Clash detection in Navisworks: a beginner's step-by-step guide
Clash detection finds places where building elements from different disciplines collide or sit too close together, before anything is built. In Navisworks Manage you append the discipline models, set up a Clash Detective test with a type and tolerance, run it, group the results and report them so each trade can fix their model.
What is clash detection?
Every building is designed by several teams: architects, structural engineers and MEP engineers. Each produces its own model. When those models are combined, problems appear: a duct passes through a beam, a drain pipe hits a column, a cable tray leaves no room for a sprinkler. On site, each of these means rework, delays and cost. Clash detection catches them in the model instead.
You can try a live version of this in the BIM Crafter Clash Lab, which runs a real geometric clash test in your browser.
Hard clashes vs clearance clashes
| Type | What it means | Example |
|---|---|---|
| Hard | Two objects physically overlap. | A supply duct running through a concrete beam. |
| Clearance | Objects are closer than an allowed distance. | A chilled water pipe 20 mm from a cable tray when 50 mm is needed for insulation and access. |
| Duplicate | The same object exists twice. | A pipe copied onto itself during modeling. |
Hard clashes are the obvious ones. Clearance clashes are the ones that look fine on screen but cause trouble during installation and maintenance.
Step-by-step: running your first clash test
Step 1: Prepare and export the models
From Revit, export each discipline to NWC (Navisworks cache) format. Make sure every model uses the same shared coordinates, otherwise they will not line up when combined.
Step 2: Append into one federated model
Open Navisworks Manage and append the architectural, structural and MEP files. Save the combined file as an NWF so it references the latest exports each time.
Step 3: Build selection sets
Create search or selection sets for what you want to test: for example "Ducts", "Pipes – CHW", "Structural Framing". Good sets make tests repeatable every week.
Step 4: Set up the test in Clash Detective
- Choose Selection A (e.g. all ducts) and Selection B (e.g. structural framing).
- Choose the type: Hard, Hard (Conservative), Clearance or Duplicates.
- Set the tolerance. Small tolerances filter out tiny, meaningless overlaps for hard tests; for clearance tests the tolerance is the minimum gap you require.
Step 5: Run and review
Run the test. Each clash gets an ID, a status (New, Active, Reviewed, Approved, Resolved) and a viewpoint. Step through them and confirm which are real.
Step 6: Group related clashes
One duct crossing ten beams can produce ten results. Group them into a single issue so the fix is assigned once, not ten times.
Step 7: Assign and report
Assign each group to a discipline and export an HTML or PDF report with viewpoints. That report becomes the agenda for the coordination meeting.
Step 8: Fix, re-export, re-run
Modelers fix their Revit models, re-export the NWCs, and you refresh and re-run. Navisworks marks clashes that no longer occur as Resolved.
How clashes are typically resolved
- Reroute the service around the obstruction.
- Resize: a wider, flatter duct can pass under a beam where a tall one cannot.
- Re-level: change the invert or bottom-of-duct height.
- Provide an opening: a sleeve through a beam, approved by the structural engineer.
- Change priority: gravity drainage usually wins because it must slope; flexible services move first.
Common beginner mistakes
- Testing everything against everything and drowning in thousands of results.
- Forgetting insulation thickness when setting clearance tolerances.
- Not grouping clashes, so the report is unreadable.
- Models on different coordinates, producing fake clashes everywhere.
Week 10 of both BIM Crafter streams is dedicated to Navisworks clash detection, and during the 6-month internship you work on real clash reports for live projects.
Apply for the next batch