Every contractor says they are BIM-enabled. Fewer can tell you what a clash costs them, which is the only number that matters when you are deciding how many coordinators to put on a job.
Four places a clash can be found
A clash between a duct and a beam can surface in four places, and the cost multiplies at each step:
- In the model, during coordination — roughly an hour of a coordinator’s time to reroute and re-issue.
- In the shop drawing, before fabrication — half a day, plus a re-approval cycle.
- On site, before installation — two to three days, because the material is already delivered and the crew is already scheduled.
- On site, after installation — a fortnight, once you count demolition, re-fabrication, re-inspection and the trades queued behind you.
That is not a smooth curve. It is a step function, and the biggest step is between “before installation” and “after.”
The arithmetic we actually use
On a mid-size build we expect somewhere between 900 and 1,400 genuine clashes across architecture, structure and MEP. Not the 40,000 the software reports before filtering — the real ones, after tolerance rules and duplicate grouping.
If 95% are resolved in the model and 5% escape to site, that is roughly 50 site-found clashes. At two days each, that is 100 days of disruption spread across the programme.
Push model resolution to 99% and you have 10 site-found clashes instead of 50. The difference — 80 days — pays for a coordination team several times over, which is the entire business case in one line.
Why the last percent is hard
Getting from 95% to 99% has very little to do with software. It comes down to three habits:
- Model discipline from subcontractors. A federated model is only as good as the worst model in it. We now make model quality a subcontract deliverable with payment attached, because asking politely does not work.
- Tolerance rules that match reality. A 5 mm clash between a hanger and a conduit is noise. A 5 mm clash between a fire damper and a wall opening is a hold point. Rules that do not distinguish these produce reports nobody reads.
- Closing the register. Every clash needs an owner and a date. An open clash with no owner is a site instruction waiting to happen.
The uncomfortable part
Coordination catches geometry. It does not catch sequence. A model can be clash-free and still describe a building you cannot physically construct in the order you have planned, because the crane cannot reach or the room cannot be closed until something inside it is commissioned.
We now run a separate constructability review alongside clash detection, with the site team in the room rather than the design team. It finds a different class of problem, and it finds it early enough to matter.