Execution classes EXC1 to EXC4 are the four levels of stringency defined in BS EN 1090-2, the standard that governs how structural steel is fabricated and erected in the UK. EXC1 is the least demanding and EXC4 the most, and the class assigned to your project decides how tightly a fabricator’s welding, inspection, tolerances and paperwork are controlled. If you have seen “EXC2” or “EXC3” written on a specification and want to know what it actually changes on the shop floor, this guide sets it out in plain terms.
The classes sit inside BS EN 1090, the wider standard that also covers Factory Production Control and declaring components as construction products. Where BS EN 1090-1 is about declaring the product, BS EN 1090-2 and its execution classes are about how the work is actually carried out. T C Rowan is CE approved and works to the BS EN 1090 execution standards; the detail below is general guidance, not a substitute for your structural engineer.
What an execution class actually controls
The execution class is not a label stuck on the drawing for show. It sets concrete requirements at every stage of fabrication:
- Welding procedure qualification, the tests that prove a given weld procedure produces sound welds before it is used on the job
- Welder qualification, proof that the individual welder is coded and competent for the process and joint type
- Welding coordination, run under BS EN ISO 3834 quality requirements and overseen by a welding coordinator qualified to BS EN ISO 14731
- Inspection and testing, including the level of non-destructive testing (NDT) applied to welds
- Dimensional tolerances, how tightly the finished steelwork must match the drawing
- Material traceability, with certificates to BS EN 10204 3.1 tying each component back to its cast and test results
- Documentation, the paper trail an auditor or building control officer can ask to see
A higher class does not mean “better welding” in some vague sense. It means a stricter, more documented version of the same controls, with more testing and less tolerance for drift. For the material-certificate side of this, our guide on S355JR, S355J0 and S355J2 covers what a 3.1 certificate actually shows.
Who decides the class, and how
The execution class is not something a fabricator picks to suit itself. It is assigned by the specifying structural engineer, combining three factors:
- Consequence class (from BS EN 1990): CC1 low consequence of failure, CC2 medium, CC3 high
- Service category: SC1 for static or quasi-static loading, SC2 for fatigue or dynamic loading, such as crane runways, bridges or seismic design
- Production category: PC1 for non-welded components or welded components below grade S355, PC2 for welded S355 and above, or certain site welding and thermal treatment
Combine those three and you get the execution class. If the designer has not stated one, BS EN 1090-2 sets a default: EXC2 applies. That is worth knowing if you are reading a specification that is silent on the point, because “not mentioned” does not mean “not controlled”. This assignment sits alongside the wider CE marking and UKCA picture, since both trace back to the same underlying standard.
Execution classes EXC1 to EXC4 at a glance
| Class | Typical structures | What it demands of the fabricator |
|---|---|---|
| EXC1 | Low-consequence structures, e.g. simple agricultural buildings rarely entered by people, basic secondary steelwork | Lightest level of welding qualification, inspection and documentation |
| EXC2 | Most UK buildings: houses, offices, industrial units, mezzanines, portal frames. The UK default when no class is specified | Qualified welders and procedures, routine inspection, traceable material, standard documentation |
| EXC3 | High-consequence or dynamically loaded structures, e.g. bridges, crane-supporting structures, stadia, tall buildings (selection guided by PD 6705-2) | Tighter welding qualification, increased NDT, closer tolerances, fuller documentation |
| EXC4 | Structures where failure has extreme consequences, e.g. major bridges, nuclear or other safety-critical facilities | The most demanding welding, inspection and traceability regime in the standard |
EXC2 vs EXC3: why the gap matters
EXC2 is where the bulk of structural steelwork in the UK sits, portal frames, office fit-outs, mezzanine floors and light industrial units among them. EXC3 is a different order of control, reserved for structures where a failure would be far more serious or where the loading is dynamic rather than static. The step from EXC2 to EXC3 typically brings more welding procedure testing, a higher level of non-destructive testing on completed welds, tighter dimensional tolerances and a fuller documentation trail for every stage of fabrication. That extra control is not optional paperwork; it exists because the consequence of getting a crane-supporting beam or a bridge girder wrong is much higher than getting a domestic portal frame wrong.
Why the right class matters, both ways
Getting the execution class right cuts both ways. Specifying EXC3 for a simple agricultural shed adds cost and time for control the structure does not need. Specifying EXC1 for a crane beam or anything carrying dynamic or fatigue loading is unsafe, because the welding and inspection regime will not have caught the faults that matter for that kind of loading. The designer’s specification is the one that governs, and a genuine fabricator works to whatever class is stated rather than defaulting to whichever is cheapest to deliver.
How to check a fabricator’s execution class
A fabricator’s BS EN 1090-1 Factory Production Control certificate, issued by a notified or approved body, states the highest execution class that fabricator is certified to work to. A fabricator may only CE or UKCA mark steelwork up to that class, no higher. So the practical check is straightforward: ask any fabricator for their FPC certificate and check the execution class printed on it against what your engineer has specified for the job. For more on vetting a fabricator generally, see our guide on how to choose a steel fabricator. Wider background on execution and conformity marking sits with the Steel Construction Institute.
T C Rowan is CE approved and works to the BS EN 1090 execution standards, with in-house design, its own Banbury workshop and its own erection team taking a job from drawing to final bolt. If you want the execution class question answered against your actual drawings, get in touch for a free quote.
Frequently asked questions
What are execution classes EXC1 to EXC4?
Execution classes are four levels of stringency defined in BS EN 1090-2, the execution standard behind UK structural steelwork. EXC1 is the least demanding and EXC4 the most, and the class sets how tightly welding qualification, inspection, tolerances and material traceability are controlled on a given project. Most everyday UK buildings sit at EXC2, which also applies by default if a designer has not specified a class.
Who decides the execution class for my project?
The specifying structural engineer or designer sets the execution class, not the fabricator. It is worked out from the consequence class (how serious failure would be, from BS EN 1990), the service category (static loading against fatigue or dynamic loading) and the production category (how the steel is welded and what grade it is). If a designer does not state a class, BS EN 1090-2 says EXC2 applies by default.
What is the difference between EXC2 and EXC3?
EXC2 covers most everyday UK buildings: houses, offices, industrial units, mezzanines and portal frames, and is the default class when none is specified. EXC3 applies to structures with a higher consequence of failure or dynamic loading, such as bridges, crane-supporting structures, stadia and tall buildings, and demands a tighter level of welding qualification, non-destructive testing and documentation than EXC2. UK guidance in PD 6705-2 sets out how to select between them.
How do I check which execution class a fabricator can certify to?
Ask for the fabricator’s BS EN 1090-1 Factory Production Control certificate, issued by a notified or approved body. It states the highest execution class the fabricator is certified to work to, and a fabricator may only CE or UKCA mark steelwork up to that class. Ask for the certificate and check the class printed on it against what your engineer has specified for the job.