Clash detection

Established

Also: Hard clash, Soft clash, Clearance clash, Interference checking

The automated checking of a federated model to find places where elements conflict physically, violate required clearances or conflict in time.

Explanation

Clash tests run between selected sets of elements in a federated model with defined tolerances. Practitioners commonly distinguish: hard clashes (geometry intersects); soft or clearance clashes (an element intrudes into a required clearance zone, such as maintenance access or insulation); and workflow or 4D clashes (conflicts in sequencing, delivery or temporary works). Raw results are grouped, filtered and assigned; issues are often exchanged as BCF topics so they can be tracked across tools.

Clash detection is one part of model coordination. A model can be clash-free and still uncoordinated — for example, if a required element is missing entirely.

Practical example

Before the level 4 rough-in, the mechanical coordinator runs ductwork against structure (hard, 25 mm tolerance) and against sprinkler mains (clearance, 50 mm). 312 raw clashes group into 18 issues, which are assigned in BCF to the responsible trades and resolved before shop drawings are issued.

Why it matters

Resolving conflicts in the model reduces field rework, RFIs and schedule delays, especially for MEP-dense buildings such as hospitals and laboratories.

Common confusion

Hard clash vs soft clash: a hard clash is actual intersection; a soft clash is an intrusion into a required space. Many "clashes" are false positives (for example insulation intentionally passing through a sleeve) and need tolerance rules.

Standards and references

  1. BIM Collaboration Format (BCF). buildingSMART International.Widely accepted professional interpretation
    Supports: Exchange of model-based issues (topics) referencing IFC elements between tools

VDC Stack.io writes its own explanations and does not reproduce copyrighted standards text. Consult the cited standard for normative wording.

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VDC Stack.io. (2026). Clash detection. In VDC Stack.io Knowledge Dictionary. Retrieved from https://vdcstack.io/dictionary/clash-detection/