A subcontractor interface register for principal contractors

Two subcontractors can each have a reasonable task plan and still create an unsafe combined operation. One trade may need a loading route while another erects a scaffold; a temporary power shutdown may affect lifting equipment; a design change may alter access for a crew that never saw the new drawing. An interface register captures where the plans meet, who decides the sequence and what must be communicated before work proceeds.

This is a practical coordination record for multi-contractor projects in Great Britain, not a replacement for the construction phase plan or each contractor's RAMS. The HSE's principal contractor guidance describes the duty to plan, manage, monitor and coordinate the construction phase. The existing how-to-write RAMS guide owns the method-statement question; this page addresses conflicts between methods and teams.

Identify an interface before it becomes an incident

Map shared locations, routes, services, equipment and times. Look ahead to the next sequence of work, not only trades already on site. An interface arises when one activity changes another party's access, controls, information or ability to work safely. Record the work package, area, expected dates, parties involved and the precise dependency. “Electrical and roofing conflict” is too broad; “roof access closed while power is isolated for the hoist” gives the team a decision it can make.

Ask subcontractors what they need from other trades and what their work will take away from them. A supervisor may know about a temporary exclusion zone that is absent from a high-level programme. Check the current drawings and construction phase plan where they govern the area. Do not assume that a meeting attendance list proves every worker has received the resulting instruction.

Give each interface a decision owner

Name the person responsible for coordinating the interface and the people authorised to agree changes for each trade. The principal contractor may coordinate the overall sequence, while a competent specialist decides technical controls for lifting, temporary works or electrical isolation. Do not let a generic “site team” owner conceal who can actually approve the next step. State what decision is needed, by when and what happens if it is not made.

An interface register should not silently convert a technical question into a programme choice. If the teams disagree about whether a working platform can support a revised activity, hold the affected work and obtain the appropriate design or engineering assessment. Keep that specialist decision linked to the register entry, rather than replacing it with a green coordination status.

Record the agreed sequence and controls

For each interface, state the order of work, access restrictions, shared controls, communication route and evidence required before release. The result may be a changed programme, revised RAMS, new drawing, permit or briefing. Reference the current version. If a control applies only for a short period, include its start, end and handback criteria so it does not remain assumed after the site changes.

The HSE's CDM guidance emphasises cooperation and coordination between parties. The register is a way to make that cooperation visible, but a record cannot replace supervisors checking what is actually happening. Walk the work area when a paper agreement depends on physical space or equipment that may have moved.

Brief the affected people and verify understanding

Send the decision to supervisors and crews whose work changes. Identify who was briefed, the document or drawing version, any questions raised and how a late-arriving worker will receive the instruction. Use a suitable site induction for new workers and ongoing briefings for changing conditions, consistent with HSE site rules and induction guidance. A signature is evidence of attendance, not proof that every worker understood the control; ask people to explain the relevant limit or demonstrate it at the workface.

Check subcontractors who are not in the immediate meeting but use the same route or service. A lift plan change may affect delivery drivers and workers on a lower level. If the decision changes the construction phase plan or another controlled document, assign the update explicitly. Avoid keeping a fresh interface decision in a private message while the site plan still shows the old arrangement.

Reopen the interface when conditions change

Set triggers for review: a different trade sequence, weather restriction, new plant, design revision, delayed delivery, changed shift or unexpected defect. Record the old assumption and the new fact. The person coordinating the interface should decide which prior controls remain valid and which people need a revised briefing. Preserve earlier decisions for audit, while making the current instruction obvious to the site team.

Do not close a register item only because both trades have finished their immediate tasks. Confirm that shared equipment and access were handed back as agreed. If a temporary barrier, lockout or permit remains in place, record its continuing owner. A closed entry that leaves an unowned restriction can create the next hazard.

Worked example: shared loading and facade work

Facade installers need a ground-level exclusion zone while materials are lifted. The mechanical contractor has a scheduled delivery through the same route. Both tasks were individually planned. The interface register identifies the shared route, time and parties, then asks the site logistics lead and lifting supervisor to agree the sequence. The delivery is moved to a defined window, the exclusion zone is shown on the current site plan, and the delivery driver is given the revised route before arrival.

At the start of the lift, the supervisor checks the physical barrier and confirms the delivery has not entered. If the delivery is delayed, the teams revisit the sequence rather than relying on yesterday's agreement. The record includes the decision, briefing and workface verification. It does not purport to approve the technical lifting plan.

For a current-product discussion, see Complys RAMS and ask whether the demonstrated configuration can connect an interface decision to the affected documents and human owners. This article does not claim automated CDM coordination or technical design approval.