1. What is a technical interface?
A technical interface is the zone where two trades, two systems or two parties connect. Ideally every interface is documented: a detail drawing sets out who does what, up to which point, with which fixings and which tolerances. In reality interfaces are often implicit, each trade assumes the other is dealing with them, and nobody does.
The fragmentation of French construction into separate trade packages, structure, façades, roofing, mechanical, electrical, partitions, finishes, is a structural source of interface risk. Each package goes to a different contractor with its own contract, its own drawings and its own programme. Coordinating between them is nominally the lead architect's responsibility, but the tools for it (coordinated drawings, coordination meetings, interface schedules) are rarely used with the rigour required.
ℹ The 3 % rule for untreated interfaces On a standard 2 M€ project with 12 trade packages, untreated interfaces statistically generate 3 to 6 % of variations for remedial work, corrections or coordination attributable to contractual grey areas. On 2 M€ that is 60 000 to 120 000 € of extra cost that coordinated drawings produced at detailed design stage would have avoided. This figure is not an unforeseeable hazard, it is the predictable result of a systemic coordination failure. Producing a coordinated services drawing and an interface schedule costs 3 000 to 8 000 € of economist and architect time. The return is ten to twenty times that.
2. The eight interfaces that generate the most extra cost
| Critical interface | Packages involved | Typical failure | Extra cost if mishandled | How to secure it |
|---|---|---|---|---|
| Façade / roof weatherproofing | Façade contractor + roofer | Each package assumes the other handles the valley junction | + 15 – 40 k€ | Façade and roofing specifications both name the responsible party explicitly |
| Structural openings for services | Concrete frame + mechanical + electrical | Openings not set out on the pour drawings before concreting | + 20 – 60 k€ | Coordinated openings drawing signed off by all consultants before the pour |
| Suspended ceilings / services zone | Ceiling contractor + mechanical + electrical + fire alarm | Not enough ceiling void for all the services | + 10 – 30 k€ | Coordinated services drawing at detailed design, clear heights verified |
| Movement joints structure / finishes | Bricklayer + tiler + drylining | Movement joint not carried through the finishes, cracking under warranty | + 5 – 20 k€ | Structural joints transferred onto every finishes drawing |
| External insulation / windows | Insulation contractor + window contractor | Thermal bridge at the reveal junction left untreated | + 10 – 25 k€ | Insulation-to-window junction detail drawn at detailed design and checked |
| Plumbing / tiling / tanking | Plumber + tiler | Floor gullies set out after the tiling is laid, requiring rework | + 8 – 20 k€ | Floor layout with gully positions signed off before tiling |
| Lift / structure | Concrete frame + lift supplier | Lift shaft built outside the manufacturer's dimensions | + 15 – 50 k€ | Lift manufacturer's drawings issued before the formwork drawings |
| External works / architecture | External works contractor + architect | Pavement channel level incompatible with the intended ground floor level | + 10 – 40 k€ | Principle sections through external works and building drawn at developed design |
The two most expensive interfaces when mishandled are structural openings for services (up to + 60 k€ if the concrete is already poured when the missing openings are discovered) and the lift shaft against the frame (+ 15 to 50 k€ if the supplier's dimensions were never fed into the formwork drawings). Both share the same characteristic: once the concrete is poured it is too late, and correction means demolition and rebuilding.
3. The contractual grey areas: who is responsible for what?
The primary cause of interface problems is contractual: each package's technical specification stops at the nominal boundary of the package without describing what happens at the junction. The result is that every contractor reads the boundary in its own favour.
The façade / roof example
The valley between façade and roof is the classic case. The façade specification says the cladding runs "up to the valley". The roofing specification says the waterproofing runs "up to the junction with the façade". Nobody describes that junction, and each contractor takes the view that it belongs to the other. On site the valley leaks, and a dispute over liability begins.
The services in the ceiling void example
Mechanical, electrical, fire alarm and plumbing services all overlap in the ceiling void. Each consultant sized its own services independently, assuming the full void height was available. When all the consultants' drawings are finally overlaid, often only during construction, the real clear height is insufficient. Either the ceiling has to be dropped, with an impact on the contractual clear heights, or the services routes have to change, with variations on three or four packages.
4. Roles and responsibilities: who coordinates what?
| Party | Nominal responsibility | Frequent grey areas | What the economist checks |
|---|---|---|---|
| Architect (lead consultant) | Overall design team coordination, drawing integration | Technical interfaces between packages, often not covered by the appointment | That the budget carries the interfaces |
| Structural engineer | Structure, foundations, stability | Openings for services through concrete walls and slabs | That the openings are set out before the pour |
| Mechanical engineer | Heating, cooling, ventilation, plumbing | Coordination with electrical services in ceilings and cable routes | That the clear heights are budgeted |
| Electrical engineer | Power, data, fire alarm, building management | Interfaces with mechanical and ceiling contractors in the void | The same; a coordinated services drawing is indispensable |
| Acoustic consultant | Verification of the statutory acoustic levels | Interfaces with partitions, floating floors and windows | That the acoustic linings are budgeted |
| Programme coordinator | Programme, coordination of contractors on site | Technical interfaces between packages during construction | Flags the unresolved interfaces in cost terms |
| Approved inspector | Verification of structure, safety and accessibility | Inspector's observations can force expensive changes during construction | Carries an allowance for inspector's observations in the detailed design budget |
What this table shows is that coordination is nominally the lead architect's responsibility, but the tools of coordination, coordinated drawings, interface schedules, consultant coordination meetings, are not always the subject of an explicit fee. A lead architect with no explicit integration duty will produce drawings without overlaying them on the consultants' drawings, and the interfaces will only surface on site.
ℹ The programme coordinator: necessary but not sufficient Appointing a programme coordinator is often presented as the answer to coordination problems. It is a partial answer: that role coordinates the programme and the logistics of the contractors on site, it does not produce coordinated drawings and it does not resolve technical interfaces during the design stages. Technical coordination between packages has to be handled at detailed design, before the tender, before the contracts are signed. Discovering an interface on site is always more expensive than resolving it in the design.
5. Coordination milestones: what to produce and when
| Coordination milestone | Stage | Required deliverable | Risk if it is missing |
|---|---|---|---|
| Consultant coordination meeting | Outline design | Coordination minutes plus updated estimate | Consultants work in silos, surprises at developed design are guaranteed |
| Coordinated structural openings drawing | Developed design | Openings in walls and slabs signed off by every consultant | Unplanned openings mean core drilling through concrete, + 20 k€ minimum |
| Coordinated services drawing (clear heights) | Detailed design | 1:20 sections with all services overlaid | Ceiling installed before verification, everything has to come down |
| Interface verification by package | Detailed design | Interface schedule signed by the architect and the consultants | An untreated interface becomes a contractor dispute on site |
| Integration meeting before tender | Detailed design / tender | Confirmation that the specifications cover every interface | A contractor discovering the interface on site raises a variation |
| Inspector check before the application | Developed design | Preliminary inspector's report reflected in the estimate | Late observations mean redesign plus unbudgeted cost |
| Site kick-off meeting | Contract award | Inter-package coordination programme posted in the site office | Poor sequencing of packages means delay and extra cost |
| Weekly site meetings | Construction | Site minutes recording the live interface issues | Interfaces found too late mean heavy variations |
The coordinated services drawing at detailed design is the most profitable deliverable of the whole design appointment on any project with mechanical services, electrical services, sprinklers and suspended ceilings. Producing it systematically, with 1:20 sections overlaying every service, catches the height conflicts before the tender, while correcting them still costs nothing but drawing time.
6. How the economist carries interface risk in the estimate
The economist cannot price undocumented interfaces; you cannot cost what you do not know. What the economist can do is carry a systematic interface allowance in the estimate, calibrated on the type of project and on the quality of the coordinated drawings available.
- Simple project (apartment building, coordinated drawings produced): interface allowance of 1 to 2 % of the works cost, covering the unavoidable minor interfaces.
- Complex project (offices, buildings open to the public, healthcare, no coordinated drawings): interface allowance of 3 to 6 %; projects dense with technical services and no coordinated drawings at detailed design are the most exposed.
- Refurbishment with poorly documented existing fabric: interface allowance of 5 to 10 %; discoveries in the existing building generate unpredictable interfaces between demolition, structure and finishing packages.
This allowance must sit apart from the general contingency. It is specifically dedicated to coordination overruns, and it should be revised downwards once the coordinated detailed design drawings have been produced and signed off.
In summary
Technical interfaces are the silent cause of 2 to 8 % of overrun on every construction project. The eight interfaces that generate the most extra cost run from façade-to-roof weatherproofing to structural openings for services, worth up to + 60 k€ once the concrete is poured. The cause is contractual: specifications that stop at the nominal boundary without describing the junction. The cure sits at detailed design: a coordinated services drawing with 1:20 sections, an interface schedule signed by every consultant, verification before the tender. The economist carries an interface allowance of 1 to 6 % depending on the complexity of the project and the quality of the coordinated drawings available.
The nature of the figures quoted
The percentages and amounts quoted on this page are orders of magnitude for framing, not measurements. They illustrate mechanisms and proportions, and they substitute neither for a tender exercise nor for an estimate prepared on drawings.
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Sources: the BT construction cost indices published monthly by INSEE on its 2010 base, used to update the figures · construction cost statistics from SDES, the statistical service of the ministry responsible for construction · regulatory texts published in the Journal officiel and consolidated on Légifrance for the requirements cited.