ifc-syntax-materials

IFC Syntax : Materials

IFC carries the material of an element with two parts : a material definition (what the element is made of) and an objectified relationship (IfcRelAssociatesMaterial) that binds that definition to one or more elements. The material definition is never a child of the element ; it floats in the file and is linked.

This skill covers the plain IfcMaterial, the three structured material families (layered, profiled, constituent), the legacy IfcMaterialList, and IfcMaterialProperties.

Applies to IFC2x3, IFC4, and IFC4.3. The profiled and constituent families and several attributes are IFC4 additions : see the Version Differences section.

Quick Reference

The association pattern

IfcRelAssociatesMaterial binds a material definition to elements. Verified verbatim (IFC4.3) :

ENTITY IfcRelAssociatesMaterial
 SUBTYPE OF (IfcRelAssociates);
	RelatingMaterial : IfcMaterialSelect;
 WHERE
	AllowedElements : ...RelatedObjects restricted to IfcElement,
		IfcElementType, IfcStructuralMember, IfcPort;
	NoVoidElement : RelatedObjects must not contain
		IfcFeatureElementSubtraction or IfcVirtualElement;
END_ENTITY;

• RelatedObjects (SET [1:?] OF IfcDefinitionSelect, inherited from IfcRelAssociates) : the elements or element types receiving the material.
• RelatingMaterial (IfcMaterialSelect) : the material definition assigned.

ALWAYS share one IfcRelAssociatesMaterial across many objects when they take the same material : RelatedObjects is a set. NEVER point RelatedObjects at an IfcFeatureElementSubtraction (an opening) or an IfcVirtualElement : the NoVoidElement rule rejects it.

IfcMaterialSelect

IfcMaterialSelect is the abstraction in RelatingMaterial. The assigned material is not always a plain IfcMaterial :

IfcMaterialSelect (SELECT)
├── IfcMaterial                    one homogeneous substance
├── IfcMaterialLayerSet            an ordered layer stack          (for types)
├── IfcMaterialLayerSetUsage       a positioned layer stack        (for occurrences)
├── IfcMaterialProfileSet          a set of cross-section profiles (for types)   [IFC4+]
├── IfcMaterialProfileSetUsage     a positioned profile set        (for occurrences) [IFC4+]
├── IfcMaterialConstituentSet      named composite parts                          [IFC4+]
└── IfcMaterialList                a bare list of materials                  (legacy)

The two usage entities (IfcMaterialLayerSetUsage, IfcMaterialProfileSetUsage) are restricted to element occurrences ; the bare set entities go on the element type. See the set-on-type rule below.

IfcMaterial

Verified verbatim (IFC4.3) :

ENTITY IfcMaterial
 SUBTYPE OF (IfcMaterialDefinition);
	Name : IfcLabel;
	Description : OPTIONAL IfcText;
	Category : OPTIONAL IfcLabel;
 INVERSE
	HasRepresentation : SET [0:1] OF IfcMaterialDefinitionRepresentation FOR RepresentedMaterial;
	IsRelatedWith : SET [0:?] OF IfcMaterialRelationship FOR RelatedMaterials;
	RelatesTo : SET [0:1] OF IfcMaterialRelationship FOR RelatingMaterial;
END_ENTITY;

• Name is required ; Description and Category are optional.
• Category is a broad grouping ('concrete', 'steel', 'aluminium').
• In IFC2x3 IfcMaterial had only Name : Description and Category are IFC4 additions.

Layered materials : walls, slabs, roofs, plates

IfcMaterialLayerSet is the ordered stack ; IfcMaterialLayer is one layer ; IfcMaterialLayerSetUsage positions the stack relative to the element.

ENTITY IfcMaterialLayerSet
 SUBTYPE OF (IfcMaterialDefinition);
	MaterialLayers : LIST [1:?] OF IfcMaterialLayer;
	LayerSetName : OPTIONAL IfcLabel;
	Description : OPTIONAL IfcText;
 DERIVE
	TotalThickness : IfcLengthMeasure := IfcMlsTotalThickness(SELF);
END_ENTITY;

TotalThickness is DERIVED : it is computed from the layer thicknesses by the function IfcMlsTotalThickness. NEVER serialize a value for it.

ENTITY IfcMaterialLayer
 SUBTYPE OF (IfcMaterialDefinition);
	Material : OPTIONAL IfcMaterial;
	LayerThickness : IfcNonNegativeLengthMeasure;
	IsVentilated : OPTIONAL IfcLogical;
	Name : OPTIONAL IfcLabel;
	Description : OPTIONAL IfcText;
	Category : OPTIONAL IfcLabel;
	Priority : OPTIONAL IfcInteger;
 WHERE
	NormalizedPriority : NOT(EXISTS(Priority)) OR {0 <= Priority <= 100};
END_ENTITY;

• LayerThickness is required. IsVentilated is TRUE for a ventilated air gap, UNKNOWN for a non-ventilated air gap, FALSE for solid material.
• Priority (when present) MUST be in [0..100] (NormalizedPriority rule) ; it governs which layer wins at element joints.
• IfcMaterialLayerSetUsage carries ForLayerSet, LayerSetDirection (IfcLayerSetDirectionEnum : AXIS1 / AXIS2 / AXIS3), DirectionSense (IfcDirectionSenseEnum : POSITIVE / NEGATIVE), OffsetFromReferenceLine (IfcLengthMeasure), and optional ReferenceExtent.

In IFC2x3 IfcMaterialLayer had only Material, LayerThickness (typed IfcPositiveLengthMeasure), and IsVentilated : Name, Description, Category, and Priority are IFC4 additions, and LayerThickness was widened to IfcNonNegativeLengthMeasure in IFC4.

Full layered-family signatures : see references/methods.md.

Profiled materials : beams, columns, members, piles (IFC4+)

The profiled family was introduced in IFC4 and does NOT exist in IFC2x3.

ENTITY IfcMaterialProfileSet
 SUBTYPE OF (IfcMaterialDefinition);
	Name : OPTIONAL IfcLabel;
	Description : OPTIONAL IfcText;
	MaterialProfiles : LIST [1:?] OF IfcMaterialProfile;
	CompositeProfile : OPTIONAL IfcCompositeProfileDef;
END_ENTITY;

ENTITY IfcMaterialProfile
 SUBTYPE OF (IfcMaterialDefinition);
	Name : OPTIONAL IfcLabel;
	Description : OPTIONAL IfcText;
	Material : OPTIONAL IfcMaterial;
	Profile : IfcProfileDef;
	Priority : OPTIONAL IfcInteger;
	Category : OPTIONAL IfcLabel;
 WHERE
	NormalizedPriority : NOT(EXISTS(Priority)) OR {0 <= Priority <= 100};
END_ENTITY;

• Profile (IfcProfileDef) is the required cross-section. See ifc-syntax-geometry-profiles for IfcProfileDef.
• IfcMaterialProfileSetUsage carries ForProfileSet, optional CardinalPoint (IfcCardinalPointReference), and optional ReferenceExtent.

IfcCardinalPointReference is a defined type over INTEGER, NOT an enumeration :

TYPE IfcCardinalPointReference = INTEGER;
 WHERE
	GreaterThanZero : SELF > 0;
END_TYPE;

Index meaning : 1-9 bounding-box points (bottom / mid-depth / top, each left / centre / right) ; 10-14 geometric centroid and aligned variants ; 15-19 shear-centre and aligned variants ; 20 lowest point inside a pipe or channel profile. Other index values are outside the specification's scope.

Constituent materials : windows, doors, composite parts (IFC4+)

For composite elements that are neither layered nor profiled (a window : frame + glazing + sealant). IfcMaterialConstituentSet and IfcMaterialConstituent were introduced in IFC4 ; they replace IfcMaterialList for this purpose.

ENTITY IfcMaterialConstituentSet
 SUBTYPE OF (IfcMaterialDefinition);
	Name : OPTIONAL IfcLabel;
	Description : OPTIONAL IfcText;
	MaterialConstituents : OPTIONAL SET [1:?] OF IfcMaterialConstituent;
END_ENTITY;

ENTITY IfcMaterialConstituent
 SUBTYPE OF (IfcMaterialDefinition);
	Name : OPTIONAL IfcLabel;
	Description : OPTIONAL IfcText;
	Material : IfcMaterial;
	Fraction : OPTIONAL IfcNormalisedRatioMeasure;
	Category : OPTIONAL IfcLabel;
END_ENTITY;

IfcMaterialConstituent.Material is required ; Fraction is the fraction of total volume or weight as an IfcNormalisedRatioMeasure ([0..1]).

IfcMaterialList : the IFC2x3 legacy mechanism

ENTITY IfcMaterialList;
	Materials : LIST [1:?] OF IfcMaterial;
END_ENTITY;

IfcMaterialList is a bare ordered list of materials with no per-part semantics, fractions, thicknesses, or geometry. It is the IFC2x3 composite mechanism. It is retained in IFC4 only for round-tripping IFC2x3 data. ALWAYS use IfcMaterialConstituentSet for a new IFC4+ composite. An IfcMaterialList in a new IFC4 file is a smell : see references/anti-patterns.md.

IfcMaterialProperties

ENTITY IfcMaterialProperties
 SUBTYPE OF (IfcExtendedProperties);
	Material : IfcMaterialDefinition;
END_ENTITY;

Supertype chain : IfcPropertyAbstraction → IfcExtendedProperties → IfcMaterialProperties. It inherits Name (OPTIONAL IfcIdentifier), Description (OPTIONAL IfcText), and Properties (SET [1:?] OF IfcProperty).

IFC4 change : in IFC2x3 material properties used specialised abstract subtypes (IfcMechanicalMaterialProperties, IfcThermalMaterialProperties, IfcHygroscopicMaterialProperties, IfcGeneralMaterialProperties, IfcOpticalMaterialProperties). IFC4 deleted all of those, made IfcMaterialProperties a single non-abstract entity, and widened Material from IfcMaterial to IfcMaterialDefinition so properties can attach to layers, profiles, constituents, and their sets. NEVER write the deleted subtypes in IFC4+.

The set-on-type, usage-on-occurrence rule

For layered and profiled elements the model splits the material across two levels :

• The element type (IfcWallType, IfcBeamType, ...) is associated with the bare set (IfcMaterialLayerSet or IfcMaterialProfileSet).
• Each element occurrence (IfcWall, IfcBeam, ...) is associated with a usage (IfcMaterialLayerSetUsage / IfcMaterialProfileSetUsage) that points back at the same set via ForLayerSet / ForProfileSet and adds placement.

ALWAYS put the usage on the occurrence and the bare set on the type. NEVER associate an IfcMaterialLayerSetUsage or IfcMaterialProfileSetUsage with an element type : the IFC specification restricts the usage entities to occurrences.

Version differences (compact)

Item	IFC2x3	IFC4	IFC4.3
IfcMaterial.Description, .Category	absent	present	present
IfcMaterialLayer.Name/.Description/.Category/.Priority	absent	present	present
IfcMaterialLayer.LayerThickness type	IfcPositiveLengthMeasure	IfcNonNegativeLengthMeasure	IfcNonNegativeLengthMeasure
IfcMaterialLayerSet.Description	absent	present	present
Profiled family (IfcMaterialProfileSet etc.)	absent	present	present
Constituent family (IfcMaterialConstituentSet etc.)	absent	present	present
IfcMaterialProperties	abstract, typed subtypes	one non-abstract entity	one non-abstract entity
IfcMaterialList	composite mechanism	legacy, retained	legacy, retained

Decision Trees

Which material definition for a given element?

What is the element made of?
|
+-- One homogeneous substance (a solid concrete footing)
|   --> IfcMaterial, associated directly.
|
+-- Parallel layers with thickness (wall, slab, roof, plate)
|   --> IfcMaterialLayerSet on the type;
|       IfcMaterialLayerSetUsage on each occurrence.
|
+-- A cross-section profile extruded along an axis (beam, column, pile)  [IFC4+]
|   --> IfcMaterialProfileSet on the type;
|       IfcMaterialProfileSetUsage on each occurrence.
|
+-- Named composite parts, not layered, not profiled (window, door)  [IFC4+]
|   --> IfcMaterialConstituentSet with one IfcMaterialConstituent per part.
|
+-- A plain list of materials, IFC2x3 round-trip only
    --> IfcMaterialList. NEVER author this fresh in IFC4+.

Set or usage : which goes where?

Are you associating a layer-set or profile-set?
|
+-- The target is an element TYPE (IfcWallType, IfcBeamType)
|   --> associate the bare set (IfcMaterialLayerSet / IfcMaterialProfileSet).
|
+-- The target is an element OCCURRENCE (IfcWall, IfcBeam)
    --> associate the usage (IfcMaterialLayerSetUsage / IfcMaterialProfileSetUsage),
        whose ForLayerSet / ForProfileSet points back at the type's set.

Is this attribute mine to write?

About to write a value for IfcMaterialLayerSet.TotalThickness?
|
+-- ALWAYS stop : TotalThickness is DERIVED (IfcMlsTotalThickness).
    The reader computes it from the layer thicknesses. NEVER serialize it.

Patterns

Pattern : Associate a plain material

ALWAYS create one IfcMaterial with a required Name, then bind it with one IfcRelAssociatesMaterial whose RelatingMaterial is that IfcMaterial and whose RelatedObjects lists the elements. ALWAYS reuse one IfcRelAssociatesMaterial across every element that shares the material : RelatedObjects is a set. NEVER duplicate an identical IfcMaterial per element : create it once and reference it.

Pattern : Build a layered wall or slab

ALWAYS model a layered element with IfcMaterialLayerSet holding an ordered MaterialLayers list ; each IfcMaterialLayer carries a Material and a LayerThickness. ALWAYS associate the bare IfcMaterialLayerSet with the element type and an IfcMaterialLayerSetUsage with each occurrence ; the usage's ForLayerSet points at the same IfcMaterialLayerSet. NEVER write a value for the derived TotalThickness. NEVER attach the IfcMaterialLayerSetUsage to the type : usage is occurrence-only.

Pattern : Build a profiled beam or column (IFC4+)

ALWAYS model a profiled element with IfcMaterialProfileSet holding MaterialProfiles ; each IfcMaterialProfile carries a required Profile (IfcProfileDef) and an optional Material. ALWAYS associate the bare IfcMaterialProfileSet with the element type and an IfcMaterialProfileSetUsage with each occurrence ; set CardinalPoint to align the profile to the member axis when needed. NEVER use the profiled family in an IFC2x3 file : it does not exist before IFC4.

Pattern : Build a constituent set for a window or door (IFC4+)

ALWAYS model a non-layered, non-profiled composite with IfcMaterialConstituentSet ; add one IfcMaterialConstituent per part (frame, glazing, sealant), each with a required Material and an optional Name naming the part. NEVER use IfcMaterialList for a new composite : IfcMaterialConstituentSet carries the per-part names, fractions, and categories that IfcMaterialList cannot.

Pattern : Attach material properties

ALWAYS attach material properties with IfcMaterialProperties, whose Material references the IfcMaterialDefinition (a material, layer, profile, or constituent) and whose Properties set holds IfcProperty instances. ALWAYS use the single non-abstract IfcMaterialProperties entity in IFC4 and IFC4.3. NEVER write IfcMechanicalMaterialProperties, IfcThermalMaterialProperties, IfcHygroscopicMaterialProperties, IfcGeneralMaterialProperties, or IfcOpticalMaterialProperties in IFC4+ : those subtypes were deleted in IFC4.

Pattern : Honour the version differences

ALWAYS check the target schema before writing a material structure : the profiled and constituent families and several attributes are IFC4 additions. ALWAYS write only IfcMaterial.Name in IFC2x3 : Description and Category were added in IFC4. NEVER carry an IfcMaterialList into a freshly authored IFC4+ model : it is a legacy artefact kept only for round-tripping IFC2x3 data.

Reference Links

• references/methods.md : the complete verified entity definitions for the layered, profiled, and constituent families side by side, IfcRelAssociatesMaterial, IfcMaterial, IfcMaterialProperties, IfcMaterialList, the IfcMaterialSelect and IfcMaterialDefinition trees, the IfcCardinalPointReference index table, and the per-version difference matrix.
• references/examples.md : worked STEP scenarios : a plain material association, a three-layer wall with type and occurrence, a steel beam profile set, a window constituent set, material properties, and an IFC2x3 IfcMaterialList.
• references/anti-patterns.md : the failure modes this skill prevents, each with the reason it fails.

Official sources

• IfcMaterial : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterial.htm
• IfcRelAssociatesMaterial : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcRelAssociatesMaterial.htm
• IfcMaterialLayerSet : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialLayerSet.htm
• IfcMaterialLayer : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialLayer.htm
• IfcMaterialLayerSetUsage : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialLayerSetUsage.htm
• IfcMaterialProfileSet : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialProfileSet.htm
• IfcMaterialProfile : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialProfile.htm
• IfcMaterialProfileSetUsage : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialProfileSetUsage.htm
• IfcCardinalPointReference : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcCardinalPointReference.htm
• IfcMaterialConstituentSet : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialConstituentSet.htm
• IfcMaterialConstituent : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialConstituent.htm
• IfcMaterialProperties : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialProperties.htm
• IfcMaterialList (IFC2x3) : https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/TC1/HTML/ifcmaterialresource/lexical/ifcmateriallist.htm

Related skills

• ifc-syntax-geometry-profiles : IfcProfileDef, the cross-section that IfcMaterialProfile.Profile requires.
• ifc-syntax-property-sets : IfcProperty and the property entities that IfcMaterialProperties.Properties holds.
• ifc-impl-data-enrichment : the workflow for adding materials to an existing model.
• ifc-errors-property-set-mistakes : material and property failure modes in real files.