The Hidden Risk in Your Building’s Drainage System
With increasingly unpredictable weather patterns and more intense rainfall events becoming the norm across the UK, the protection of commercial buildings from water ingress has never been more critical. February 2025 saw the publication of BS 8490:2025, introducing significant changes to siphonic drainage standards that building owners and facilities managers need to understand to ensure their properties remain protected and compliant. In this blog post we’ll walk you through what these changes mean.
Why Rainfall is Becoming More Intense
Recent data shows that the UK experienced 40-60% more rainfall across all regions in 2024 compared to the 1991-2020 averages. This dramatic increase is consistent with climate change projections that predict:
- More frequent and intense summer thunderstorms
- Increased winter precipitation
- Greater variability in rainfall patterns
- More “cloudburst” events where extreme amounts of rain fall in very short periods
Climate scientists have been warning for years that global warming leads to more water vapour in the atmosphere—approximately 7% more moisture for each degree Celsius of warming. This increased atmospheric moisture results in more intense precipitation events when conditions are right for rainfall.
The construction industry and building standards are now adapting to this new reality, with the BS 8490:2025 being a prime example of how climate change is directly influencing building regulations and best practices.
What is Siphonic Drainage?
Unlike traditional gravity drainage systems, siphonic drainage uses a negative pressure principle to efficiently remove large volumes of rainwater. The system includes special baffle plates fitted to rainwater outlets that prevent air from entering pipes during intense storms, creating a siphonic action that dramatically increases water flow rates.
This approach offers several advantages:
- Higher capacity to handle intense rainfall
- Fewer, smaller diameter pipes required
- More flexible placement of pipework
- Self-cleansing properties
- Reduced underground drainage requirements
Critical Changes in BS 8490:2025
The new standard, which supersedes BS 8490:2007, introduces several important updates:
1. Mandatory Secondary Systems for Internal Gutters
The most significant change affects clause 8.9.2. Previously, the 2007 standard indicated that difficult layouts with high water volumes required a secondary system, while presenting “Primary & Secondary” as an alternative method.
The 2025 standard now explicitly states that “where systems can block and cause ingress to buildings, a Primary & Secondary should be provided.” This means that all internal gutters should now have a secondary siphonic system to comply with the new standards.
Key Takeaway: If your building has internal gutters with only a primary siphonic system, it no longer meets current standards regardless of whether it satisfies rainfall flow rate requirements.
2. Enhanced Maintenance Requirements
While maintenance was mentioned in the 2007 standard, the 2025 version places greater emphasis on regular inspection and maintenance of siphonic systems. This includes specific requirements for checking bracketry, fixings, and system integrity.
The new standard recommends more comprehensive preventative maintenance schedules, providing an opportunity for building owners to implement more robust maintenance protocols.
3. Exceptions to Secondary System Requirements
It’s worth noting that not all roof types require secondary systems under the new standards. Flat sealed roofs (membrane/felt) and outboard eave gutters are exempt from the secondary system requirement.
What Does This Mean for Building Owners?
If your building was constructed before February 2025, there’s a high likelihood that your siphonic drainage system no longer meets current standards, particularly if you have internal gutters with only a primary system.
Buildings with older systems face several risks:
- Non-compliance with current standards
- Increased vulnerability to water ingress during intense rainfall
- Potential property damage and business interruption
- Possible implications for insurance coverage
Case Study: Proactive Compliance
Billy Gordon, Technical Manager at NSS: “We recently worked with a major national retailer to upgrade their siphonic drainage systems across multiple UK stores. Our initial surveys revealed that most locations had internal gutters with primary-only systems. While these were functioning adequately during normal conditions, they were clearly non-compliant with the new BS 8490:2025 standards.
“What made this project particularly interesting was that we were able to implement secondary siphonic systems with minimal disruption to their day-to-day operations. The client was initially concerned about potential trading interruptions, but our approach allowed them to maintain normal business hours throughout the upgrade process.
“One location in the North West experienced significant water ingress during summer 2023 due to their primary-only system being overwhelmed during an intense rainfall event. After our upgrades, they weathered several comparable storms in 2024 without issue. This real-world validation of the secondary system’s effectiveness has prompted them to accelerate the upgrade programme across their remaining properties.”
Next Steps for Building Owners
- Assessment: Arrange for a professional review of your current siphonic drainage systems, particularly if they were designed before February 2025
- Compliance Planning: If your system only has primary drainage for internal gutters, develop a plan to add secondary systems
- Maintenance Review: Update your preventative maintenance schedules to meet the enhanced requirements in the new standard
- Future-Proofing: Consider climate projections when upgrading systems, as rainfall intensities are expected to continue increasing due to climate change
The Cost of Inaction
Failing to adapt to these new standards and climate realities can have significant consequences:
- Property damage: Water ingress can damage building structures, equipment, inventory, and systems
- Business interruption: Flooding can force temporary closures, resulting in lost revenue and customer dissatisfaction
- Insurance implications: Insurers are increasingly scrutinising flood risk management measures, and non-compliance with current standards could affect coverage or premiums
- Reputational damage: Building failures reflect poorly on organisational management and preparedness
The cost of implementing secondary siphonic systems is typically far less than the potential losses from a significant water ingress event. Additionally, implementation can often be phased to manage capital expenditure while prioritising the most vulnerable areas.
Expert Support
Given the specialised nature of siphonic drainage systems, it’s essential to work with accredited specialists who understand the new standards and can provide appropriate solutions. Look for contractors who are members of the Siphonic Roof Drainage Association (SRDA) and have experience in retrofitting secondary systems to existing buildings.
As a member of the SRDA with extensive experience in designing, installing, and maintaining siphonic drainage systems, NSS is well-positioned to help building owners navigate these new requirements. Our team of specialists can conduct thorough assessments, provide detailed recommendations, and implement compliant solutions with minimal disruption to your operations.
NSS is a nationwide specialist in maintenance, cleaning, hire, and testing services, delivering comprehensive solutions for commercial properties across the UK. As members of the Siphonic Roof Drainage Association with over 50 years of collective industry experience, our in-house team of NICEIC-approved technicians brings unparalleled expertise to every project. Operating with our own fleet of vehicle-mounted access platforms, we provide both planned maintenance and rapid emergency response without relying on subcontractors.
Whether you need roof maintenance, siphonic drainage upgrades, or specialist cleaning services, NSS is committed to protecting your business operations with dependable service that sets new standards for safety, speed, and transparency. Contact us today to learn how we can help safeguard your property and reputation.
Click here to learn more about the new Siphonic Drainage Standard BS 8490: 2025 at our event.