Interference Checks — Run interference checks conceptually...

SECTION 1

Historical Context & Motivation

In the architecture, engineering, and construction (AEC) industry, one of the most persistent and costly problems has been spatial conflicts — situations in which two or more building systems physically occupy the same space. Imagine a large HVAC duct routed directly through a structural steel beam, or a plumbing chase that collides with an electrical conduit run. Before digital modeling tools existed, these clashes were discovered on the construction site, where resolving them required expensive rework, schedule delays, and strained relationships among trades. The desire to catch these errors before a single brick was laid drove the development of what we now call interference checking, sometimes referred to as clash detection.

For Visual Arts students who work in spatial design, set design, exhibition planning, or architectural visualization, understanding interference checks is essential. Your creative intent can be undermined by physical impossibilities that only reveal themselves during fabrication. Interference checking bridges the gap between the beauty of a design concept and the reality of building it.

1980s

2D CAD Coordination

Architects overlaid translucent drawings on light tables to visually spot conflicts between structural, mechanical, and electrical systems. This manual process was slow, error-prone, and depended heavily on human perception.

1990s

Early 3D Modeling & Navisworks

Software like Navisworks emerged, enabling teams to federate 3D models from different disciplines and run automated clash detection algorithms. This marked the first time interference checking became computational rather than purely visual.

2004

Revit Embraces BIM Coordination

Autodesk Revit matured as a Building Information Modeling (BIM) platform with built-in interference checking, allowing architects to detect clashes within a single parametric model without exporting to external software.

2010s

Cloud-Based & Real-Time Clash Detection

With Autodesk BIM 360 and integrated cloud workflows, interference checks could be run across geographically distributed teams in near real-time, accelerating coordination on complex global projects.

2020s

AI-Assisted Conflict Resolution

Machine learning algorithms now prioritize clashes by severity and suggest routing alternatives, pushing interference checking from mere detection toward intelligent, automated resolution.

The central question that interference checking addresses is deceptively simple: Do all the parts of this building actually fit together without colliding? Despite its simplicity, answering this question rigorously across thousands of elements in a model requires systematic methodology — precisely what Revit's interference check workflow provides.

SECTION 2

Core Principles & Definitions

Before diving into the workflow, it is important to establish a shared vocabulary. Interference checking in Revit rests on several foundational ideas that structure the entire process, from selecting which elements to test to interpreting the results. These principles apply whether you are coordinating a gallery installation, a theater set with integrated lighting rigs, or a full-scale architectural project.

Clash / Interference

A clash occurs when the solid geometry of one element intersects the solid geometry of another. Revit tests bounding volumes and then exact geometry to confirm a true overlap.

Category Pairs

Revit organizes elements into categories (walls, ducts, pipes, structural framing, etc.). An interference check always compares elements from one category selection against elements from another, forming a category pair.

Hard vs. Soft Clashes

A hard clash is a physical intersection of solid geometry. A soft clash involves clearance or tolerance violations — elements that are too close even if they do not literally overlap.

Clash Report

After running the check, Revit generates a clash report — an interactive list of every detected interference, sortable by element ID, category, and location. You can click any item to zoom directly to the conflict in the 3D view.

Resolution Workflow

Detection alone is not enough. Each clash must be resolved — by moving elements, resizing components, rerouting systems, or adding penetrations — and then the check is re-run to confirm the fix.

✦ KEY TAKEAWAY

Think of interference checking like a dress rehearsal for a stage production. You would never wait until opening night to discover that a lighting rig collides with a set piece; instead, you rehearse — you simulate the full assembly in advance so that every element occupies its own space. Revit's interference check is your digital dress rehearsal for construction.

SECTION 3

Visual Explanation — Anatomy of a Clash

The following diagram illustrates the most common type of interference encountered in building design: a mechanical duct routed through a structural beam. In this scenario, two separate disciplines — structural engineering and HVAC design — have each placed elements in the model without full awareness of the other's work. The diagram shows the overlapping zone that Revit identifies as a clash, along with the bounding volumes the software uses to accelerate detection.

The structural beam (purple) spans horizontally while the HVAC duct (cyan) runs vertically. Their intersection — the clash zone (red dashed rectangle) — represents geometry that cannot physically coexist. Revit first tests the dashed bounding boxes for overlap and, upon finding an intersection, performs an exact geometry check to confirm the clash.

Notice how each element carries its own axis-aligned bounding box (the dashed outlines). Revit uses a two-phase detection strategy. In the first, coarse phase, the software checks whether bounding boxes overlap — a computationally cheap test. Only when bounding boxes intersect does Revit proceed to the second, fine phase, where it tests exact solid geometry for true penetration. This two-phase approach makes it practical to check thousands of elements within seconds.

SECTION 4

How Interference Checking Works in Revit

While interference checking in Revit does not require manual mathematical computation on the designer's part, understanding the underlying logic deepens your ability to interpret results and set up checks strategically. The process follows a deterministic pipeline: selection → bounding-box test → exact geometry test → report generation.

Step-by-Step Pipeline

Define Category Pair: The user selects two sets of elements — for example, all structural framing elements versus all mechanical ducts. Revit will compare every element in Set A against every element in Set B.
Bounding-Box Overlap (Broad Phase): For each pair, Revit checks whether the axis-aligned bounding boxes intersect. If they do not, the pair is immediately excluded — no further computation is needed.
Exact Geometry Intersection (Narrow Phase): For pairs that pass the broad phase, Revit evaluates the actual solid geometry using Boolean intersection operations. A non-zero intersection volume confirms a hard clash.
Report Assembly: Every confirmed clash is logged with the element IDs of both offending components, their categories, the location of the intersection, and the approximate volume of overlap.

BROAD-PHASE OVERLAP CONDITION

Overlap = (Aₘᵢₙ.x ≤ Bₘₐₓ.x) ∧ (Aₘₐₓ.x ≥ Bₘᵢₙ.x) ∧ (Aₘᵢₙ.y ≤ Bₘₐₓ.y) ∧ (Aₘₐₓ.y ≥ Bₘᵢₙ.y) ∧ (Aₘᵢₙ.z ≤ Bₘₐₓ.z) ∧ (Aₘₐₓ.z ≥ Bₘᵢₙ.z)

A and B are the axis-aligned bounding boxes of two elements. The subscripts min and max denote the minimum and maximum extents along each axis (x, y, z). All six conditions must be true (∧ = logical AND) for the bounding boxes to intersect.

CLASH CONFIRMATION (NARROW PHASE)

V_clash = Volume(Solid_A ∩ Solid_B) > 0

If the intersection volume V_clash of the two solids is greater than zero, a hard clash is confirmed. Revit computes this via Boolean intersection of the parametric geometry kernels.

💡 Why This Matters for Visual Arts Students

You will likely never need to code these Boolean operations yourself. However, understanding the two-phase logic explains why narrowing your category selections dramatically speeds up the check. Testing "everything vs. everything" forces Revit to evaluate an enormous number of bounding-box pairs. Selecting only the relevant categories — for instance, lighting fixtures vs. structural columns — keeps the process fast and the results actionable.

SECTION 5

Detailed Breakdown — Types of Clashes

Not all clashes are created equal. In professional practice, interference issues are commonly classified into three categories based on severity and the nature of the conflict. Understanding these distinctions helps you prioritize your resolution efforts — a critical skill when a clash report returns hundreds of items and you need to decide which ones threaten the integrity of your design.

The three categories of clashes ranked by severity. Hard clashes involve physical overlap and must always be resolved. Soft clashes violate required clearances. Workflow clashes stem from coordination failures between disciplines rather than geometry.

Comparison of clash types, detection methods, and typical resolutions

Clash Type	Detection Method	Typical Resolution	Revit Native?
Hard Clash	Solid geometry Boolean intersection	Move, resize, or reroute one element	Yes — Collaborate tab → Interference Check
Soft Clash	Clearance / tolerance buffer around elements	Increase spacing; verify code compliance	Limited — more robust in Navisworks
Workflow Clash	Manual review of model versions and schedules	Update linked models; synchronize with central	Partially — Copy/Monitor assists

SECTION 6

Worked Example — Running an Interference Check in Revit

Suppose you are designing a small contemporary art gallery. The model includes architectural walls and floors, structural columns, an HVAC system for climate control (critical for art preservation), and a custom lighting rig suspended from the ceiling. You suspect that the lighting designer's suspended track fixtures may conflict with the HVAC ductwork. Here is how you would run an interference check conceptually within Revit.

Gallery Lighting vs. HVAC Ductwork — Interference Check

Step 1 — Open the Interference Check DialogNavigate to the Collaborate tab on the Revit ribbon. In the Coordinate panel, click Interference Check → Run Interference Check. This opens the dialog where you define the two sets of elements to compare.

Dialog opens with two panes: Categories from Set A (left) and Categories from Set B (right).

Step 2 — Define Category Set AIn the left pane, expand the category tree and select Mechanical Equipment and Ducts. You can also include Duct Fittings and Duct Accessories for comprehensive coverage. This constitutes Set A — the HVAC system.

Set A populated: Ducts, Duct Fittings, Duct Accessories, Mechanical Equipment.

Step 3 — Define Category Set BIn the right pane, select Lighting Fixtures. If your gallery uses custom families for the suspended track system, make sure those families are categorized as Lighting Fixtures in the Family Editor; otherwise, they will not appear in this selection. This is Set B — the lighting rig.

Set B populated: Lighting Fixtures.

Step 4 — Run the CheckClick OK to execute the interference check. Revit iterates through every possible pair of elements — one from Set A, one from Set B — performing the broad-phase bounding-box test followed by the narrow-phase exact geometry test. On a moderately sized model, this typically completes within a few seconds.

Revit completes the check and opens the Interference Report dialog.

Step 5 — Interpret the ReportThe report lists three clashes. Clash #1: a 600 mm × 400 mm supply duct intersects a recessed track light in Gallery Room B. Clash #2: a duct elbow fitting clips the edge of a pendant fixture in the main corridor. Clash #3: the return air grille overlaps with a wall-wash fixture mounting bracket. Clicking any clash highlights both elements in the 3D view and zooms to the conflict.

3 hard clashes identified. Priority: Clash #1 (largest overlap volume) → Clash #2 → Clash #3.

Step 6 — Resolve and Re-RunFor Clash #1, you lower the lighting track by 150 mm to clear the duct. For Clash #2, you reroute the duct elbow with a wider turn radius. For Clash #3, you move the grille 200 mm to the east. After each modification, re-run the interference check with the same category pair to confirm the clash has been resolved. The goal is a zero-clash report.

Final result: 0 clashes detected. The lighting and HVAC systems are fully coordinated.

SECTION 7

Strengths, Limitations & Comparisons

Revit's built-in interference check is a powerful first line of defense, but it is not the only clash detection tool available. Understanding its strengths and limitations helps you decide when to use it and when to escalate to external software such as Navisworks Manage or BIM 360 Coordinate.

Revit Interference Check vs. Navisworks Clash Detective

Aspect	Revit Interference Check	Navisworks Clash Detective
Clash Types	Hard clashes only (geometry-to-geometry)	Hard, soft (clearance), and time-based (4D) clashes
Model Scope	Single Revit model or linked Revit models	Federated models from Revit, AutoCAD, IFC, and other formats
Tolerance Settings	No user-defined tolerance buffer	Fully customizable tolerance and search sets
Report Export	HTML report; limited formatting	Detailed reports with screenshots, BCF, and issue-tracking integration
Best For	Quick, in-model checks during design development	Large-scale, multi-discipline coordination meetings
Cost	Included with Revit license	Separate license required

✦ KEY TAKEAWAY

Think of Revit's interference check as a spell checker and Navisworks as the professional editor. The spell checker catches obvious errors quickly and is always at hand. The professional editor provides nuanced, comprehensive review. You should use both: run Revit checks frequently during design, and use Navisworks at major coordination milestones.

SECTION 8

Connection to Advanced Coordination Workflows

Revit's interference check is the conceptual gateway to a broader ecosystem of coordination technologies. As your projects grow in complexity — from gallery renovations to multi-story mixed-use buildings or film studio back-lot facilities — you will encounter increasingly sophisticated tools that build on the same principles of spatial conflict detection.

Revit interference checking vs. advanced coordination ecosystems

Feature	Revit Interference Check	Advanced Coordination Tools
Clash Grouping	Flat list — each clash listed individually	Smart grouping by proximity, system, or grid location
Issue Tracking	Manual — export HTML and track externally	BCF (BIM Collaboration Format) with status, assignee, and due date
4D Integration	Not supported	Time-based clash detection against construction schedule
AI Assistance	None	Machine learning prioritization; suggested rerouting paths
Multi-Format Federation	Revit and linked Revit files only	Revit, AutoCAD, IFC, SketchUp, Rhino, point clouds

For Visual Arts students, the most relevant advanced workflow is likely BIM Collaboration Format (BCF). BCF allows clash issues to be exported as structured data packages that any BIM-compatible viewer can open. This means you can flag a lighting-versus-structure conflict in Revit and have the structural engineer see it in their native software — eliminating miscommunication. As your career advances into production design, exhibition architecture, or interdisciplinary creative practice, BCF fluency will be a professional differentiator.

🚀 Looking Ahead

Emerging generative design tools can automatically propose system routing alternatives that avoid clashes entirely. Imagine telling the software, "Route all ducts while avoiding all structural members and staying above the ceiling grid," and receiving three viable options ranked by material cost and installation complexity. This is the future that today's conceptual interference checking is evolving toward.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

Explain, in your own words, the difference between the broad-phase (bounding-box) test and the narrow-phase (exact geometry) test in Revit's interference check pipeline. Why does the software use two phases instead of jumping directly to exact geometry comparison?

PROBLEM 2 — BASIC CALCULATION

A Revit model contains 120 duct elements in Set A and 85 structural framing elements in Set B. How many element-pair comparisons must the broad-phase algorithm evaluate? If 95% of those pairs fail the bounding-box test, how many pairs proceed to the narrow phase?

PROBLEM 3 — INTERMEDIATE

You are coordinating an exhibition hall model that includes architectural walls, structural columns, mechanical ducts, electrical conduit, and plumbing pipes. You want to check all MEP (Mechanical, Electrical, Plumbing) systems against all structural elements. How many distinct category-pair interference checks would you set up, and why is it better to run them separately rather than as one massive check?

PROBLEM 4 — APPLIED

You are designing a museum with climate-controlled galleries. After running an interference check between HVAC ducts and the custom ceiling grid system (modeled as Generic Models), the report shows 14 hard clashes in Gallery Wing A. However, 8 of those clashes occur at penetration sleeves that were intentionally designed to allow ducts to pass through the ceiling grid. How should you handle these false positives in Revit, and what workflow strategy would prevent them from cluttering future reports?

PROBLEM 5 — CRITICAL THINKING

Consider a scenario in which you are collaborating on a large performing arts center. The architect works in Revit, the structural engineer works in Revit, but the theatrical rigging consultant delivers their model as an IFC file. Revit's native interference check cannot evaluate IFC geometry directly. Propose a comprehensive coordination strategy that ensures spatial conflicts between the theatrical rigging and other building systems are detected and resolved. Discuss at least two tools or workflows you would employ.

SUMMARY

Lesson Summary

Interference checking is the process of detecting spatial conflicts between building elements before construction. In Autodesk Revit, you access this tool via Collaborate → Interference Check → Run Interference Check, where you define two category sets (Set A and Set B) to compare. Revit uses a two-phase detection pipeline — a fast bounding-box broad phase followed by an exact-geometry narrow phase — to efficiently identify hard clashes where solid geometry intersects.

Clashes fall into three categories: hard clashes (physical overlap), soft clashes (clearance violations), and workflow clashes (version and schedule conflicts). Revit's native tool excels at quick, in-model hard-clash detection, while Navisworks and BIM Collaborate Pro extend capabilities to soft clashes, multi-format federation, and issue tracking. The goal of every interference check is a zero-clash report — proof that your design can be built as modeled.