EN 1090 is the European standard that governs how structural steel and aluminium components are manufactured, marked and declared. It assigns every structural component to one of four execution classes (EXC-1 to EXC-4), which define the level of rigour required in detailing, welding, inspection and documentation. The execution class is not chosen by building type. It is derived from the structural design, following a defined procedure set out in EN 1990 and EN 1993-1-1.
What EN 1090 actually is
EN 1090-1 is the product standard. It requires manufacturers of structural components to operate a Factory Production Control system, declare performance, and CE-mark the output. EN 1090-2 is the execution standard for steel; EN 1090-3 covers aluminium. Together they replace the earlier national codes (BS 5950, DIN 18800 and others) with a single European rulebook. Without a valid EN 1090 certification, a structural-steel manufacturer cannot legally place product on the EU market.
That is the regulatory view. The engineering view is that EN 1090 formalises the level of rigour applied to a piece of structural steel. It puts a measurable grade on fabrication craft, tied to the consequences and loading conditions the structure is designed for.
The four execution classes
- EXC-1: lowest requirements. Minimal welding procedure control, no impact toughness testing, visual inspection only. Typical for agricultural and non-critical secondary structures.
- EXC-2: documented welding procedures (WPS/PQR), partial non-destructive testing, basic dimensional tolerance control. Typical for the majority of normal steel structures.
- EXC-3: full welding-procedure qualification, impact toughness testing of parent steel, non-destructive testing on all full-penetration welds, tighter dimensional tolerances, full traceability from mill to member. Typical for structures where fatigue, dynamic loading or consequence of failure is significant.
- EXC-4: EXC-3 requirements plus witness inspection and third-party hold points. Reserved for structures where failure would have extreme consequences (nuclear, offshore primary structure, certain defence-critical assets).
Each step up adds testing scope, documentation depth and inspection intensity. The increment from EXC-2 to EXC-3 is the largest, because it introduces 100% NDT of full-penetration welds, impact testing of the parent material, and an annually re-qualified welder regime. It is also the point at which structural reliability becomes fully measurable rather than inferred.
How execution class is determined
EXC is not assigned by calling a structure "modular", "residential" or "military". It is derived from three parameters defined in EN 1990 and EN 1993-1-1 Annex A (and referenced by EN 1090-2 Annex B):
- Consequence Class (CC1 / CC2 / CC3): the risk to human life and the economic, social or environmental consequences if the structure were to fail. CC1 covers low-consequence structures, CC3 covers structures where failure would lead to high loss of life or major consequences.
- Service Category (SC1 / SC2): the nature of the actions on the structure. SC1 covers predominantly quasi-static loading. SC2 covers structures subjected to fatigue actions, seismic actions in ductile zones, or other significant dynamic effects.
- Production Category (PC1 / PC2): the complexity of fabrication. PC1 covers welded non-alloy steels up to S355 with no special fabrication demands. PC2 covers welded high-strength steels (from S355 upwards in certain conditions, and above S460), hot-formed components, and fabrications with complex welding or post-weld treatment.
The recommended execution class is then read from a matrix that combines these three inputs. CC3 combined with SC2 and PC2 drives EXC-4. CC1 with SC1 and PC1 drives EXC-1. Most real-world structures fall somewhere in between, and the assignment has to be made explicit by the structural designer in the project specification.
The project specifier, meaning the structural designer or the client's consulting engineer, is the party responsible for fixing CC, SC and PC, and therefore EXC, in the contract documents. The fabricator executes to the EXC stated in the specification, audits to it, and declares it on the CE-mark Declaration of Performance.
Three practical examples
The same fabrication technique, applied to the same type of modular unit, can land in different execution classes once the design parameters are fixed. Three illustrative cases:
- A single-storey modular office for a construction site. Normal occupancy, quasi-static loading in service, S235 or S355 steel, straightforward welded details. Typical assignment: CC1, SC1, PC1, resulting in EXC-1 or EXC-2 depending on span, occupancy and site seismic requirements.
- A mobile hospital module used in disaster response. Higher occupancy, essential service in aftermath scenarios, repeated relocation, crane lifts as a design load case, potential seismic exposure at deployment sites. Typical assignment: CC2 or CC3 (loss of this facility in an ongoing disaster increases the consequence), SC2 (repeated lift and transport cycles generate fatigue actions), PC1 or PC2 depending on steel grade. This commonly drives EXC-3.
- A heavy-duty modular command or weapons-storage unit. High consequence of failure, possible blast-resistant detailing, high-strength steels (S460 or above), tight geometric tolerances for integration with hardened systems. Typical assignment: CC3, SC2, PC2, which drives EXC-3 and, in some defence frameworks, EXC-4.
None of these outcomes follows from the word "modular" alone. Each follows from the combination of consequence, loading and fabrication complexity stated by the designer of record.
What this means for modular buildings
Modular buildings share some attributes that tend to push Service Category towards SC2: they are lifted by crane during production, during shipping and during installation; they may travel thousands of kilometres by road or sea; and over a service life of fifteen to thirty years they may be relocated and reconfigured multiple times. Each lift and transport cycle is a cyclic action on the chassis, corner posts and lifting lugs.
This does not automatically place every modular unit in EXC-3. It means that, for the elements that actually carry the transport and lifting loads, the fatigue assessment under EN 1993-1-9 has to be done explicitly, and the resulting Service Category has to be reflected in the execution class the designer specifies. For a light, single-storey office unit moved once in its life, EXC-2 with documented lifting procedures may be the correct outcome. For a multi-storey hospital that will be lifted and stacked repeatedly, EXC-3 for the primary framing, chassis, stacking corners and lifting lugs is a common and defensible outcome.
The controlling principle is simple: the execution class is an output of the structural design process, not an input to it, and certainly not a marketing attribute of the building type.
The welding requirements under the hood
Once an execution class is fixed, it drives the welding quality management system. EXC-2 and below typically require compliance with EN ISO 3834-3 (standard welding quality requirements). EXC-3 and EXC-4 require compliance with EN ISO 3834-2 (comprehensive welding quality requirements).
Under EN ISO 3834-2, every welder on the job is qualified to a specific Welding Procedure Specification (WPS) backed by a Procedure Qualification Record (PQR) that tested that procedure on a coupon plate. Welders are re-tested annually. A Responsible Welding Coordinator (IWE, IWT or equivalent) signs off every procedure. The welding log is traceable from each weld back to the WPS, the welder, the consumables and the parent-steel mill certificate.
This is not paperwork for its own sake. When a welded joint fails in service, the first thing a failure investigation needs is the traceability chain. Without it, the failure cannot be diagnosed and the affected fleet cannot be cleared for continued service.
Reading a Declaration of Performance
Every CE-marked structural component ships with a Declaration of Performance (DoP). On the DoP you will find the execution class (EXC-1, EXC-2, EXC-3 or EXC-4), the steel grade (for example S355J2+N), the weld quality level (typically B or C per EN ISO 5817), and the notified body number.
When specifying modular buildings, require your structural designer to state the intended execution class in the technical specification, and require the fabricator to declare that execution class on the DoP. Those two documents (specification in, DoP out) are the only defensible record that EN 1090 was applied correctly on the project.
Disclaimer. The execution class for any given project is defined by the structural designer of record based on the structure's Consequence Class, Service Category and Production Category, together with applicable national and project requirements. Figures and examples above are illustrative and do not substitute for project-specific structural design.
Bir mühendisle konuşun→If you are scoping a project and need the execution class set correctly from the start, we will work with your structural designer (or provide ours) to document CC, SC, PC and the resulting EXC before any steel is cut.