage details to a holistic water management approach, emphasizing the interconnectedness of drainage with accessibility, slip resistance, waterproofing, and maintenance. Key changes include making overflow design mandatory, updating valley gutter calculations, and specifying appliance connections. The presentation stresses the importance of designing for blockages, coordinating multiple design requirements at building thresholds, and selecting appropriate materials and methods for durability and compliance. Practical advice is offered on detailing, installation, and maintenance to ensure effective and safe drainage solutions.
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Chapters
- The NCC 2025 and AS/NZS 3500.3:2025 introduce significant changes to surface and subsurface water management.
- These revisions integrate previously separate drainage requirements into a unified framework, particularly affecting building thresholds and public spaces.
- The building threshold is a complex interface requiring coordination between drainage, accessibility, slip resistance, waterproofing, and maintenance.
Understanding these updated standards is crucial for engineers to ensure compliance and design effective, safe drainage systems that prevent common failures.
NCC 2025 consolidates surface (F1) and subsurface (F3) water management into a single framework, explicitly including balconies and podiums.
- Appendix F, concerning overflow and eave gutters, is now normative, requiring explicit calculations for overflow pathways.
- Valley gutter design is enhanced with a graphical method (Figure 3.6.2) allowing for more accurate sizing based on actual catchment areas.
- New Clause 5.3.8 mandates explicit requirements for connecting miscellaneous appliances like HVAC condensate and pool overflows to the stormwater system.
These changes move overflow and appliance drainage from advisory guidance to mandatory design requirements, necessitating a more rigorous and calculated approach.
Previously advisory overflow details are now normative, meaning engineers must select a device type and verify its capacity against the required overflow rate (Q*).
- Threshold design must integrate drainage performance with accessibility (e.g., Livable Housing Design Guidelines), slip resistance (AS 4586), waterproofing continuity, and maintenance access.
- The primary drainage path must handle design flow, and overflow paths must be designed to function even under partial blockage.
- Designing for blockages is paramount; blockage factors (e.g., 0.5 for sag pits, 0.8 for on-grade pits) must be applied to sizing calculations.
Failure to coordinate these elements at the threshold leads to common issues like flooding, trip hazards, and waterproofing failures, impacting building integrity and user safety.
A recessed door slab or local set-down creates a drainage recess without introducing a step, while a linear slot drain immediately outside intercepts surface runoff.
- The five-step design process includes selecting the Annual Exceedance Probability (AEP) based on consequence, determining rainfall intensity, calculating runoff coefficients, sizing primary drainage with blockage factors, and designing overflow paths.
- Runoff coefficients vary by surface type (e.g., 1.0 for roofs, 0.85-0.9 for impervious paved surfaces).
- Minimum pipe sizes are specified (e.g., 90mm for residential, 150mm downstream of pits), and surcharge outlets have strict exit velocity limits (0.15 m/s).
Following a systematic design process ensures that drainage systems are adequately sized for both peak flows and potential blockages, preventing system failure.
For overland flow in pedestrian zones, limits include a maximum ponding depth of 50mm and a maximum depth-times-velocity product of 0.4 m²/s.
- Slip resistance is directly linked to drainage; surfaces that remain wet or accumulate debris become slip risks, requiring design for wet conditions (AS 4586 P4/P5 ratings).
- Maintenance access is now a direct NCC 2025 requirement, necessitating visible surcharge outlets and provision for inspection and cleaning.
- As-built verification before handover is essential to catch and correct deviations from the design, such as altered levels or fall directions.
Ensuring adequate slip resistance and planning for maintenance are critical for long-term safety and functionality, preventing premature failure and costly repairs.
Surcharge outlets must be visible, noticeable, clear of buildings, and within the property boundary, with exit velocity not exceeding 0.15 m/s.
- Threshold drains are primarily designed to capture wind-driven rain, not large volumes of catchment runoff.
- The design must accommodate door types, slab depth, and waterproofing details, often requiring purpose-designed shallow profile drains.
- Key considerations include the drain's interface with the door track, sill drainability, and hydraulic capacity, with non-uniform flow principles being essential for accurate sizing.
Specialized threshold drainage is vital for preventing water ingress at building entrances, especially with the prevalence of level thresholds, and requires careful product selection based on specific site conditions.
Sliding or bifold doors with recess mullions often require an integrated linear drain that interfaces directly with the door track to capture water through weep holes.
- Trench drain hydraulics are non-uniform, meaning water depth and velocity change along the channel; V-shaped channels offer better self-cleaning than U-shaped ones.
- Channel slope and run length significantly impact performance; shorter runs and built-in falls improve water evacuation.
- Drainage elements must be strong (AS 3996 load classes), durable (corrosion-resistant materials like stainless steel or polymer concrete), and properly encased in concrete for structural integrity.
Understanding hydraulic principles and selecting durable materials ensures the drainage system functions effectively over its design life and withstands environmental and load stresses.
The EN 1433 standard provides load classifications for trench drains and grates, tested up to 900 kN, crucial for public spaces and areas with vehicle access.
- Proper installation sequencing, especially the interface between waterproofing, drains, and door systems, is critical.
- Boltless locking systems for grates facilitate easy maintenance and cleaning, reducing the risk of loose grates.
- Grate design must consider heel safety (typically 8-10mm slots) and orientation (slots across the direction of travel for AS 1428.1 compliance) to prevent trip hazards and entrapment.
Correct installation and features that promote easy maintenance and user safety are essential for the long-term effectiveness and reliability of drainage systems.
Slots between 8-13mm must run across the dominant direction of travel to prevent wheels, canes, and mobility aids from tracking into the openings, as per AS 1428.1.
Key takeaways
- NCC 2025 and AS/NZS 3500.3:2025 mandate a more integrated and calculated approach to water management, especially at building thresholds.
- Overflow drainage is no longer optional; it must be designed and calculated according to normative requirements.
- The building threshold is a multi-disciplinary coordination challenge requiring simultaneous consideration of drainage, accessibility, waterproofing, and safety.
- Designing for blockage is as critical as designing for peak flow; apply blockage factors to all drainage sizing.
- Slip resistance is directly tied to drainage performance; design for wet conditions and ensure grate materials match pavement slip characteristics.
- Adequate maintenance access and clear overland flow paths are essential to prevent flooding and ensure long-term system functionality.
- Threshold drains are specialized for wind-driven rain and must be carefully detailed to integrate with door systems and waterproofing.
Key terms
NCC 2025AS/NZS 3500.3:2025Building ThresholdSurface DrainageSubsurface Water ManagementNormative Appendix FOverflow PathwayDeemed-to-Satisfy (DTS)Performance Solution (PS)Annual Exceedance Probability (AEP)Blockage FactorsSlip Resistance (AS 4586)Waterproofing MembraneWind-Driven RainNon-Uniform FlowHydraulic PerformanceLoad Classes (AS 3996)Heel Safe Slot SizeMaintenance AccessOverland Flow Path
Test your understanding
- How has NCC 2025 changed the approach to water management compared to previous versions, particularly concerning building thresholds?
- Explain why Appendix F of AS/NZS 3500.3:2025 moving from informative to normative significantly impacts drainage design.
- What are the five key design requirements that must be coordinated at a building threshold, and why can't they be resolved in isolation?
- Describe the importance of designing for blockages in surface water drainage and provide examples of blockage factors mentioned in the video.
- How does the design of threshold drainage differ from general surface water drainage, and what specific challenges does it address?