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ifc-errors-property-set-mistakes

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Updated 10-15 22:18

About this skill

IFC Property Set Mistakes

Diagnose and fix the six recurring mistakes in IFC property sets, quantity sets and material associations : a reserved-prefix name collision, a property set attached to a type through the wrong relationship, a material-usage entity on a type, a hand-written derived attribute, a quantity carrying the wrong unit type, and the legacy IfcMaterialList in a modern file.

Two of these are schema-valid but wrong (a "smell") ; three break a WHERE rule (a hard error) ; one is a derived-attribute violation. ALWAYS classify the mistake before fixing it.

For the property and quantity entity model see ifc-syntax-property-sets. For the material entity model see ifc-syntax-materials. For units see ifc-syntax-units. For schema-validation mechanics see ifc-errors-schema-validation.

Quick Reference

The six mistakes

# Mistake Severity Rule or mechanism Fix
1 Custom property set named Pset_... Smell : schema-valid, collides with the standard library Pset_ is reserved for published sets Rename without the Pset_ prefix
2 IfcRelDefinesByProperties RelatedObjects holds an IfcTypeObject Hard error (IFC4+) WHERE rule NoRelatedTypeObject Use IfcTypeObject.HasPropertySets for types
3 IfcMaterialLayerSetUsage / IfcMaterialProfileSetUsage assigned to an element type Hard error Usage entities are occurrence-only Put the bare set on the type, the usage on the occurrence
4 A literal value written into the derived IfcMaterialLayerSet.TotalThickness Hard error : derived slot The value is computed by IfcMlsTotalThickness Write * in the STEP slot ; never a number
5 A length unit on IfcQuantityArea (or any quantity unit mismatch) Hard error WHERE rule WR21 : the unit must match the quantity type Use an AREAUNIT, or omit Unit to inherit the project unit
6 IfcMaterialList in a new IFC4 / IFC4.3 file Smell : schema-valid, legacy mechanism Superseded by IfcMaterialConstituentSet Replace with IfcMaterialConstituentSet

Severity, restated

  • Hard error : a schema validator rejects the file (mistakes 2, 3, 5) or the STEP line is malformed (mistake 4). The file does not pass validation.
  • Smell : the file passes validation, but the model is wrong by convention and breaks downstream tools or data exchange (mistakes 1, 6). NEVER read "validation passed" as "the property data is correct."

Property set versus quantity set : same attachment, different content

IfcPropertySet (named properties) and IfcElementQuantity (measured quantities) are both subtypes of IfcPropertySetDefinition. Both attach to an occurrence through IfcRelDefinesByProperties, and to a type through IfcTypeObject.HasPropertySets. The naming conventions are parallel : Pset_ for property sets, Qto_ for quantity sets. Every rule in this skill that names Pset_ applies equally to Qto_.

The reserved prefixes

Prefix Reserved for A custom name MUST
Pset_ buildingSMART published property sets NEVER start with Pset_
Qto_ buildingSMART published quantity sets NEVER start with Qto_

A published set name is an exact identifier, for example Pset_WallCommon ("properties common to all occurrences of IfcWall") or Qto_WallBaseQuantities. A custom set uses a project or vendor prefix instead, for example ACME_WallThermal.

Decision Trees

Naming a property set or quantity set

You are naming an IfcPropertySet or IfcElementQuantity.
|
+-- Is the name an EXACT published Pset_ / Qto_ identifier for this element,
|   verified against the buildingSMART property library?
|     YES -> use it as-is. The content must match the published template.
|     NO  -> continue.
|
+-- Does the name start with Pset_ or Qto_ ?
      YES -> COLLISION (mistake 1). Rename with a project / vendor prefix.
      NO  -> custom set, correctly named. Proceed.

Attaching a property set to the right host

You are attaching an IfcPropertySet (or IfcElementQuantity).
|
+-- Is the host an occurrence (IfcWall, IfcSlab, ... an IfcObject) ?
|     -> create IfcRelDefinesByProperties
|        RelatedObjects = the occurrence(s),
|        RelatingPropertyDefinition = the set.
|
+-- Is the host a type object (IfcWallType, ... an IfcTypeObject) ?
      -> add the set to IfcTypeObject.HasPropertySets directly.
      -> NEVER put the type into IfcRelDefinesByProperties.RelatedObjects
         (mistake 2 : WHERE rule NoRelatedTypeObject, IFC4+).

Associating a structured material

You are associating a material that has internal structure.
|
+-- Layered element (wall, slab, roof, plate) ?
|     type      -> IfcRelAssociatesMaterial.RelatingMaterial = IfcMaterialLayerSet
|     occurrence-> IfcRelAssociatesMaterial.RelatingMaterial = IfcMaterialLayerSetUsage
|                  (the usage ForLayerSet points back at the same set)
|
+-- Profiled element (beam, column, member, pile) ? [IFC4+ only]
|     type      -> RelatingMaterial = IfcMaterialProfileSet
|     occurrence-> RelatingMaterial = IfcMaterialProfileSetUsage
|
+-- Non-layered, non-profiled composite (window, door) ?
      IFC4+  -> IfcMaterialConstituentSet
      IFC2x3 -> IfcMaterialList (the only option in 2x3)

NEVER put a *Usage entity on a type : it is occurrence-only (mistake 3).

Patterns

Pattern 1 : Name a custom property set

ALWAYS reserve Pset_ and Qto_ for buildingSMART published sets. A custom set ALWAYS uses a distinguishing project or vendor prefix.

  • Correct published use : Pset_WallCommon, content matching the published template (each property name, type and value type as specified).
  • Correct custom use : ACME_WallAcoustics, ProjectX_CostData.
  • NEVER : Pset_WallAcoustics for a custom set. It is schema-valid, so no validator stops it, but it shadows or collides with the standard library and receiving software cannot tell custom data from standard data.

A custom Pset_ name is mistake 1. The fix is a rename ; see references/examples.md Example 1.

Pattern 2 : Attach a property set to a type, not through the wrong relationship

IfcRelDefinesByProperties attaches a set to occurrences. In IFC4 and IFC4.3 its RelatedObjects is SET [1:?] OF IfcObjectDefinition, which syntactically admits an IfcTypeObject ; the WHERE rule NoRelatedTypeObject ("There shall be no related object being of type IfcTypeObject") forbids it.

ALWAYS attach a type's property sets through the HasPropertySets aggregation on IfcTypeObject itself. NEVER list a type in IfcRelDefinesByProperties.RelatedObjects. See references/examples.md Example 2.

Version note : in IFC2x3 RelatedObjects is SET [1:?] OF IfcObject, and IfcTypeObject is not an IfcObject, so the schema type alone blocks the mistake and no WHERE rule is needed. The widening to IfcObjectDefinition in IFC4 is exactly why NoRelatedTypeObject was added.

Pattern 3 : Bare set on the type, usage on the occurrence

IfcMaterialLayerSet and IfcMaterialProfileSet are the bare structured sets. IfcMaterialLayerSetUsage and IfcMaterialProfileSetUsage add placement and are occurrence-only : the IFC4.3 specification states the usage "is always assigned to an individual occurrence object (and only to relevant subtypes of IfcElement)."

ALWAYS associate the element TYPE (IfcWallType, IfcBeamType) with the bare IfcMaterialLayerSet / IfcMaterialProfileSet. ALWAYS associate each OCCURRENCE with a ...SetUsage whose ForLayerSet / ForProfileSet points back at that same bare set. NEVER associate a ...SetUsage with a type. See references/examples.md Example 3.

Pattern 4 : Leave TotalThickness derived

IfcMaterialLayerSet.TotalThickness is a derived attribute : its value is computed from the layer thicknesses by the schema function IfcMlsTotalThickness. A derived attribute is never stored. In a STEP file its positional slot ALWAYS holds the token *. Writing a literal number there is mistake 4 : the value is either rejected as malformed or silently recomputed and discarded.

ALWAYS write * in the TotalThickness slot. To change the total thickness, change the LayerThickness of the layers. See references/examples.md Example 4.

Pattern 5 : Match the quantity unit to the quantity type

IfcQuantityArea carries WHERE rule WR21 : "If a unit is given, the unit type shall be area unit" (Unit.UnitType = IfcUnitEnum.AREAUNIT). It also carries WR22 : AreaValue >= 0. The sibling quantities behave the same way : IfcQuantityLength needs a LENGTHUNIT, IfcQuantityVolume a VOLUMEUNIT, and so on.

The Unit attribute of every IfcPhysicalSimpleQuantity is OPTIONAL. ALWAYS EITHER omit Unit, so the quantity inherits the matching unit from the project IfcUnitAssignment, OR supply a unit whose UnitType matches the quantity entity. NEVER put a LENGTHUNIT on IfcQuantityArea. See references/examples.md Example 5.

Pattern 6 : Replace IfcMaterialList in IFC4 and IFC4.3

IfcMaterialList (single attribute Materials : LIST [1:?] OF IfcMaterial) is a bare ordered list with no per-part semantics, fractions or geometry. It is the IFC2x3 mechanism for a non-layered composite. It remains in IFC4 and IFC4.3 ONLY for round-tripping IFC2x3 data.

ALWAYS use IfcMaterialConstituentSet for a non-layered, non-profiled composite in a new IFC4 or IFC4.3 file : each IfcMaterialConstituent names a part, its Material, an optional Fraction and Category. An IfcMaterialList in a new IFC4+ file is mistake 6 : schema-valid, but a smell that loses constituent semantics. See references/examples.md Example 6.

Version Notes

Entity / rule IFC2x3 IFC4 / IFC4.3
IfcRelDefinesByProperties.RelatedObjects SET [1:?] OF IfcObject ; no WHERE rule needed SET [1:?] OF IfcObjectDefinition ; WHERE rule NoRelatedTypeObject
IfcMaterialProfileSet family does not exist introduced in IFC4
IfcMaterialConstituentSet family does not exist introduced in IFC4
IfcMaterialList the standard non-layered mechanism legacy ; superseded by IfcMaterialConstituentSet
Quantity-set prefix BaseQuantities convention Qto_ prefix (IFC4+)
IfcQuantityNumber does not exist introduced in IFC4
IfcMaterialProperties split into typed subtypes (IfcMechanicalMaterialProperties, ...) one consolidated non-abstract entity

Material usage entities (IfcMaterialLayerSetUsage, IfcMaterialProfileSetUsage) are occurrence-only in every version that defines them. The element-supertype rename IfcBuildingElement to IfcBuiltElement (IFC4.3) does not affect any entity in this skill : property sets, quantities and materials attach through IfcObject / IfcObjectDefinition and IfcTypeObject, whose names are stable.

Reference Links

  • references/methods.md : entity and attribute signatures for IfcPropertySet, IfcRelDefinesByProperties, IfcElementQuantity, the quantity subtypes, the material set and usage entities, with cardinalities and WHERE rules.
  • references/examples.md : the wrong STEP and the corrected STEP for each of the six mistakes, plus ifcopenshell detection snippets.
  • references/anti-patterns.md : the mistakes in detail, why each fails, and the deterministic rule.

Official sources (verified 2026-05-20)