Headroom Clash Detection: The 450mm Rule Explained

Headroom Clash Detection is a coordination check most teams skip until someone’s head almost hits a duct under a stair landing during a site walk. By then the duct is welded, the ceiling is framed, and the fix costs ten times what it would have cost in the model.

What a headroom clash actually is

A headroom clash happens where a sloped element (a stair, a ramp) passes under a fixed obstruction (a beam, a duct, a pipe rack) and the vertical clearance between them drops below what’s usable — not just below a code minimum, but below what a person can walk under without ducking.

Standard clash detection tools (Navisworks Clash Detective, Revit’s own Interference Check) test hard clashes — geometry that physically intersects. A headroom problem is a soft clash: the duct and the stair don’t touch, there’s a gap, but the gap is too small to be usable. Out-of-the-box clash rules don’t catch this at all.

The 450mm rule

This isn’t a universal code number — it’s a practical buffer used across most Vietnamese and regional building codes’ minimum headroom requirement (typically 2000-2100mm clear under a stair flight), applied as a safety margin between MEP and structural coordination, not a clearance-to-occupant number.

Rule: if the vertical gap between the underside of any duct/pipe/cable-tray and the top of the finished stair nosing (or ramp surface) is less than 450mm above the code-minimum headroom line, flag it for review — even if it technically passes code.

Why 450mm specifically:

• 100-150mm for finish buildup you haven’t modeled yet (ceiling tile, fireproofing, insulation wrap)
• 100mm for the fact that ductwork routing is rarely final at the stage teams run this check
• 150-200mm safety margin because moving a duct after slab pour is a demolition job, not a redesign

If your clearance math leaves less than 450mm of slack above the legal minimum, that clash will become a real problem the moment insulation or fireproofing gets added — which nobody checks in the model because it’s not modeled yet.

How to actually find these in Revit + Navisworks

1. In Revit: create a 3D section box that follows the stair stringer diagonally — a straight vertical section will miss the sloped clearance zone under a landing.
2. Model a clearance volume, not just the stair geometry — an in-place mass or generic model extruded along the stair slope, offset by the code-minimum headroom, gives clash detection something solid to test against instead of relying on visual inspection.
3. In Navisworks Clash Detective: run the clash test between this clearance volume and every MEP category (ducts, pipes, cable trays, conduit) — not the stair geometry itself. This converts a soft clash into a hard clash the tool can actually catch.
4. Filter by tolerance: set the clash test tolerance to 450mm (not 0mm) so the report flags near-misses, not just overlaps. A 0mm tolerance test will report zero clashes even when you’re 50mm away from a real problem.

Common mistake

Teams run the clash test against the stair’s structural geometry (stringers, treads) instead of a dedicated clearance volume. The stair geometry itself rarely overlaps ductwork — the space above it does, and that space isn’t modeled by default. If your clash report comes back clean under every stair on a project with active MEP routing above it, that’s a signal the test is checking the wrong geometry, not that the coordination is actually fine.