Last updated: September 2026 

EN 1090 is the European standard for structural steel and aluminium. Execution Class 4, or EXC4, is its  strictest tier. It applies to structures where failure would be catastrophic, such as major bridges and nuclear  components. It demands the toughest welding, inspection, traceability and documentation controls. 

Get the execution class wrong and a steel project can unravel fast. The wrong class brings wasted cost, failed audits and delays. You need to know what each class means before you specify or buy. 

This guide explains EN 1090, all four execution classes, and what really needs EXC4. It is written for  specifiers, contractors and buyers in plain terms. 

What is EN 1090? 

BS EN 1090 is the standard that governs how structural steel and aluminium are fabricated and erected in  the UK. It sets the quality, welding and inspection rules a fabricator must meet. Meeting those rules lets a  fabricator place structural components on the market legally. 

The standard covers load-bearing steel and aluminium parts built into permanent structures. Compliance has been required for structural steelwork across Europe since 2014. 

EN 1090 works alongside the Eurocodes, which cover structural design. Design rules sit in Eurocode 3 (EN  1993). Fabrication rules sit in EN 1090. 

What are the three parts of EN 1090? 

Three parts make up EN 1090. EN 1090-1 covers conformity assessment and CE or UKCA marking, including  Factory Production Control. EN 1090-2 sets the technical rules for steel and defines the execution classes. EN 1090-3 does the same for aluminium. Execution Class 4 sits in Part 2. 

Factory Production Control (FPC) is a fabricator’s documented system for keeping quality consistent. An  approved body audits that system before a fabricator can mark its work. 

CE marking shows a product meets European rules. UKCA marking is the UK equivalent, introduced after  Brexit. Both marks rely on the same EN 1090 requirements. 

What is an execution class? 

An execution class is a grade in EN 1090-2 that sets how strict a structure’s fabrication and inspection must  be. It reflects the consequences of failure. There are four classes, EXC1 to EXC4, from lightest to strictest.  The rules are cumulative, so each class includes everything below it. 

The correct abbreviation is EXC, so the classes are EXC1, EXC2, EXC3 and EXC4. A designer chooses the class  during design, not on the workshop floor. 

Higher classes mean more welding controls, tighter welder qualification and more testing. They also mean  more paperwork. Non-destructive testing (NDT) checks welds without damaging them, and its extent rises  with the class.

What is the difference between EXC1, EXC2, EXC3 and EXC4? 

EXC1 is the lightest class, for low-risk structures. EXC2 is the everyday default for ordinary buildings. EXC3  covers higher-risk or fatigue-loaded structures like bridges and stadiums. EXC4 is the strictest, for structures  where failure would be catastrophic. Controls on welding, testing and records rise at each step. 

The table below sums up how the four classes compare. 

Source: BS EN 1090-2 and guidance from the British Constructional Steelwork Association (BCSA). 

What is Execution Class 4, and why is it the strictest? 

Execution Class 4, or EXC4, is the highest class in EN 1090-2. It applies where a failure would have extreme  consequences for people or the environment. It demands the toughest welding and welder qualification, the widest testing, comprehensive ISO 3834-2 welding-quality management, and the fullest traceability and  records of any class. 

Moving from EXC3 to EXC4 raises the bar again. More welds get non-destructive testing. Inspection records  must be fuller and better controlled. 

Documentation for public infrastructure is often kept for at least ten years. Independent oversight of the  fabricator also increases at EXC4. 

What structures actually need Execution Class 4? 

EXC4 is rare in practice. It usually applies to road and rail bridges over crowded areas or high-hazard plants.  It also covers safety-critical nuclear components and structures facing extreme earthquake loads. Most  everyday steel is Execution Class 2, so EXC4 is reserved for the highest-risk work. 

Examples of EXC4 work include: 

• Road or rail bridges over dense population or hazardous plants 

• Safety-critical components in nuclear power stations 

• Major long-span bridges 

• Structures designed for extreme seismic loads 

A designer can also apply EXC4 to specific parts of a structure. The rest of the frame can sit at a lower class.  This keeps cost proportionate to risk. 

At our Southampton workshop, EXC4 certification shapes daily practice, from how each weld is planned to  the records we keep. We deliver marine and industrial steelwork from that certified facility. 

According to [full name], Responsible Welding Coordinator at EAP Ltd, EXC4 work is exacting. Speaking in  September 2026, they put it plainly: “Every weld is planned, checked and logged before the next run starts.” 

Who decides the execution class, and how? 

Designers set the execution class, not the fabricator or the client. Under EN 1090-2 the class comes from  three inputs: the consequence class, the service category for loading, and the production category. If nobody specifies a class, EXC2 applies by default.

The consequence class rates what happens if the structure fails, from CC1 to CC3. The service category rates  the loading, either mostly static or fatigue and seismic. A production category rates how the parts are made. 

Together these inputs point to the right class. Buyers should always confirm a supplier holds the class the  designer specified. 

How does the execution class link to ISO 3834? 

Each execution class maps to a level of ISO 3834 welding-quality management. EXC1 maps to the elementary level, ISO 3834-4. EXC2 maps to the standard level, ISO 3834-3. EXC3 and EXC4 both map to the  comprehensive level, ISO 3834-2. EXC2 and above also need a welding coordinator. 

ISO 3834 is the standard for quality in fusion welding. It sets three tiers, from elementary to comprehensive. 

Many fabricators certify to the comprehensive tier, ISO 3834-2, even when their current work does not  require it. That tier is the one EXC3 and EXC4 demand. 

Do you still need CE or UKCA marking in the UK? 

In Great Britain, structural steel must carry CE or UKCA marking under BS EN 1090-1. The plan to make UKCA  compulsory from mid-2025 was dropped. As of 2026, the CE mark stays accepted in Great Britain, and firms  can use either mark. Northern Ireland uses CE, or CE with UKNI. 

The government confirmed this in a written statement on 2 September 2024. As a result, CE marking  continues to be available in Great Britain with no set end date. 

You should still check the current rules before you specify. Reform of the wider regime is under way. 

Is EC4 the same as Eurocode 4? 

No. People sometimes write EC4 for Execution Class 4, but EC4 officially means Eurocode 4. Eurocode 4, or  EN 1994, is the standard for designing composite steel and concrete structures. Execution Class 4 is a  fabrication-quality tier in EN 1090-2. Use EXC4 for the execution class to stay clear. 

These two standards do different jobs. Eurocode 4 guides design. EN 1090-2 governs fabrication and its  execution classes. 

How do you check a fabricator holds the right execution class? 

Ask the fabricator for three documents. First, their Factory Production Control certificate from an approved  body. Second, their welding certificate stating the execution class achieved. Third, their Declaration of  Performance. Check the certified class matches or beats the class your designer specified. 

1. Factory Production Control (FPC) certificate from an approved body 

2. Welding certificate stating the execution class achieved 

3. Declaration of Performance (DoP) for the components supplied 

A workshop certified only to EXC2 cannot legally supply EXC4 work. The British Constructional Steelwork  Association explains the verification process for structural steelwork in detail. 

The takeaway 

EN 1090 sets the quality rules for structural steel, and the execution class decides how strict those rules are.  EXC4 is the rare, safety-critical top tier, while EXC2 covers most everyday work. Choosing the right class  protects safety, cost and programme. 

EAP’s Southampton workshop is certified to Execution Class 4 under BS EN 1090-2:2018. If your project calls  for EXC3 or EXC4 steelwork, contact our team to plan it properly. 

Not sure which execution class your project needs? Tell us the structure and we will point you to the right  class.