What Is Clash Detection and Why It Matters

Clash detection is the automated process of identifying spatial conflicts between building elements modeled by different disciplines — finding where a structural beam intersects a duct, where a sprinkler pipe runs through a concrete column, or where two conduit runs fight for the same ceiling space. In traditional 2D coordination, these conflicts were discovered in the field during construction, where fixing them costs 10 to 100 times more than resolving them in the model. A McKinsey analysis of large construction projects found that 98% of megaprojects exceed budget, and rework driven by coordination failures is consistently cited as a top-five cost driver.

BIM clash detection shifts conflict resolution to the design phase. The ROI is well-documented: Holder Construction reported $10 saved for every $1 spent on BIM coordination. A Stanford CIFE study found BIM reduced construction errors and omissions by up to 40%. On a $100M healthcare project with typical MEP density, a thorough coordination process can identify several hundred clashes worth $2M–$5M in potential rework before a single bolt is tightened on site.

The Three Types of Clashes

Not all conflicts are created equal. BIM coordination platforms classify clashes into three categories:

  • Hard Clashes (Intersecting): Two solid objects occupy the same physical space — a duct penetrating a beam flange, a pipe running through a concrete shear wall without a sleeve, a cable tray buried in a structural column. These are physically impossible to build as modeled and must be resolved before construction. Hard clash detection is the baseline expectation on every BIM project.
  • Soft Clashes / Clearance Clashes: Two objects do not intersect but violate a required clearance buffer — insufficient insulation clearance around a chilled water pipe, a duct too close to a sprinkler head, a transformer without the NEC-required 3-foot front clearance. Soft clash detection requires defining tolerance distances and is typically configured for maintenance access envelopes, fire ratings, and code-required clearances. These are often more valuable to find than hard clashes on well-modeled projects.
  • Workflow / 4D Clashes: Conflicts in the construction sequence — two subcontractors scheduled to occupy the same area simultaneously, a crane swing radius overlapping an occupied building. 4D clash detection requires linking schedule data (from Primavera P6, MS Project, or Asta) to model elements and analyzing conflicts through time. This is the frontier of BIM coordination; relatively few projects run full 4D clash detection but the practice is growing rapidly on large infrastructure projects.

Revit's Built-In Interference Check

Revit includes a basic clash detection tool at Collaborate > Interference Check > Run Interference Check. You select two sets of categories to check against each other (for example, Structural Framing vs. Ducts), run the check, and get a list of intersecting element pairs. Clicking any result highlights both elements in the model and allows you to navigate directly to the conflict.

The Interference Check is useful for quick in-model checks but has significant limitations for professional coordination:

  • It can only check within one model at a time — it reads linked model elements but cannot generate a persistent cross-model report.
  • There is no tolerance/clearance setting; it only finds hard intersections.
  • It cannot group, filter, prioritize, or assign clashes to team members.
  • Reports export only to HTML; there is no integration with an issue management system.
  • It does not track resolution — there is no way to mark a clash as reviewed, accepted, or resolved within the Revit interface.

Use the Revit Interference Check for discipline-internal QC (checking that your own ducts do not intersect your own structural beams after routing changes) rather than for cross-discipline coordination.

Navisworks Clash Detective: Professional Coordination Workflow

Autodesk Navisworks Manage is the industry standard desktop tool for multi-discipline clash detection. Its workflow begins with NWC (Navisworks Cache) exports from Revit.

Exporting NWC Files from Revit

In Revit, go to File > Export > NWC. In the NWC Export Settings dialog, configure:

  • Coordinates: Set to Shared Coordinates so all discipline NWC files align when appended in Navisworks.
  • Export scope: Entire project or current view. For coordination, export the entire project with all links converted (converts each linked model into a separate NWC).
  • Convert URLs, properties, and room/area data: Enable all three to carry Revit element properties (type, family, system, room number) into Navisworks for use in clash grouping and reports.
  • Faceting factor: Controls mesh resolution on curved surfaces. The default (1.0) is adequate for coordination; do not set below 0.5 as it inflates file size without meaningful accuracy gain.

Append all discipline NWC files into a single Navisworks NWD (Navisworks Document) aggregated model. Save this NWD as the coordination model. All team members should work from the same NWD for a given coordination cycle.

Setting Up Clash Detective Tests

In Navisworks, open Home > Clash Detective. Each test represents one discipline pair to check. A typical project might have:

  • HVAC Ductwork vs. Structural Framing
  • Mechanical Piping vs. Structural Framing
  • Electrical Conduit/Cable Tray vs. Structural Framing
  • HVAC Ductwork vs. Mechanical Piping
  • HVAC Ductwork vs. Electrical Conduit/Cable Tray
  • All MEP vs. Architectural (ceilings, walls, columns)

For each test, define Selection Sets A and B. Use Navisworks Search Sets (created from Revit category/parameter data) or Selection Sets based on NWC file source. Set the Type (Hard, Clearance, or Duplicates) and the Tolerance (for clearance checks, typically 2 inches for insulated piping, 4 inches for maintenance access). Run all tests; Navisworks processes in parallel and typically completes a full-building check in under 2 minutes for a 500-element model.

Working the Clash Results

After running tests, the Results tab shows each clash with a unique ID, status (New, Active, Reviewed, Approved, Resolved), assigned user, and a description field. Key workflow steps:

  • Group clashes: Right-click results and use Group by > Selection A + Selection B to bundle similar conflicts. A duct running 40 feet through a zone and clashing with 12 joists is one coordination issue, not 12 separate clashes.
  • Add viewpoints: Each clash auto-generates a viewpoint; annotate with red-line markups for coordination meeting export.
  • Export reports: Clash Detective exports HTML, XML, or tabular CSV reports. The HTML report with embedded viewpoint images is the standard format for distribution to discipline engineers at coordination meetings.
  • Status tracking: As engineers resolve clashes in their Revit models and re-export NWC files, rerun the Navisworks tests. Clashes that no longer exist flip to Resolved automatically. Previously active clashes that still exist remain Active.

Autodesk Construction Cloud (ACC) Model Coordination

ACC Model Coordination (formerly BIM 360 Model Coordination) is Autodesk's cloud-based clash detection platform. Rather than manual NWC exports and desktop Navisworks, teams upload their Revit models directly to the cloud project; ACC automatically converts them and runs clash detection on a schedule or on demand.

The coordination workflow in ACC:

  1. Model Upload: Team members sync their Revit models to the ACC project's Design Collaboration module. Models are published to the Shared folder at agreed milestones.
  2. Automatic Clash Detection: ACC runs clash detection whenever a new model version is published. Results are available within minutes for typical building models.
  3. Issue Creation: Reviewers open Model Coordination, view clash results in the 3D browser, and convert clashes to Issues. Issues include a description, assigned discipline/person, due date, and root-cause tag.
  4. Issue Markup: Using the markup toolbar, coordinators add annotations, dimensions, and notes directly in the 3D viewer. Markups are stored with the issue.
  5. Resolution Tracking: Assigned team members receive email or ACC notification. They resolve the clash in their Revit model, publish a new model version, and close the issue. ACC automatically retests closed issues on the next model update and reopens them if the clash persists.

Tool Comparison: Revit vs. Navisworks vs. ACC

Feature Revit Interference Check Navisworks Manage ACC Model Coordination
Hard clash detection Yes (limited) Yes (full) Yes (full)
Clearance/soft clash No Yes Yes
4D/schedule clash No Yes Limited
Multi-model federation Limited (in-host) Yes Yes
Issue management No Basic Full (assign, track, close)
Clash status tracking No Manual Automated on re-upload
Cost Included in Revit Separate license (~$2,800/yr) Included in ACC subscription
Best use Quick in-model QC Complex projects, 4D, GC teams Cloud-first teams, owner-driven

The Coordination Meeting Workflow

Clash reports are tools for conversation, not just documentation. Effective coordination meetings follow a structured agenda: distribute the clash report 48 hours before the meeting so engineers can pre-review; during the meeting, screen-share the Navisworks or ACC model; walk through clash groups by priority (structural conflicts first, then tight MEP zones); assign each open clash a responsible party and resolution deadline; and close the meeting with a revised clash count that will be measured at the next meeting.

The metric that matters is clash closure rate — the percentage of active clashes resolved from one meeting cycle to the next. A stalling closure rate (below 60% week-over-week in the design development phase) is an early warning signal that coordination is behind schedule and field conflicts are likely.