BS 8490:2025: What the New Siphonic Drainage Standard Means for Your Building
When it comes to commercial roof drainage, performance and compliance go hand in hand. With heavier rainfall events and more complex roof structures becoming the norm, effective roof drainage systems are no longer a nice-to-have, they’re essential.
That’s why the newly released BS 8490:2025 is such an important update for the built environment. This latest British Standard provides detailed guidance on siphonic roof drainage systems, setting out best practices for design, installation, validation, and maintenance.
So what’s changed and what do you need to do to stay compliant?
What is BS 8490:2025?
Published in February 2025, BS 8490:2025 is the updated version of the UK’s guidance document for siphonic roof drainage systems, replacing BS 8490:2007.
Siphonic systems use full-bore flow to rapidly remove rainwater from flat or low-pitched roofs. When designed and maintained correctly, they can be more efficient, space-saving, and cost-effective than traditional gravity systems, particularly for large commercial or industrial buildings.
The updated standard outlines key criteria for performance, resilience, and safety – all of which are increasingly vital as UK weather patterns shift towards more frequent intense rainfall.
“Poorly maintained or under-designed drainage systems don’t just risk damage, they risk disruption, liability, and cost.”
What’s new in the 2025 edition?
Secondary Drainage Now Mandatory
One of the biggest changes in BS 8490:2025 is the requirement for secondary siphonic drainage systems on buildings with internal gutters.
Previously only recommended in specific cases, secondary systems are now mandatory, offering added protection against blockages or extreme weather events.
Improved Design Guidance
The standard includes updated parameters for rainfall intensity, flow rates, pipe sizing, velocity, and system priming. This ensures better performance and system balance, and helps mitigate the risk of overflows or flooding.
Clearer Installation and Handover Requirements
From pipe fixings and jointing to insulation and acoustic dampening, BS 8490:2025 gives detailed installation best practices. It also outlines steps for handover testing, including pressure tests and flow validation, key to confirming a system is fit for purpose.
Emphasis on Ongoing Maintenance
The standard places greater focus on inspection and maintenance, stressing the need for regular checks, cleaning, and system reviews to ensure long-term reliability. Poor maintenance can lead to blockages, backflow, or structural water damage, all avoidable with the right schedule in place.
Why This Matters for Building Owners and Facilities Managers
BS 8490:2025 isn’t just for designers and engineers. If you own or manage a building that uses siphonic drainage, this update affects you directly.
Here’s why:
Non-compliance could lead to insurance issues or failed inspections
Inadequate drainage may result in costly water damage, tenant disruption, or safety risks
Secondary drainage systems may now need to be retrofitted to existing buildings
Maintenance plans may need updating to align with the standard’s recommendations
How RPMG Can Help
At RPMG, we specialise in maintaining and upgrading commercial roof drainage systems across the UK. Our teams stay ahead of the latest standards and work closely with property owners, contractors, and surveyors to ensure full compliance.
Whether you’re planning a new roof drainage system or need to assess existing infrastructure against BS 8490:2025, we offer:
- Siphonic system inspections
- Secondary drainage design and retrofitting
- Drainage performance testing and reporting
- Preventative maintenance packages tailored to compliance
Need support? Get in touch to book a drainage survey or speak to one of our technical specialists.