How to Manage MEP Clashes in Navisworks
No one looks through a clash report that has thousands of rows. Foremen quickly scan the page shut the file and go back, to the job site. Real conflicts stay buried next to harmless overlaps. The test setup causes this.
A blanket Navisworks test compares every model against every other model, so teams receive everything it finds. An MEP clash detection matrix fixes that by deciding upfront which trades get tested, in what order and at what tolerance. Building one starts with understanding why raw results fail.
Why Raw Navisworks
Tests Create So Many Junk Clashes
Navisworks' Clash Detective compares
geometry and nothing else, so coordinators must supply the judgment. Three
modeling habits make the noise worse:
Placeholder duplicates: Architectural
models carry generic fixtures that overlap the subcontractor’s fabrication
models.
Insulation: Tests treat jacketing as solid, so
pipes with enough room still register clashes.
Missing openings: Early slabs and
walls have no cutouts, so every riser reports a conflict.
The cost reaches
past the model. A 2018 Autodesk
and FMI survey linked 48% of U.S. construction rework to poor project data and
miscommunication, about $31.3 billion. That is why a clean Clash Detection
Workflow needs rules before the first test runs.
Rank Trades First
in Your MEP Clash Detection Matrix
Rules start with a ranking that every trade accepts. The trade that
can move least gets the space first. Coordinators record that order in an MEP
Coordination Matrix, and it drives every later test. A simple MEP trade
priority matrix runs from least to most flexible:
Structure and
post-tension slabs: These stay fixed without a structural
engineer’s stamped approval.
Gravity plumbing: Drain lines need continuous slope, typically 1/8 to 1/4 inch per
foot.
Large HVAC
ductwork: Wide trunks need deep ceiling space.
Pressurized
pipework: Chilled water, fire protection and domestic
lines can offset around obstacles.
Electrical trays
and conduit: Branch runs bend in the field, so they
move first.
Once the order is set, the next question is how close is too close.
Set Limits with a
BIM Clash Tolerance Table
A ranking tells who
turn is next. Tolerances tell you when a conflict is worth bringing up. A BIM
clash tolerance table gives a single number for each pair of trades to test
against. Hard Clashes are real overlaps and Soft Clashes are clearance
failures, e.g. a Pipe crowding a Valve Handle. Navisworks names these tests
Hard and Clearance. A hard test ignores overlaps smaller than the tolerance. A
clearance test reports anything closer than it.
Starting values
vary by project, so adjust them to the specification:
Structure vs. primary duct: Hard, 0 mm
Pipe vs. cable tray: Clearance, 50 mm
VAV box access zone: Clearance, 600 to
900 mm
Navisworks also
includes an Insulation Thickness rule for clearance tests. Coordinators can
apply it to drop pipes that already have room for their jacketing.
Build Search Sets
and a Navisworks Clash Matrix Template
A tolerance table
does nothing until each row becomes a test, and Search Sets let coordinators
build those tests. A Selection Set freezes specific objects, so it breaks when
a subcontractor re-exports a model. A Search Set stores a rule, such as
Category equals Ducts, and finds new geometry when models update. Base the
rules on three Revit properties:
Category: Ducts, Pipes, Cable Trays
System Type: Supply Air, Sanitary, Chilled Water
Reference Level: Level 02, Level 03
Coordinators export
the sets and batch tests to XML, then import them on the next project. That
shared library works as a reusable Navisworks clash matrix template.
Group Clashes and
Track Issues Through BCF
Running tests is
the easy part. Triage takes the effort, because single pins overwhelm the
foremen who must act on them. Coordinators fix this by combining related pins
into one issue:
By run: Eight sprinkler branches crossing one tray
become one issue.
By level and grid: Tag each issue
with an intersection such as L03_Grid_F4-G5.
By zone: Isolate mechanical rooms so one contractor
closes a plant room in one pass.
This is where MEP coordination for contractors pays off, since each foreman sees only their own
zone. BCF, the open BIM Collaboration Format, then carries each issue’s 3D
view, assignee, due date and status into Revizto, Procore or Autodesk
Construction Cloud. Providers of MEP BIM coordination services usually set the
due dates at this stage.
When to Bring In
Outside Help
Some projects
outgrow internal hours. Contractors then hire MEP clash detection services to
set up the matrix and handle triage. Others fold this work into wider MEP BIM
services that run from modeling through coordination. Either way, ask for proof
before signing:
A documented tolerance table
A reusable search set library
A BCF issue flow with clear due dates
A provider who can
show all three has already solved the setup problem.
Conclusion
A matrix turns
coordination from cleanup into a planned process. Coordinators rank the trades,
set tolerances, build search sets, group results, and track fixes in BCF. Each
step removes noise before it reaches the foreman. The full blog includes a free
Excel template that packages the whole setup.
Stop
sorting junk clashes. Read the full step-by-step guide to building your matrix :
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