Key takeaways

  • Most MEPF clashes are not exotic — five categories repeat across pharma, industrial, and commercial projects.
  • A BIM Execution Plan that locks discipline priority (HVAC main > sprinkler main > cable tray > gravity drainage) prevents about 70 per cent of clashes before any modelling starts.
  • Clash detection in Navisworks is necessary but not sufficient — soft clashes (clearance for valve operation, BMS sensor access) need an experienced reviewer, not just the report.
  • Resolving design-stage clashes costs hours. Resolving the same clash on site costs days, plus an RFI, plus a variation order.

If you have ever sat through a Navisworks clash review meeting, you know how the conversation goes. The HVAC designer says the duct cannot move. The sprinkler designer says the main cannot move. The electrical designer says the cable tray was there first. Somebody from the structural team points out that the slab cutout is already cast. The PM asks how this got through coordination. Everyone looks at the BIM coordinator.

Most of these meetings are unnecessary, because most of these clashes are predictable. After dozens of MEPF coordination cycles on pharma plants, hospitals, automotive launch lines, and data centres, we see the same five collision patterns repeat almost every time. Knowing what they are turns coordination from reactive firefighting into a routine quality gate.

1. HVAC supply duct vs sprinkler main at the corridor crossing

Corridors are where everything wants to live. The HVAC main is running parallel to the corridor centreline. The sprinkler main wants to run parallel to the corridor centreline. The cable tray wants to run parallel to the corridor centreline. At the perpendicular crossing — typically at a fire compartment wall — all three need to cross each other in roughly 350 mm of vertical clearance.

The fix is design-stage discipline stacking: in our default BIM Execution Plan, HVAC supply duct holds the highest soffit elevation, sprinkler main is second, cable tray is third, and gravity drainage (where applicable) is fourth. The corridor false ceiling is sized to accommodate this stack plus 50 mm clearance. When this stacking is documented in the BEP and locked before modelling begins, this clash disappears — every discipline knows where its envelope is.

2. Fire damper vs structural slab cutout

Every duct that crosses a fire compartment wall or a fire-rated floor needs a fire damper rated to the same fire rating as the partition. The damper requires a slab cutout that is 50-100 mm larger than the duct on each side to allow the damper sleeve. On about half the projects we audit, the structural team has cast a slab cutout that matches the duct dimensions exactly — leaving no room for the damper sleeve.

This is technically not a clash that Navisworks flags by default, because the damper is often not modelled until shop drawing stage. It is a coordination omission. The fix: add fire dampers to the MEP coordination model at concept stage with their full sleeve dimensions, share the slab cutout schedule with structural before slab pour, and verify damper-to-cutout clearance on every fire-rated penetration in the model walk-down.

3. Cable tray vs sprinkler branch line above a corridor

The HVAC main and sprinkler main are usually coordinated above the corridor. The clash that gets missed is between branches — a sprinkler branch dropping down from the main to a sprinkler head, and a small cable tray feeding a wall-mounted DB or a CCTV camera near the same location.

This happens because branch routing is usually done at shop drawing stage by the trade contractor, not by the design consultant. By then, the cable tray has already been modelled and the sprinkler branch is doing whatever physics allows. The fix is to require sprinkler branches to be modelled to the head location at design stage, not just the main, and to run clash detection on branches before the issued-for-construction set is locked.

4. Valve access clearance — the "soft clash" nobody flags

Navisworks Clash Detective will flag a physical interference where two solids occupy the same coordinates. It does not flag the case where a butterfly valve handle needs 400 mm of swing radius to operate but is installed 200 mm from a wall.

This soft clash is one of the most common reasons for first-year operations complaints — the valve was installed exactly where the model showed it, but it cannot be opened or closed without removing adjacent pipework. The fix is twofold: include valve handle/actuator envelopes as separate clearance volumes in the model, and require an experienced MEPF reviewer (not just the BIM coordinator) to walk the model from the perspective of the maintenance technician who will operate it.

This is especially important for fire pump rooms, AHU rooms, chiller plant rooms, and electrical rooms where maintenance frequency is high. The cost of a poorly placed valve in design is half an hour. The cost of relocating it after commissioning is a four-week pipe rework.

5. Variable refrigerant flow (VRF) refrigerant piping vs ceiling void services

VRF and split-DX systems are becoming the default for office fit-outs and smaller commercial spaces in India. The refrigerant piping runs in the ceiling void back to outdoor units, often crossing multiple discipline zones along the way. Because VRF is typically a specialist trade contractor scope (Daikin, Mitsubishi, LG), the refrigerant routing is usually NOT in the initial MEPF coordination model — it gets added at shop drawing stage after the rest of the ceiling is fixed.

By then, the available ceiling void is whatever is left over. We have seen projects where VRF refrigerant piping had to cross above fire ducts at impossible angles because the routing was never coordinated. The fix is to require the VRF contractor to submit a coordination model at scope-issue stage, not shop-drawing stage, and to integrate refrigerant routing into the main MEPF clash detection from day one of the trade scope.

The process change that prevents most of this

None of these five clashes are technically hard. What makes them keep happening is a process where coordination is treated as a Navisworks review at the end of design, instead of a discipline-stacking discipline at the beginning. A few practices that have worked for us:

  • BIM Execution Plan with locked discipline priority. Before any modeller opens Revit, the BEP documents which discipline gets which elevation band in corridors, plant rooms, and risers.
  • Soft clash review by an MEPF engineer, not the BIM coordinator alone. Navisworks generates the report. A human who has operated these systems decides what is real.
  • Coordination is iterative, not a single checkpoint. Run clash detection at LOD 200, again at LOD 300, again before issued-for-construction. The cost is low. The catch rate is high.
  • Trade contractors model their scope into the central model, not in isolation. Especially VRF, specialist piping, and electrical containment for kitchen / IT / process systems.

BIM coordination is one of the cheapest insurance policies in a multi-discipline project. The cost of running another clash detection cycle is a few engineering hours. The cost of finding the same clash on site is days of rework, an RFI, a variation, and a slipping handover date. Treating coordination as a design quality gate rather than a meeting to manage is what separates projects that finish on schedule from those that do not.

Frequently asked questions

What is the difference between a hard clash and a soft clash?

A hard clash is a physical interference — two solid objects occupy the same coordinates. Navisworks Clash Detective flags these automatically. A soft clash is a clearance or operational issue — for example, a valve that has the handle envelope blocked by an adjacent wall, or a duct that meets minimum clearance to a cable tray but leaves no room for insulation. Soft clashes need human review.

At what LOD should clash detection start?

We run an initial coordination check at LOD 200 (generic systems with approximate sizing). The catch rate is moderate but the cost of fixes is very low at that stage. We re-run at LOD 300 (specific systems with accurate sizing) when most clashes are caught and fixed. A final check at LOD 350+ before issued-for-construction catches the last 5-10 per cent. Skipping the LOD 200 check is the most common mistake.

Should the architect run clash detection or the MEP contractor?

Whoever holds the central federated model runs the detection. On a design-bid-build contract that is usually the architect or design consultant. On a design-build or EPC contract that is usually the MEPF contractor. The important thing is that one party owns the federated model and runs detection as a regular cadence, not a one-time review.

Is IFC sufficient or do we need native Revit files?

IFC works for clash detection when models come from different authoring tools (Revit, AutoCAD MEP, ArchiCAD). For coordination cycles that involve geometry edits, native Revit files are still more efficient because round-tripping through IFC loses parametric intelligence. For final hand-over, IFC 4.3 has become the standard for asset information delivery.