20 KiB

BIM Specifications

Originally elaborated by OpeningDesign / Ryan Schultz / Yorik van Havre and licensed under CreativeCommons. Feel free to use, distribute or contribute to improve the process.


This BIM specification, unlike most other BIM specifications available, is based on real-life, tested methods to get the job done and guarantee as much as possible the primary aim of BIM--real collaboration between the different actors of a building project.

This specification consists of simple rules to guarantee a maximum of fidelity in data transfers using the IFC file format. The following rules work with any version of the IFC schema.

The correct implementation of these rules implies a good understanding of the IFC format, and ways to verify the contents of IFC files others than in the software where it was authored. Several free or open-source applications allow to open, visualize and inspect IFC files and make sure that the rules below were correctly implemented.


Model Lines: Lines in 3-dimensional space

Platform Native Functionality Import Export
FreeCAD Any Part-based object which contains edges and no faces, such as Draft or Sketch objects IfcAnnotation objects are imported as simple, non-parametric Part objects. Color and line style currently not supported. Part-based objects with edges and no faces are exported as IfcAnnotation
Revit Model Lines ✔️ - Name, Color, & Pattern, - Weight Had to use an MVD that supports the export of lines (Coordination View 2.0 does not)

Test folder: ./Specifications test files/Model Lines


Platform Native Functionality Import Export
FreeCAD Walls IfcWall and IfcStandardWall are imported as wall objects all walls and any other Arch-based object which Role property is set as Wall, is exported as an IfcWall.
Revit Walls from FreeCAD, the wall imports as a in-place family, that is, not an intelligent wall ✔️

Test folder: ./Specifications test files/Wall

To do:

  • Check if there is any possibility to recreate a working family from an IFC file


The notion of material in IFC is spread over different types: objects can have a IfcStyledItem applied directly to them, that allows to set for example a color, an IfcMaterial, that actually contains little more (it also contains an IfcStyledItem), and different IfcProperties that is where the physical properties of a material can be stored.

Any Ifc object can have any of the above. For the material to work in Revit, it must have a IfcStyledItem directly applied on the object, and an IfcMaterial. Both the IfcStyledItem and the IfcMaterial contain an IfcSurfaceStyle. It is important that both these IfcSurfaceStyle have the same name as the material. So for any given object with a material, you need three IFC entities with the same name.

Platform Native Functionality Import Export
FreeCAD Material For objects with an associated IfcMaterial, a FreeCAD material is created and linked to the object. For objects that only have a Surface Style directly applied, no material is created in FreeCAD, but the object takes the color of the Surface Style. If FreeCAD objects have a material, it is exported as an IfcMaterial with corresponding Surface Style. Otherwise, only the shape color is exported as Surface Style.

Name and Description: All objects and materials should have a human-readable name or description

Objects should have a name that allows a human being to understand what it is, in case the software that reads the IFC file fails to recognize or categorize it appropriately. For example, bad names are "Object00014", "Material43". Good names are "Kitchen chair", "Grey concrete", "East living room wall"

All IFC objects have a Name and a Description. Any of them can be used for this purpose.

This rule mainly applies to IfcProduct entities in IFC: They are the final, individual objects that compose a building. For example, a wall, a column, a chair, a window. It doesn't consider objects that are part of a final product, when they are composed of multiple objects, for example a leg of a chair or a brick of a wall. It is sufficient to have the chair correctly named, not necessarily each component of the chair.

Platform Native Functionality Import Export
FreeCAD Label property (all FreeCAD objects), Description property (only Arch objects) IFC name translated to FreeCAD object label ('My test wall'), IFC description ('This is a test wall to check rule number one') imported into FreeCAD object description if available FreeCAD object label exported as IFC name, FreeCAD object description, if present, exported as IFC description
Revit Upon import, name and description is not accessible to modify from Revit UI. (See Screenshots 1 & 2) Name and description exports out correctly.

Test folder: ./Specifications test files/Name and Description

To do:

  • Check where name and description are stored in Revit, maybe that could be accessed by dynamo or something. See if Revit doesn't provide any alternative (different ways to "label" objects)

Nested Groups: All objects should be grouped in meaningful ways

Grouping objects using IfcGroups allows a human being to clearly recognize objects as being part of a same area, function or category. Groups can be nested inside other groups. A same object cannot be part of several groups.

Platform Native Functionality Import Export
FreeCAD Groups IFC groups translated to FreeCAD groups. Nesting is respected. FreeCAD groups are exported to IFC groups, but groups are not part of IfcBuildingStoreys (Problem: IfcGroups cannot be nested into IfcBuildingStoreys)
Revit Groups of Elements Does not import IFCgroups into Revit Groups Yes, exports #253= IFCGROUP('2wfBgyl9H71872FVeaZPs0',#41,'Model Group:Test Revit Group:149951',$,'Model Group:Test Revit Group');

Test folder: ./Specifications test files/Nested Groups

To do:

  • IFC offers two different "structures" to group and organize contents: space-related (Building->BuildingStorey->Zone->Space) and non-space-related (Group ). These two structures are fully independent and cannot be combined (you cannot add a group to a spatial structure element). Check if using IFC Groups is the most adequate form, and if it wouldn't be better to switch to a full space-related system.

Duplicatable components

A way to be able to group a series of objects into a same structure, let's call it a component, and duplicate that component in the model. If one object is changed, added or removed from/to the base component, all instances of that component should update automatically. This is typically how "blocks" work in AutoCAD, or "components" in SketchUp, or "compounds" in FreeCAD. An example would be a restroom stall, with all its parts and accessories: door, wc basin, paper hanger, sink, etc.

Platform Native Functionality Import Export
FreeCAD Compound
Revit Groups ✔️
Revit Families
Blender Groups/Collections

Generic components

IFC objects that are not of a specific type (wall, beam, etc) are usually saved into IFC as BuildingElementProxy. This is often used by applications that don't classify geometric objects by type (most non-BIM applications like Blender or Sketchup), but can be also used when you want a specific object to be rendered as less parametric as possible.

Platform Native Functionality Import Export
FreeCAD Part shape BuildingElementProxy objects are rendered either as simple, non-parametric Part shapes, or as generic Arch components depending on the import preference settings. The Arch component will retain the IFC properties and material attached to the object, while the Part shape not. Generic Arch components and any other FreeCAD object that is not one of the Arch building elements (Wall, structure, etc) will be exported as BuildingElementPRoxy.
Revit Generic component

Safe geometry types

To be developed:

Some geometry types, although they import correctly in all applications, are sometimes not editable (the concept of what editable means needs to be developed as well). This item should identify the geometry types that are "safe".

Note that in any application, safe geometry only means that: The geometry will be safely reproduced. All the semantic layers on top of the geometry (type, properties, materials, etc) is independent from the geometry itself. A faithfully rendered geometry doesn't necessarily mean that all these pieces of information will be reproduced correctly as well.

Safe geometry for Revit

  • Extrusions: Revit will treat all extrusion (derived from If ExtrudedAreaSolid ) as native Revit extusions, which is the basic condition to be editable.
  • Faceted Breps: Revit is also able sometimes to recognize extrusions from very simple and prismatic IfcFacetedBrep objects, and therefore consider them as editable too. All the rest will be non-editable (excluding untested types below).

Safe geometry for FreeCAD

  • A priori, anything is safe in FreeCAD. The IfcOpenShell engine, which is responsible for importing and exporting IFC files in FreeCAD, supports almost all the possible IFC geometry types and creates native FreeCAD geometry from them. However, FreeCAD doesn't feature the same "editability" concept as Revit. Objects are not editable or not by nature. However, the nature of FreeCAD objects, which are all breakable/reconstructable, allows in theory any object to be modified. However a couple of types will render in a more parametric way:
  • Extrusions: all extrusion (derived from If ExtrudedAreaSolid ) objects will be rendered as Part Extrusions or, if they are of a type whose corresponding Arch equivalent is extrudable (Structure or Wall), as such.

To be tested:

  • Vertical (Z-axis) extrusions of any profile (straight/only lines, complex/curved, and predefined types)
  • Arbitrary (any direction) extrusions of any profile
  • Booleans (unions, differences, intersections)
  • Faceted Brep shapes
  • Advanced Brep shapes


Use free/open-source IFC applications to validate the data inside IFC files.

It is fundamental for the author of an IFC file to be fully aware of what has been included in that file. Therefore, it is essential to be able to verify the contents of the file in a neutral manner (independent of the application that exported it). It should also be possible for other people to easily open that file, and verify its contents, independently of the application used to import it.

Items that should be checked on opening an IFC file for verification:

  • The application used for verification reports no error on opening the file
  • The total number of objects informed by the application used for verification matches the number of objects informed by the application used for exporting
  • All the geometry is there when inspecting the 3D model in the application used for verification
  • All the geometry is at its correct location when inspecting the 3D model in the application used for verification

To be developed:

  • For now the only reliable one I know that is open-source and cross-platform is IfcPlusPlus which does a fairly good job. If it prints no error, and all objects appear in place, it generally means the data is of very good quality. BimServer might become a perfect option once it has good data validation plugins. IfcPlusPlus doesn't support IfcAdvancedBrep (Still not checked with BimServer).