CDM 2015 for
Mechanical Engineers —
What You Actually Need to Know

The Construction (Design and Management) Regulations 2015 apply to a much broader range of engineering work than most mechanical engineers realise. If you design mechanical systems that will be installed, maintained, or decommissioned on a construction site — which includes most process plant, industrial building services, and installed equipment — CDM 2015 creates specific legal duties that fall on you as a designer. Ignorance of those duties is not a defence.

This article covers the structure of CDM 2015, the duty holder roles, what the designer's duties specifically require, the distinction between notifiable and non-notifiable projects, and the practical documentation that CDM requires. It is aimed at practising mechanical design engineers, not health and safety specialists.

What CDM 2015 Is and Why It Exists

The Construction (Design and Management) Regulations 2015 are the UK's primary regulatory framework for managing health and safety in construction projects. They implement EU Directive 92/57/EEC (the Temporary or Mobile Construction Sites Directive) and replace earlier CDM regulations from 1994 and 2007.

The fundamental premise of CDM 2015 is that the decisions made during the design phase of a project have the greatest influence on the health and safety risks that workers, users and maintainers will face throughout the structure's or system's lifetime. A designer who specifies heavy components that require two-person lifts, locates valves in confined and inaccessible positions, or designs pipework systems with no provision for in-service inspection, has created risks that downstream health and safety management will struggle to control. CDM places the duty to address these risks at the design stage, where they can be eliminated or reduced at the lowest cost.

What Counts as Construction Work

The definition of "construction work" under CDM 2015 is broader than it appears and catches much mechanical engineering work that engineers do not typically think of as construction:

"Construction work means the carrying out of any building, civil engineering or engineering construction work and includes: the construction, alteration, conversion, fitting out, commissioning, renovation, repair, upkeep, redecoration or other maintenance... and decommissioning, demolition or dismantling..."

The critical phrase is "engineering construction work." Under CDM, this includes:

• Installation of process plant, vessels, reactors, tanks
• Installation of mechanical services (HVAC, pipework, pumping systems)
• Construction of offshore oil and gas structures
• Erection of steelwork, structures and supporting frames
• Installation of mechanical handling equipment
• Maintenance and modification of existing plant and equipment

CDM does not apply to purely manufacturing or supply activities — designing a pump in a workshop for supply to a client is not itself construction work. But once that pump is being specified as part of an installation project, CDM applies to the installation project and the designer providing the installation specification has designer duties under CDM.

The Duty Holder Roles

CDM 2015 identifies five duty holder roles. A single organisation can hold more than one role on a project.

Client

The organisation or individual for whom the project is carried out. The client has overarching duties to make suitable arrangements for managing the project, including appointing the Principal Designer and Principal Contractor, ensuring pre-construction information is provided, and ensuring the project is notified to the HSE where required. A domestic client (individual having work done on their home) has significantly reduced duties compared to a commercial client.

Principal Designer (PD)

The designer appointed by the client to plan, manage, monitor and coordinate health and safety during the pre-construction phase — from concept design through to the point when the Principal Contractor takes over. The PD must be a designer (not a project manager without design competence) and must have control over the pre-construction phase. The PD role is mandatory on all notifiable projects, where more than one contractor will be working.

The Principal Designer role is frequently misunderstood — it does not mean the lead design organisation does all the design. It means that one organisation has the coordination responsibility for health and safety across all designers on the project during the pre-construction phase. A mechanical engineer or engineering consultancy can be appointed Principal Designer if they have the appropriate skills and experience.

Designer

Any person who, in the course of a business, prepares or modifies a design for a structure — including drawings, specifications, calculations, or the specification of articles and substances. This definition explicitly includes mechanical and structural engineers, architects, building services engineers, and specialist equipment designers. If you produce drawings or specifications that will be used in construction work, you are a designer under CDM and you have designer duties regardless of your title or your firm's size.

Principal Contractor (PC)

The contractor appointed by the client to plan, manage, monitor and coordinate health and safety during the construction phase. The PC takes over coordination from the PD when construction begins. On projects where only one contractor is involved, that contractor acts as the contractor (not the Principal Contractor).

Contractor

Any individual or organisation who carries out, manages or controls construction work. Subcontractors, specialist installers, and self-employed tradespeople carrying out construction work are contractors under CDM.

The Designer's Duties — The Core Obligation

Regulation 9 of CDM 2015 sets out the designer's duties. These apply to every designer, on every project, regardless of size or whether the project is notifiable. They are not optional and they cannot be delegated to the client or the Principal Designer:

1. Not to commence work unless satisfied the client is aware of their duties

Before beginning design work, the designer must take reasonable steps to ensure the client is aware of their CDM duties. In practice this means raising CDM in the initial project discussions and confirming it in writing — not assuming someone else has covered it.

2. Eliminate foreseeable risks during design

The primary duty — and the one most directly relevant to mechanical engineering practice. The designer must, so far as is reasonably practicable, eliminate risks to the health and safety of persons:

• Carrying out or liable to be affected by construction work
• Maintaining or cleaning a structure
• Using a structure designed as a workplace

This is not a duty to eliminate all risk — it is a duty to eliminate foreseeable risks so far as is reasonably practicable. The test of reasonable practicability requires weighing the cost and difficulty of eliminating the risk against the magnitude and likelihood of the harm. High-severity risks with a high likelihood of occurring must be eliminated even at significant cost; low-severity risks with a low likelihood may be acceptable to leave with appropriate information.

3. Reduce risks that cannot be eliminated

Where a foreseeable risk cannot be eliminated, the designer must reduce it so far as is reasonably practicable — through design choices that make the hazardous activity safer, less frequent, or more controlled. This is the substitution and engineering controls tier of the hierarchy.

4. Provide information about remaining risks

Where risks remain after elimination and reduction efforts, the designer must communicate those residual risks to others who need that information — through drawings, specifications, residual risk registers, and the health and safety file. Providing information is the last resort, not the first response to a risk.

The Design Risk Management Hierarchy — ERIC

The hierarchy for managing risk through design is commonly expressed as ERIC:

Information and control are not substitutes for elimination and reduction. The CDM guidance is explicit: designers who jump to providing information about hazards without first attempting to eliminate or reduce them are not fulfilling their duty.

Notifiable Projects

CDM 2015 applies to all construction projects regardless of size. However, additional requirements — principally the mandatory appointment of a Principal Designer and Principal Contractor, and formal HSE notification — apply to notifiable projects:

A project is notifiable if the construction phase is expected to:

• Last longer than 30 working days and have more than 20 workers simultaneously at any point, OR
• Exceed 500 person-days of construction work in total

For notifiable projects, the client must notify the HSE using the F10 notification form before construction work begins. The Principal Designer and Principal Contractor must be formally appointed in writing. The construction phase plan and health and safety file are mandatory deliverables. On non-notifiable projects, these documents are still good practice but not a formal regulatory requirement in the same way.

The Key Documents

Pre-Construction Information

The client must provide all designers and contractors with pre-construction information — existing information about the site and structure that is relevant to health and safety planning. For mechanical engineering projects on existing sites this typically includes: existing services drawings (underground and overhead), confined space identification, hazardous area classification drawings, previous asbestos surveys, site emergency plans, and any known structural constraints on the installation.

The designer uses this information to design safely — not designing penetrations through structural members, not routing pipe through identified confined spaces, avoiding hazardous areas where possible. If pre-construction information is inadequate, the designer should request it from the client rather than designing on assumptions that may prove incorrect and dangerous.

The Construction Phase Plan

Produced by the Principal Contractor (or contractor on non-notifiable projects), the Construction Phase Plan describes how the construction work will be managed safely. As a designer, you are not responsible for producing this document — but you may be asked to provide design-stage information to inform it, such as the sequence in which components must be installed, or access requirements for specific operations.

The Health and Safety File

The Health and Safety File is the most directly relevant CDM document for the mechanical design engineer. It is a record of information that will be needed by anyone responsible for the structure in the future — those who carry out cleaning, maintenance, alteration, refurbishment, repair, or demolition. The Principal Designer is responsible for ensuring it is prepared and handed to the client at project completion.

For mechanical systems, the H&S file typically contains:

• As-built drawings of the installed system
• Equipment schedules and datasheets
• Material specifications and mill certificates for pressure-containing components
• Maintenance requirements — intervals, access requirements, specific tools or procedures
• Residual risks that were identified during design but could not be eliminated — confined spaces, weight of components requiring lifting equipment, substances requiring specific handling procedures
• Decommissioning information — how the system is to be safely isolated, drained and removed
• Inspection records and test certificates

The H&S file is a living document — it should be updated whenever the structure is modified. A mechanical engineer performing a design modification to an existing system should ensure the file is updated as part of the modification scope, not left to reflect the original installation only.

Design Risk Register

The design risk register is the working tool through which designers document their application of the CDM hierarchy. It is not specifically mandated by CDM 2015, but it is standard practice and a practical demonstration of how the designer has fulfilled their duties. A design risk register typically records:

• The hazard identified during design
• Who is at risk and when (during construction, during maintenance, during operation)
• The design action taken to eliminate or reduce the risk
• The residual risk after design action
• Any information required by others to manage the residual risk
• Who the information has been provided to

The risk register should be started at concept design and updated iteratively as the design develops. Risks identified early are cheapest to eliminate; risks identified at detailed design stage may still be reducible; risks identified after construction has begun can usually only be controlled, at much greater cost.

Practical Implications for Mechanical Design Engineers

The following design decisions are directly influenced by CDM duties — they are not optional health and safety additions, they are part of the design scope:

Maintenance access

Every component that requires periodic inspection, maintenance, or replacement must be accessible. Accessible means: reachable without undue risk, with sufficient working space, by the appropriate equipment. A pump impeller that can only be accessed by removing the entire pump skid, or a heat exchanger bundle that can only be pulled with a crane that cannot reach the installed location, is a foreseeable maintenance risk that the designer must address. Design the maintenance access first; fit the equipment around it.

Manual handling

Equipment and components above approximately 25kg that must be handled manually during installation or maintenance represent a foreseeable risk. The designer should specify lifting points, consider the use of lighter materials or smaller modular assemblies, and where handling is unavoidable, provide information about lifting requirements in the design documentation.

Working at height

Equipment located above ground level generates working at height risk for installation and maintenance. Where equipment can be located at ground level, it should be. Where elevation is necessary, the design should provide permanent safe access — platforms, guardrails, fixed ladders — rather than assuming temporary access equipment will be available and safe. Specifying "access by scaffold" in a drawing note is not an adequate response to a foreseeable working at height risk that could have been designed out.

Confined spaces

Vessels, tanks, ducts, and enclosed structures may constitute confined spaces under the Confined Spaces Regulations 1997. The designer should identify where confined space entry will be required for inspection, cleaning or maintenance, note this in the H&S file, and where possible design the confined space entry requirement out — through self-cleaning geometry, external inspection ports, or access arrangements that avoid the need for entry.

Hazardous substances

Process fluids, cleaning chemicals, insulation materials (particularly legacy asbestos, but also ceramic fibre and MMMF which are also hazardous), coatings, and welding fumes all represent foreseeable risks. Where the designer specifies a hazardous material, the duty to inform requires that the hazard is communicated in the design documentation and H&S file.

Common Misunderstandings

1. "CDM only applies to buildings." It applies to all construction work, which includes mechanical engineering construction. Process plant installation, mechanical services, and structural steelwork are all within scope.
2. "We're not the Principal Designer so CDM doesn't apply to us." Every designer has designer duties under Regulation 9 regardless of whether they hold the PD role. The PD coordinates — but all designers must apply the design hierarchy to their own work.
3. "We put a note on the drawing so we've discharged our duty." Informing others about a risk is the last resort, not the first response. The duty hierarchy requires elimination first, then reduction, then information. A drawing note saying "this valve requires a two-person lift" when it could have been specified 30% lighter is not adequate fulfilment of the duty.
4. "CDM doesn't apply to us — we're a small practice." There is no size threshold for designer duties. The complexity of the duty (and particularly the PD role) scales with project size, but every designer on every construction project has duties.
5. "The contractor will sort out health and safety on site." The contractor manages health and safety during construction — they cannot design out risks that were created in the design phase. Once the design is issued for construction, opportunities to eliminate hazards through design are largely closed.

Summary

CDM 2015 creates legally enforceable duties for mechanical design engineers on construction projects. Those duties require — in priority order — eliminating foreseeable risks through design decisions, reducing risks that cannot be eliminated, and communicating residual risks to those who need to manage them. The duties apply to every designer on every construction project, regardless of project size, firm size, or the designer's title.

In practical terms, this means that maintenance access, manual handling, working at height, confined spaces and hazardous substances must be considered as design constraints, not site management problems. A mechanical engineer who designs plant without considering how it will be maintained, modified and eventually decommissioned is not meeting their CDM duty — and if something goes wrong as a result, the regulatory and legal exposure is real.

The good news is that CDM-compliant design is largely the same as good engineering practice. Accessible equipment is easier to maintain. Lighter components are easier to handle. Ground-level equipment is easier to install. Designing for maintenance from the outset produces better engineering outcomes as well as meeting the regulatory requirement.