Category Mapping override

There are many cases where the global mapping discussed previously needs to be overridden for specific elements. The reason behind this is that the IFC classes are often more granular than Revit categories and also have their predefined types based on their use.

This functionality has been improved since Revit 2023 and now offers a selection box for easier assignment of IFC classes and types:

Element-based IFC Mapping in Revit 2023 and newer - Notice that the same parameters exist both at the instance and type level to cover multiple use cases. If both values are set, the instance value will override the type value.

Revit 2022 and earlier is still using the IFC Class and Type manual assignment to the parameter IfcExportAs. If it does not already exist in the project, it is strongly recommended to add this parameter as a shared parameter using the shared parameter files included with Revit IFC:

In Revit 2022 the IfcExportAs parameter needs to be added to the project and filled manually using valid values for IFC Class / type. This parameter can also exist both on Instance and type level to cover multiple cases (instance value overrides the type).

The value of this parameter needs to be IfcClass.TYPE – both of these are defined in the IFC schema. Like the main mapping table, Don‘t export can exclude a particular element from export.

It is also possible to map Revit categories to other classes, however Revit will not be able to add missing properties and relationships. In the following example, a wall is mapped to IfcRailing with the predefined type BALUSTRADE:

However, if compared to a regular railing, not all custom properties defined in the IFC Schema can be automatically mapped and have to be assigned manually:

Note: There are some restrictions to the mapping of more complex system families, such as curtain walls, to other IFC classes. A current overview of the supported IFC Class mapping when exporting Revit models to IFC can be found in this AKN article.

The IFC Schema allows USERDEFINED types. The correct use of these is achieved by adding USERDEFINED as value for the actual IfcType and specifying the actual user-defined type name in IfcObjectType.

Example:

Overview of available types defined for IfcRailing in the IFC 4 documentation:

Constant	Description
HANDRAIL	A type of railing designed to serve as an optional structural support for loads applied by human occupants (at hand height). Generally located adjacent to ramps and stairs. Generally floor or wall mounted
GUARDRAIL	A type of railing designed to guard human occupants from falling off a stair, ramp or landing where there is a vertical drop at the edge of such floors/landings.
BALUSTRADE	Similar to the definitions of a guardrail except the location is at the edge of a floor, rather then a stair or ramp. Ex- amples are balustrates at roof-tops or balconies.
USERDEFINED	User-defined railing element, a term to identify the user type is given by the attribute IfcRailing.ObjectType.
NOTDEFINED	Undefined railing element, no type information available.

Userdefined type definition in Revit:

It is also possible to specify a userdefined IfcName during export using the type parameter NameOverride for the type name as well as the instance parameter IfcName to comply with specific naming conventions: