As UK weather becomes increasingly extreme and unpredictable, drainage design for commercial properties must adapt to prevent water ingress and maintain compliance. This has now been recognised by BSI with the introduction of BS 8490:2025, covering siphonic drainage. It is an important change that retail property owners must be fully aware of how it may impact their existing and future estate. We will walk you through the essential parts of this standard below to help provide a clear understanding.

The Background To The New Standard

In the UK we have very rarely been short of rain. But in recent years the volume and type of rainfall has shifted. Up to 60% more rain fell last year compared to the average amount between 1991-2020. Furthermore, climate experts have stated for some time that how rain falls will change, and we are already beginning to see this in action.  For every degree Celsius rise in temperature air moisture increases by 7%. The UK is now seeing more thunderstorms during the summer months, higher volumes in winter, and major rainstorms in short periods.

Siphonic Drainage Explained

Traditional drainage is very simple in using gravity to remove rain. However, this system can become overwhelmed when large amounts fall. One answer is to use negative pressure, where the drain effectively sucks water down rather than rely on it to just fall. Siphonic drainage consists of baffle plates within the outlet that prevents air entering, thus creating a negative pressure during high rainfall which pulls more water through the system away from the building.

Siphonic drainage has some great advantages over older gravity systems, including:

• The higher water removal rate reduces systems being overwhelmed, impacting the building
• Smaller diameter pipework, or fewer downpipes, can be installed, along with a reduction in underground pipework too
• Positioning of pipework is more flexible
• Fast-running water helps keep systems clear of blockages

BS 8490 was created to help ensure siphonic drainage in circular pipework performs effectively. It is used in conjunction with BS EN 12056-3, which deals with layout and calculation of roof drainage. BS 8490 handles design parameters, installation and maintenance, and is used for inspection of systems by specifiers and regulatory authorities. The standard BS8490:2007 was superseded by BS 8490:2025 in February.

Exempted Roof & Drainage Systems

The new standard does not apply to flat sealed roofs such as membrane/felt designs, or to outboard eave gutters designs.

Important Points in BS 8490:2025

Within the new standard you can find the following  important updates:

Mandatory Secondary Systems for Internal Gutters Designs

In BS8490:2007 the standard advised that complex drainage systems handling high volumes should have a secondary system to step in for high volumes. This has now changed, with section  8.9.2 making it compulsory for buildings with internal gutters to have a secondary siphonic system, regardless of the capacity of the main system. This means that buildings with internal gutters relying solely on a primary siphonic system, regardless of flow capacity, are now considered non-compliant under BS 8490:2025.

New Maintenance Stipulations

Although the old standard did nominally cover maintenance, B 8490:2025 has a great emphasis in this area. Regular inspection and maintenance of siphonic systems now specifically states key areas such as checking bracketry, fixings, and overall integrity of the system.

BS 8490 also recommends building owners include siphonic drainage in their preventative maintenance schedules. This places drainage within the same maintenance approach as other systems such as fire prevention and electrical testing. Failure to include siphonic drainage in scheduled maintenance could affect not only system performance but also compliance with regulatory standards, potentially impacting insurance claims in the event of water damage.

The Impact of BS 8490:2025 for Building Owners

First, if your building was constructed before February 2025, it is very likely your siphonic drainage system does not comply with BS 8490:2025, especially if your property uses internal gutters and just a primary system. This has several very important implications for your business:

• The building will be vulnerable to water ingress during high volumes of rainfall
• You run the risk of damage to the property, stock and asset, and business continuity issues
• Your insurance may be invalidated

If you are in a position where you need to replace your existing drainage with a symphonic system, along with conforming to building regulations you might also require planning permission. The threshold here will be if a new system will potentially have a negative impact on public health or the environment.

What You Must Do Now

If you are at all unsure if BS 8490 applies to any of your properties, arrange for an inspection. If it is found you do have siphonic drainage and it does not satisfy the new standard, you will need to add a secondary system. Crucially, you will need to introduce drainage to your Preventative Maintenance schedule to meet the new requirements.

Professional Support To Bring You Up To Standard

Siphonic drainage is a specialised trade and it is vital you work with the right people.  Store Maintenance has an extensive network of accredited drainage professionals who can inspect, retrofit and maintain your siphonic systems. This includes professional help in creating a drainage strategy, which will cover how your drainage system will be managed and maintained.

Complementing this is our own huge amount of experience in operating cost-effective Preventative Maintenance schedules. Together, Store Maintenance provides a complete service to ensure your siphonic systems will perform as intended to deal with the changes in weather and keep your buildings free of water ingress.

For further information or to book a review of your siphonic drainage systems contact us here.