What a clash is
A clash is a conflict between parts of a building model. A duct passes through a beam. A pipe runs through a column. Two cable trays occupy the same route. In the real building each of these means a stop on site, a redesign and a delay. Found in the model, each is a quick fix.
The three kinds
- Hard clash: two elements physically intersect. A pipe through a duct.
- Soft clash, or clearance clash: no collision, but not enough space. A valve with no room to reach it, or a duct too close to a ceiling to allow maintenance.
- Workflow clash: a conflict of sequence or information. Equipment that cannot be installed because the wall around it is built first.
How a test works
- Models from each trade are combined into a federated model, aligned to the same origin and units.
- A test compares two sets of elements, for example ducts against structure, with a tolerance.
- The software lists each clash with a location, a picture and the element IDs.
- Clashes are grouped, so ten collisions caused by one badly placed duct appear as one issue.
- Each issue is assigned to the trade that must resolve it, then the test is run again.
We run tests in Autodesk Navisworks on models authored in Revit, or exchanged as IFC.
Keeping the report useful
- Set tolerances so that trivial overlaps do not bury real problems.
- Group related clashes and report issues, not raw counts.
- Agree the level of detail for each trade first. A clash test is only as reliable as the least detailed model.
- Assign every issue to an owner and a date.
- Run rounds on a regular cadence rather than once at the end.
Zero clashes is not the goal
A model with no reported clashes is not necessarily a good model. It may simply be too rough to clash. The goal is to find and settle every conflict that matters before installation, and to agree what to accept, such as a minor overlap in a void that will be filled with insulation.
Want it done for you?
Our clash detection service covers testing, reporting, resolution proposals and re-testing across architecture, structure and MEP.
Quick questions
When should clash detection start?
As soon as there are models from more than one trade to compare, and it should repeat at each design stage.
Who fixes a clash?
The trade that owns the element that has to move, agreed in the coordination meeting. The coordinator tracks it to closure.
Published 14 October 2026 · Written by the Sactik team