The 110% Bunding Rule Explained — UK Secondary Containment Requirements
If you store oil, chemicals, or other hazardous liquids at your premises, you will almost certainly have encountered the term "110% bunding rule". It is one of the most cited — and most misunderstood — requirements in UK environmental compliance. This article explains exactly what the rule means, where it comes from in law, and how to apply it correctly to protect your business from enforcement action by the Environment Agency or HSE.
What Is Secondary Containment?
Secondary containment (also called bunding) is a physical barrier designed to contain a liquid spill if the primary container fails. The primary container might be a drum, an IBC (intermediate bulk container), a tank, or a bottle. The secondary containment is typically a bunded pallet, a drip tray, a bunded floor, or a purpose-built sump unit that surrounds it.
The principle is simple: if the primary container leaks or ruptures, the secondary containment catches the liquid before it can reach drains, watercourses, or the surrounding environment.
Where Does the 110% Rule Come From?
The 110% rule is derived from several pieces of UK legislation and guidance:
- The Control of Pollution (Oil Storage) Regulations 2001 — Applies to above-ground oil storage of 200 litres or more at industrial, commercial, and institutional sites in England. Requires secondary containment of at least 110% of the largest container, or 25% of total storage capacity (whichever is greater).
- Environment Agency Pollution Prevention Guidelines (PPG2) — Guidance on above-ground oil storage tanks and secondary containment design.
- COSHH Regulations 2002 — Requires adequate controls to prevent the release of hazardous substances, which includes containing potential spills at source.
- Environmental Permitting (England and Wales) Regulations 2016 — Operators must take all appropriate preventative measures to avoid pollution incidents.
How to Calculate 110% Bunding Capacity
The calculation is straightforward but must be applied correctly:
| Scenario | Calculation | Required Sump Capacity |
|---|---|---|
| Single 205-litre drum | 205 × 110% | 225.5 litres minimum |
| Four 205-litre drums (820 litres total) | 205 × 110% (largest single container) | 225.5 litres minimum |
| One 1,000-litre IBC | 1,000 × 110% | 1,100 litres minimum |
| Two 1,000-litre IBCs (2,000 litres total) | 1,000 × 110% (largest single container) | 1,100 litres minimum |
Note: The 110% applies to the largest single container, not the total combined volume. However, your assessment should also consider the realistic worst-case scenario — if all containers failed simultaneously, could the bund contain the total volume? Best practice is to size your bund accordingly.
What Counts as a Compliant Bund?
To be considered compliant, secondary containment must:
- Be constructed of a material impermeable to the stored substance
- Have no drain valves or openings (unless lockable and kept sealed)
- Be structurally sound and free from cracks or damage
- Be kept free of rainwater accumulation (which reduces effective capacity)
- Provide the required 110% volumetric capacity
Our range of spill pallets and bunded drum pallets are designed and tested to meet these requirements. For IBC storage, view our IBC bunds and containment units.
Common Compliance Mistakes
The most frequent errors we see during site assessments include: using standard pallets (not bunded) under drums, allowing the bund sump to fill with rainwater, placing multiple containers in a bund sized only for one, and using damaged or cracked bunding that no longer holds liquid. Each of these creates an immediate compliance breach and could result in a prosecution following a pollution incident.
Consequences of Non-Compliance
The Environment Agency takes pollution incidents seriously. A single oil or chemical spill into a watercourse can result in fines of tens of thousands of pounds, remediation costs, and reputational damage. Under the Environmental Permitting Regulations 2016, operators can be prosecuted even where no pollution actually occurs, if adequate preventative measures were not in place.
Need expert advice? Call 01744 520 110